Hello everyone,
it's a well known problem that the DMA-buf subsystem mixed synchronization and memory management requirements into the same dma_fence and dma_resv objects. Because of this dma_fence objects need to guarantee that they complete within a finite amount of time or otherwise the system can easily deadlock.
One of the few good things about this problem is that it is really good understood by now.
Daniel and others came up with some documentation: https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_b...
And Jason did an excellent presentation about that problem on last years LPC: https://lpc.events/event/11/contributions/1115/
Based on that we had been able to reject new implementations of infinite/user DMA fences and mitigate the effect of the few existing ones.
The still remaining down side is that we don't have a way of using user fences as dependency in both the explicit (sync_file, drm_syncobj) as well as the implicit (dma_resv) synchronization objects, resulting in numerous problems and limitations for things like HMM, user queues etc....
This patch set here now tries to tackle this problem by untangling the synchronization from the memory management. What it does *not* try to do is to fix the existing kernel fences, because I think we now can all agree on that this isn't really possible.
To archive this goal what I do in this patch set is to add some parallel infrastructure to cleanly separate normal kernel dma_fence objects from indefinite/user fences:
1. It introduce a DMA_FENCE_FLAG_USER define (after renaming some existing driver defines). To note that a certain dma_fence is an user fence and *must* be ignore by memory management and never used as dependency for normal none user dma_fence objects.
2. The dma_fence_array and dma_fence_chain containers are modified so that they are marked as user fences whenever any of their contained fences are an user fence.
3. The dma_resv object gets a new DMA_RESV_USAGE_USER flag which must be used with indefinite/user fences and separates those into it's own synchronization domain.
4. The existing dma_buf_poll_add_cb() function is modified so that indefinite/user fences are included in the polling.
5. The sync_file synchronization object is modified so that we essentially have two fence streams instead of just one.
6. The drm_syncobj is modified in a similar way. User fences are just ignored unless the driver explicitly states support to wait for them.
7. The DRM subsystem gains a new DRIVER_USER_FENCE flag which drivers can use to indicate the need for user fences. If user fences are used the atomic mode setting starts to support user fences as IN/OUT fences.
8. Lockdep is used at various critical locations to ensure that nobody ever tries to mix user fences with non user fences.
The general approach is to just ignore user fences unless a driver stated explicitely support for them.
On top of all of this I've hacked amdgpu so that we add the resulting CS fence only as kernel dependency to the dma_resv object and an additional wrapped up with a dma_fence_array and a stub user fence.
The result is that the newly added atomic modeset functions now correctly wait for the user fence to complete before doing the flip. And dependent CS don't pipeline any more, but rather block on the CPU before submitting work.
After tons of debugging and testing everything now seems to not go up in flames immediately and even lockdep is happy with the annotations.
I'm perfectly aware that this is probably by far the most controversial patch set I've ever created and I really wish we wouldn't need it. But we certainly have the requirement for this and I don't see much other chance to get that working in an UAPI compatible way.
Thoughts/comments?
Regards, Christian.
This is supposed to be used by device drivers.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/i915/i915_request.h | 2 +- drivers/gpu/drm/i915/i915_sw_fence_work.h | 2 +- drivers/gpu/drm/nouveau/nouveau_fence.c | 4 ++-- include/linux/dma-fence.h | 4 ++-- 4 files changed, 6 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h index 28b1f9db5487..716c8c413cc4 100644 --- a/drivers/gpu/drm/i915/i915_request.h +++ b/drivers/gpu/drm/i915/i915_request.h @@ -80,7 +80,7 @@ enum { * * See i915_request_is_active() */ - I915_FENCE_FLAG_ACTIVE = DMA_FENCE_FLAG_USER_BITS, + I915_FENCE_FLAG_ACTIVE = DMA_FENCE_FLAG_DRIVER,
/* * I915_FENCE_FLAG_PQUEUE - this request is ready for execution diff --git a/drivers/gpu/drm/i915/i915_sw_fence_work.h b/drivers/gpu/drm/i915/i915_sw_fence_work.h index d56806918d13..ece0a06e598c 100644 --- a/drivers/gpu/drm/i915/i915_sw_fence_work.h +++ b/drivers/gpu/drm/i915/i915_sw_fence_work.h @@ -33,7 +33,7 @@ struct dma_fence_work { };
enum { - DMA_FENCE_WORK_IMM = DMA_FENCE_FLAG_USER_BITS, + DMA_FENCE_WORK_IMM = DMA_FENCE_FLAG_DRIVER, };
void dma_fence_work_init(struct dma_fence_work *f, diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c index 7f01dcf81fab..e2f61b34cc1e 100644 --- a/drivers/gpu/drm/nouveau/nouveau_fence.c +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c @@ -61,7 +61,7 @@ nouveau_fence_signal(struct nouveau_fence *fence) list_del(&fence->head); rcu_assign_pointer(fence->channel, NULL);
- if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags)) { + if (test_bit(DMA_FENCE_FLAG_DRIVER, &fence->base.flags)) { struct nouveau_fence_chan *fctx = nouveau_fctx(fence);
if (!--fctx->notify_ref) @@ -510,7 +510,7 @@ static bool nouveau_fence_enable_signaling(struct dma_fence *f)
ret = nouveau_fence_no_signaling(f); if (ret) - set_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags); + set_bit(DMA_FENCE_FLAG_DRIVER, &fence->base.flags); else if (!--fctx->notify_ref) nvif_notify_put(&fctx->notify);
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index 775cdc0b4f24..afea82ec5946 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -49,7 +49,7 @@ struct dma_fence_cb; * DMA_FENCE_FLAG_SIGNALED_BIT - fence is already signaled * DMA_FENCE_FLAG_TIMESTAMP_BIT - timestamp recorded for fence signaling * DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT - enable_signaling might have been called - * DMA_FENCE_FLAG_USER_BITS - start of the unused bits, can be used by the + * DMA_FENCE_FLAG_DRIVER - start of the unused bits, can be used by the * implementer of the fence for its own purposes. Can be used in different * ways by different fence implementers, so do not rely on this. * @@ -99,7 +99,7 @@ enum dma_fence_flag_bits { DMA_FENCE_FLAG_SIGNALED_BIT, DMA_FENCE_FLAG_TIMESTAMP_BIT, DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, - DMA_FENCE_FLAG_USER_BITS, /* must always be last member */ + DMA_FENCE_FLAG_DRIVER, /* must always be last member */ };
typedef void (*dma_fence_func_t)(struct dma_fence *fence,
Start introducing a new part of the framework for handling user fences.
In strict opposition to normal fences user fences don't reliable finish in a fixed amount of time and therefore can't be used as dependency in memory management.
Because of this user fences are marked with DMA_FENCE_FLAG_USER. Lockdep is checked that we can at least fault user pages when we check them for signaling.
This patch also adds a flag to dma_fence_get_stub() so that we can retrieve a signaled user fence. This can be used together with lockdep to test the handling in code path supporting user fences.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-fence-unwrap.c | 4 +-- drivers/dma-buf/dma-fence.c | 31 ++++++++++++------- drivers/dma-buf/st-dma-fence.c | 2 +- .../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 4 +-- drivers/gpu/drm/drm_syncobj.c | 10 +++--- include/linux/dma-fence.h | 17 +++++++++- 9 files changed, 49 insertions(+), 25 deletions(-)
diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c index c9becc74896d..87ee2efced10 100644 --- a/drivers/dma-buf/dma-fence-unwrap.c +++ b/drivers/dma-buf/dma-fence-unwrap.c @@ -76,7 +76,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, }
if (count == 0) - return dma_fence_get_stub(); + return dma_fence_get_stub(false);
if (count > INT_MAX) return NULL; @@ -129,7 +129,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, } while (tmp);
if (count == 0) { - tmp = dma_fence_get_stub(); + tmp = dma_fence_get_stub(false); goto return_tmp; }
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 066400ed8841..52873f5eaeba 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -26,6 +26,7 @@ EXPORT_TRACEPOINT_SYMBOL(dma_fence_signaled);
static DEFINE_SPINLOCK(dma_fence_stub_lock); static struct dma_fence dma_fence_stub; +static struct dma_fence dma_fence_user_stub;
/* * fence context counter: each execution context should have its own @@ -123,24 +124,28 @@ static const struct dma_fence_ops dma_fence_stub_ops = {
/** * dma_fence_get_stub - return a signaled fence + * @user: if true the returned fence is an user fence * - * Return a stub fence which is already signaled. The fence's - * timestamp corresponds to the first time after boot this - * function is called. + * Return a stub fence which is already signaled. The fence's timestamp + * corresponds to the first time after boot this function is called. If @user is + * true an user fence is returned which can be used with lockdep to test user + * fence saveness in a code path. */ -struct dma_fence *dma_fence_get_stub(void) +struct dma_fence *dma_fence_get_stub(bool user) { + struct dma_fence *fence = user ? &dma_fence_stub : &dma_fence_user_stub; + spin_lock(&dma_fence_stub_lock); - if (!dma_fence_stub.ops) { - dma_fence_init(&dma_fence_stub, - &dma_fence_stub_ops, - &dma_fence_stub_lock, + if (!fence->ops) { + dma_fence_init(fence, &dma_fence_stub_ops, &dma_fence_stub_lock, 0, 0); - dma_fence_signal_locked(&dma_fence_stub); + if (user) + set_bit(DMA_FENCE_FLAG_USER, &fence->flags); + dma_fence_signal_locked(fence); } spin_unlock(&dma_fence_stub_lock);
- return dma_fence_get(&dma_fence_stub); + return dma_fence_get(fence); } EXPORT_SYMBOL(dma_fence_get_stub);
@@ -497,8 +502,9 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) return -EINVAL;
might_sleep(); - __dma_fence_might_wait(); + if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + might_fault();
trace_dma_fence_wait_start(fence); if (fence->ops->wait) @@ -870,6 +876,9 @@ dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count, for (i = 0; i < count; ++i) { struct dma_fence *fence = fences[i];
+ if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + might_fault(); + cb[i].task = current; if (dma_fence_add_callback(fence, &cb[i].base, dma_fence_default_wait_cb)) { diff --git a/drivers/dma-buf/st-dma-fence.c b/drivers/dma-buf/st-dma-fence.c index c8a12d7ad71a..50f757f75645 100644 --- a/drivers/dma-buf/st-dma-fence.c +++ b/drivers/dma-buf/st-dma-fence.c @@ -412,7 +412,7 @@ static int test_stub(void *arg) int i;
for (i = 0; i < ARRAY_SIZE(f); i++) { - f[i] = dma_fence_get_stub(); + f[i] = dma_fence_get_stub((i % 2) == 1); if (!dma_fence_is_signaled(f[i])) { pr_err("Obtained unsignaled stub fence!\n"); goto err; diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c index 64ac4f8f49be..541c59635c34 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c @@ -263,7 +263,7 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo, /* TODO: Instead of block before we should use the fence of the page * table update and TLB flush here directly. */ - replacement = dma_fence_get_stub(); + replacement = dma_fence_get_stub(false); dma_resv_replace_fences(bo->tbo.base.resv, ef->base.context, replacement, DMA_RESV_USAGE_READ); dma_fence_put(replacement); diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c index a28b7947a034..95eeab527ca9 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c @@ -1399,7 +1399,7 @@ int amdgpu_cs_fence_to_handle_ioctl(struct drm_device *dev, void *data, return PTR_ERR(fence);
if (!fence) - fence = dma_fence_get_stub(); + fence = dma_fence_get_stub(false);
switch (info->in.what) { case AMDGPU_FENCE_TO_HANDLE_GET_SYNCOBJ: diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c index 7f33ae87cb41..73165f387f3b 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c @@ -193,7 +193,7 @@ int amdgpu_ring_init(struct amdgpu_device *adev, struct amdgpu_ring *ring, adev->rings[ring->idx] = ring; ring->num_hw_submission = sched_hw_submission; ring->sched_score = sched_score; - ring->vmid_wait = dma_fence_get_stub(); + ring->vmid_wait = dma_fence_get_stub(false); r = amdgpu_fence_driver_init_ring(ring); if (r) return r; diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c index 7e5cc8323329..e5c8e72a9485 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c @@ -1689,7 +1689,7 @@ int amdgpu_vm_bo_update_mapping(struct amdgpu_device *adev, }
if (!unlocked && !dma_fence_is_signaled(vm->last_unlocked)) { - struct dma_fence *tmp = dma_fence_get_stub(); + struct dma_fence *tmp = dma_fence_get_stub(false);
amdgpu_bo_fence(vm->root.bo, vm->last_unlocked, true); swap(vm->last_unlocked, tmp); @@ -2905,7 +2905,7 @@ int amdgpu_vm_init(struct amdgpu_device *adev, struct amdgpu_vm *vm) else vm->update_funcs = &amdgpu_vm_sdma_funcs; vm->last_update = NULL; - vm->last_unlocked = dma_fence_get_stub(); + vm->last_unlocked = dma_fence_get_stub(false);
mutex_init(&vm->eviction_lock); vm->evicting = false; diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c index 7e48dcd1bee4..5a961ea90a35 100644 --- a/drivers/gpu/drm/drm_syncobj.c +++ b/drivers/gpu/drm/drm_syncobj.c @@ -255,7 +255,7 @@ static void drm_syncobj_fence_add_wait(struct drm_syncobj *syncobj, dma_fence_put(fence); list_add_tail(&wait->node, &syncobj->cb_list); } else if (!fence) { - wait->fence = dma_fence_get_stub(); + wait->fence = dma_fence_get_stub(false); } else { wait->fence = fence; } @@ -411,7 +411,7 @@ int drm_syncobj_find_fence(struct drm_file *file_private, * signalled, use a new fence instead. */ if (!*fence) - *fence = dma_fence_get_stub(); + *fence = dma_fence_get_stub(false);
goto out; } @@ -1000,7 +1000,7 @@ static void syncobj_wait_syncobj_func(struct drm_syncobj *syncobj, dma_fence_put(fence); return; } else if (!fence) { - wait->fence = dma_fence_get_stub(); + wait->fence = dma_fence_get_stub(false); } else { wait->fence = fence; } @@ -1067,7 +1067,7 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs, if (fence) entries[i].fence = fence; else - entries[i].fence = dma_fence_get_stub(); + entries[i].fence = dma_fence_get_stub(false);
if ((flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE) || dma_fence_is_signaled(entries[i].fence)) { @@ -1472,7 +1472,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data, }
for (i = 0; i < args->count_handles; i++) { - struct dma_fence *fence = dma_fence_get_stub(); + struct dma_fence *fence = dma_fence_get_stub(false);
drm_syncobj_add_point(syncobjs[i], chains[i], fence, points[i]); diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index afea82ec5946..be96687d31d8 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -99,6 +99,16 @@ enum dma_fence_flag_bits { DMA_FENCE_FLAG_SIGNALED_BIT, DMA_FENCE_FLAG_TIMESTAMP_BIT, DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, + + /** + * @DMA_FENCE_FLAG_USER: + * + * Indicates an user fence. User fences are not guaranteed to signal in + * a finite amount of time. Because of this it is not allowed to wait for user + * fences with any lock held nor depend the signaling of none user + * fences on them. + */ + DMA_FENCE_FLAG_USER, DMA_FENCE_FLAG_DRIVER, /* must always be last member */ };
@@ -398,6 +408,8 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence); static inline bool dma_fence_is_signaled_locked(struct dma_fence *fence) { + WARN_ON(test_bit(DMA_FENCE_FLAG_USER, &fence->flags)); + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true;
@@ -428,6 +440,9 @@ dma_fence_is_signaled_locked(struct dma_fence *fence) static inline bool dma_fence_is_signaled(struct dma_fence *fence) { + if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + might_fault(); + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true;
@@ -583,7 +598,7 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr) return ret < 0 ? ret : 0; }
-struct dma_fence *dma_fence_get_stub(void); +struct dma_fence *dma_fence_get_stub(bool user); struct dma_fence *dma_fence_allocate_private_stub(void); u64 dma_fence_context_alloc(unsigned num);
On 02/05/2022 17:37, Christian König wrote:
Start introducing a new part of the framework for handling user fences.
In strict opposition to normal fences user fences don't reliable finish in
reliably
a fixed amount of time and therefore can't be used as dependency in memory management.
Because of this user fences are marked with DMA_FENCE_FLAG_USER. Lockdep is checked that we can at least fault user pages when we check them for signaling.
This patch also adds a flag to dma_fence_get_stub() so that we can retrieve a signaled user fence. This can be used together with lockdep to test the handling in code path supporting user fences.
Signed-off-by: Christian König christian.koenig@amd.com
drivers/dma-buf/dma-fence-unwrap.c | 4 +-- drivers/dma-buf/dma-fence.c | 31 ++++++++++++------- drivers/dma-buf/st-dma-fence.c | 2 +- .../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 4 +-- drivers/gpu/drm/drm_syncobj.c | 10 +++--- include/linux/dma-fence.h | 17 +++++++++- 9 files changed, 49 insertions(+), 25 deletions(-)
diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c index c9becc74896d..87ee2efced10 100644 --- a/drivers/dma-buf/dma-fence-unwrap.c +++ b/drivers/dma-buf/dma-fence-unwrap.c @@ -76,7 +76,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, } if (count == 0)
return dma_fence_get_stub();
return dma_fence_get_stub(false);if (count > INT_MAX) return NULL; @@ -129,7 +129,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, } while (tmp); if (count == 0) {
tmp = dma_fence_get_stub();
tmp = dma_fence_get_stub(false);diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 066400ed8841..52873f5eaeba 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -26,6 +26,7 @@ EXPORT_TRACEPOINT_SYMBOL(dma_fence_signaled); static DEFINE_SPINLOCK(dma_fence_stub_lock); static struct dma_fence dma_fence_stub; +static struct dma_fence dma_fence_user_stub; /*
- fence context counter: each execution context should have its own
@@ -123,24 +124,28 @@ static const struct dma_fence_ops dma_fence_stub_ops = { /**
- dma_fence_get_stub - return a signaled fence
- @user: if true the returned fence is an user fence
- Return a stub fence which is already signaled. The fence's
- timestamp corresponds to the first time after boot this
- function is called.
- Return a stub fence which is already signaled. The fence's timestamp
- corresponds to the first time after boot this function is called. If @user is
- true an user fence is returned which can be used with lockdep to test user
*/
- fence saveness in a code path.
-struct dma_fence *dma_fence_get_stub(void) +struct dma_fence *dma_fence_get_stub(bool user) {
- struct dma_fence *fence = user ? &dma_fence_stub : &dma_fence_user_stub;
- spin_lock(&dma_fence_stub_lock);
- if (!dma_fence_stub.ops) {
dma_fence_init(&dma_fence_stub,&dma_fence_stub_ops,&dma_fence_stub_lock,
- if (!fence->ops) {
dma_fence_init(fence, &dma_fence_stub_ops, &dma_fence_stub_lock, 0, 0);
dma_fence_signal_locked(&dma_fence_stub);
if (user)set_bit(DMA_FENCE_FLAG_USER, &fence->flags);dma_fence_signal_locked(fence);
- return dma_fence_get(&dma_fence_stub);
- return dma_fence_get(fence); } EXPORT_SYMBOL(dma_fence_get_stub);
@@ -497,8 +502,9 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) return -EINVAL; might_sleep();
- __dma_fence_might_wait();
- if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags))
might_fault();trace_dma_fence_wait_start(fence); if (fence->ops->wait) @@ -870,6 +876,9 @@ dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count, for (i = 0; i < count; ++i) { struct dma_fence *fence = fences[i];
if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags))might_fault();- cb[i].task = current; if (dma_fence_add_callback(fence, &cb[i].base, dma_fence_default_wait_cb)) {
diff --git a/drivers/dma-buf/st-dma-fence.c b/drivers/dma-buf/st-dma-fence.c index c8a12d7ad71a..50f757f75645 100644 --- a/drivers/dma-buf/st-dma-fence.c +++ b/drivers/dma-buf/st-dma-fence.c @@ -412,7 +412,7 @@ static int test_stub(void *arg) int i; for (i = 0; i < ARRAY_SIZE(f); i++) {
f[i] = dma_fence_get_stub();
f[i] = dma_fence_get_stub((i % 2) == 1);diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c index 64ac4f8f49be..541c59635c34 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c @@ -263,7 +263,7 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo, /* TODO: Instead of block before we should use the fence of the page * table update and TLB flush here directly. */
- replacement = dma_fence_get_stub();
- replacement = dma_fence_get_stub(false); dma_resv_replace_fences(bo->tbo.base.resv, ef->base.context, replacement, DMA_RESV_USAGE_READ); dma_fence_put(replacement);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c index a28b7947a034..95eeab527ca9 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c @@ -1399,7 +1399,7 @@ int amdgpu_cs_fence_to_handle_ioctl(struct drm_device *dev, void *data, return PTR_ERR(fence); if (!fence)
fence = dma_fence_get_stub();
fence = dma_fence_get_stub(false);switch (info->in.what) { case AMDGPU_FENCE_TO_HANDLE_GET_SYNCOBJ: diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c index 7f33ae87cb41..73165f387f3b 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c @@ -193,7 +193,7 @@ int amdgpu_ring_init(struct amdgpu_device *adev, struct amdgpu_ring *ring, adev->rings[ring->idx] = ring; ring->num_hw_submission = sched_hw_submission; ring->sched_score = sched_score;
ring->vmid_wait = dma_fence_get_stub();
ring->vmid_wait = dma_fence_get_stub(false);diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c index 7e5cc8323329..e5c8e72a9485 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c @@ -1689,7 +1689,7 @@ int amdgpu_vm_bo_update_mapping(struct amdgpu_device *adev, } if (!unlocked && !dma_fence_is_signaled(vm->last_unlocked)) {
struct dma_fence *tmp = dma_fence_get_stub();
struct dma_fence *tmp = dma_fence_get_stub(false);amdgpu_bo_fence(vm->root.bo, vm->last_unlocked, true); swap(vm->last_unlocked, tmp); @@ -2905,7 +2905,7 @@ int amdgpu_vm_init(struct amdgpu_device *adev, struct amdgpu_vm *vm) else vm->update_funcs = &amdgpu_vm_sdma_funcs; vm->last_update = NULL;
- vm->last_unlocked = dma_fence_get_stub();
- vm->last_unlocked = dma_fence_get_stub(false);
mutex_init(&vm->eviction_lock); vm->evicting = false; diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c index 7e48dcd1bee4..5a961ea90a35 100644 --- a/drivers/gpu/drm/drm_syncobj.c +++ b/drivers/gpu/drm/drm_syncobj.c @@ -255,7 +255,7 @@ static void drm_syncobj_fence_add_wait(struct drm_syncobj *syncobj, dma_fence_put(fence); list_add_tail(&wait->node, &syncobj->cb_list); } else if (!fence) {
wait->fence = dma_fence_get_stub();
wait->fence = dma_fence_get_stub(false);@@ -411,7 +411,7 @@ int drm_syncobj_find_fence(struct drm_file *file_private, * signalled, use a new fence instead. */ if (!*fence)
*fence = dma_fence_get_stub();
*fence = dma_fence_get_stub(false);goto out; } @@ -1000,7 +1000,7 @@ static void syncobj_wait_syncobj_func(struct drm_syncobj *syncobj, dma_fence_put(fence); return; } else if (!fence) {
wait->fence = dma_fence_get_stub();
wait->fence = dma_fence_get_stub(false);@@ -1067,7 +1067,7 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs, if (fence) entries[i].fence = fence; else
entries[i].fence = dma_fence_get_stub();
entries[i].fence = dma_fence_get_stub(false);if ((flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE) || dma_fence_is_signaled(entries[i].fence)) { @@ -1472,7 +1472,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data, } for (i = 0; i < args->count_handles; i++) {
struct dma_fence *fence = dma_fence_get_stub();
struct dma_fence *fence = dma_fence_get_stub(false);drm_syncobj_add_point(syncobjs[i], chains[i], fence, points[i]); diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index afea82ec5946..be96687d31d8 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -99,6 +99,16 @@ enum dma_fence_flag_bits { DMA_FENCE_FLAG_SIGNALED_BIT, DMA_FENCE_FLAG_TIMESTAMP_BIT, DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
- /**
* @DMA_FENCE_FLAG_USER:** Indicates an user fence. User fences are not guaranteed to signal in* a finite amount of time. Because of this it is not allowed to wait for user* fences with any lock held nor depend the signaling of none user* fences on them.*/- DMA_FENCE_FLAG_USER, DMA_FENCE_FLAG_DRIVER, /* must always be last member */ };
@@ -398,6 +408,8 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence); static inline bool dma_fence_is_signaled_locked(struct dma_fence *fence) {
- WARN_ON(test_bit(DMA_FENCE_FLAG_USER, &fence->flags));
- if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true;
@@ -428,6 +440,9 @@ dma_fence_is_signaled_locked(struct dma_fence *fence) static inline bool dma_fence_is_signaled(struct dma_fence *fence) {
- if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags))
might_fault();
Why this one can fault and the dma_fence_signal_locked cannot? I mean obviosuly it must not in the locked section, but how do signalling callbacks know the context of the caller?
This maybe ties back to the issue I don't think I ever found an explanation to - why "is signal" helpers actually need (or want?) to do the signalling itself, and are therefore hit also with the signalling annotations?
Regards,
Tvrtko
- if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true;
@@ -583,7 +598,7 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr) return ret < 0 ? ret : 0; } -struct dma_fence *dma_fence_get_stub(void); +struct dma_fence *dma_fence_get_stub(bool user); struct dma_fence *dma_fence_allocate_private_stub(void); u64 dma_fence_context_alloc(unsigned num);
Am 04.05.22 um 09:53 schrieb Tvrtko Ursulin:
On 02/05/2022 17:37, Christian König wrote:
[SNIP] @@ -398,6 +408,8 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence); static inline bool dma_fence_is_signaled_locked(struct dma_fence *fence) { + WARN_ON(test_bit(DMA_FENCE_FLAG_USER, &fence->flags));
if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true; @@ -428,6 +440,9 @@ dma_fence_is_signaled_locked(struct dma_fence *fence) static inline bool dma_fence_is_signaled(struct dma_fence *fence) { + if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + might_fault();
Why this one can fault and the dma_fence_signal_locked cannot? I mean obviosuly it must not in the locked section, but how do signalling callbacks know the context of the caller?
Well, dma_fence_is_signaled_locked() shouldn't exists in the first place.
The locking can only be done by either the framework itself or the driver exporting the fence. And if you take at the actual users than you indeed see that the only two cases where this is used the driver knows what fence it has and should probably call it's internal function itself.
This maybe ties back to the issue I don't think I ever found an explanation to - why "is signal" helpers actually need (or want?) to do the signalling itself, and are therefore hit also with the signalling annotations?
Yeah, exactly that's what I could never figure out as well. And I think that it isn't good design to do this, because it results in the dma_fence being signaled from different sources.
Without it it would just be consistently signaled from the interrupt (or whatever context) the exporter uses to signal it.
Thanks, Christian.
Regards,
Tvrtko
if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true; @@ -583,7 +598,7 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr) return ret < 0 ? ret : 0; } -struct dma_fence *dma_fence_get_stub(void); +struct dma_fence *dma_fence_get_stub(bool user); struct dma_fence *dma_fence_allocate_private_stub(void); u64 dma_fence_context_alloc(unsigned num);
When any of the fences inside the array is an user fence the whole array is considered to be an user fence as well.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-fence-array.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c index 5c8a7084577b..de196b07d36d 100644 --- a/drivers/dma-buf/dma-fence-array.c +++ b/drivers/dma-buf/dma-fence-array.c @@ -189,8 +189,13 @@ struct dma_fence_array *dma_fence_array_create(int num_fences, * Enforce this here by checking that we don't create a dma_fence_array * with any container inside. */ - while (num_fences--) - WARN_ON(dma_fence_is_container(fences[num_fences])); + while (num_fences--) { + struct dma_fence *f = fences[num_fences]; + + WARN_ON(dma_fence_is_container(f)); + if (test_bit(DMA_FENCE_FLAG_USER, &f->flags)) + set_bit(DMA_FENCE_FLAG_USER, &array->base.flags); + }
return array; }
If either the previous fence or the newly chained on is an user fence the dma_fence chain node is considered an user fence as well.
This way the user fence status propagates through the chain.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-fence-chain.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c index 06f8ef97c6e8..421241607ac2 100644 --- a/drivers/dma-buf/dma-fence-chain.c +++ b/drivers/dma-buf/dma-fence-chain.c @@ -253,6 +253,10 @@ void dma_fence_chain_init(struct dma_fence_chain *chain, dma_fence_init(&chain->base, &dma_fence_chain_ops, &chain->lock, context, seqno);
+ if ((prev && test_bit(DMA_FENCE_FLAG_USER, &prev->flags)) || + test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + set_bit(DMA_FENCE_FLAG_USER, &chain->base.flags); + /* * Chaining dma_fence_chain container together is only allowed through * the prev fence and not through the contained fence.
This patch adds the new DMA_RESV_USAGE_USER flag to the dma_resv object which must be used with user fence objects.
In opposite to the other usage flags this one doesn't automatically return other lower classes. So when user fences are requested from the dma_resv object only user fences are returned.
Lockdep is used to enforce that user fences can only be queried while the dma_resv object is not locked. Additional to that waiting for the user fences inside a dma_resv object requires not other lock to be held.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-resv.c | 62 +++++++++++++++++++++++--------------- include/linux/dma-resv.h | 23 ++++++++++++-- 2 files changed, 58 insertions(+), 27 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index 0cce6e4ec946..da667c21ad55 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -58,7 +58,7 @@ DEFINE_WD_CLASS(reservation_ww_class); EXPORT_SYMBOL(reservation_ww_class);
/* Mask for the lower fence pointer bits */ -#define DMA_RESV_LIST_MASK 0x3 +#define DMA_RESV_LIST_MASK 0x7
struct dma_resv_list { struct rcu_head rcu; @@ -288,6 +288,10 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence, */ WARN_ON(dma_fence_is_container(fence));
+ /* User fences must be added using DMA_RESV_USAGE_USER */ + WARN_ON(test_bit(DMA_FENCE_FLAG_USER, &fence->flags) != + (usage == DMA_RESV_USAGE_USER)); + fobj = dma_resv_fences_list(obj); count = fobj->num_fences;
@@ -349,6 +353,15 @@ void dma_resv_replace_fences(struct dma_resv *obj, uint64_t context, } EXPORT_SYMBOL(dma_resv_replace_fences);
+/* Matches requested usage with the fence usage for iterators */ +static bool dma_resv_iter_match_usage(struct dma_resv_iter *cursor) +{ + if (cursor->usage == DMA_RESV_USAGE_USER) + return cursor->fence_usage == DMA_RESV_USAGE_USER; + + return cursor->usage >= cursor->fence_usage; +} + /* Restart the unlocked iteration by initializing the cursor object. */ static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor) { @@ -385,8 +398,7 @@ static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor) continue; }
- if (!dma_fence_is_signaled(cursor->fence) && - cursor->usage >= cursor->fence_usage) + if (dma_resv_iter_match_usage(cursor)) break; } while (true); } @@ -405,14 +417,9 @@ static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor) */ struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor) { - rcu_read_lock(); - do { - dma_resv_iter_restart_unlocked(cursor); - dma_resv_iter_walk_unlocked(cursor); - } while (dma_resv_fences_list(cursor->obj) != cursor->fences); - rcu_read_unlock(); - - return cursor->fence; + /* Force a restart */ + cursor->fences = NULL; + return dma_resv_iter_next_unlocked(cursor); } EXPORT_SYMBOL(dma_resv_iter_first_unlocked);
@@ -428,18 +435,21 @@ EXPORT_SYMBOL(dma_resv_iter_first_unlocked); */ struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor) { - bool restart; - - rcu_read_lock(); cursor->is_restarted = false; - restart = dma_resv_fences_list(cursor->obj) != cursor->fences; do { - if (restart) - dma_resv_iter_restart_unlocked(cursor); - dma_resv_iter_walk_unlocked(cursor); - restart = true; - } while (dma_resv_fences_list(cursor->obj) != cursor->fences); - rcu_read_unlock(); + bool restart; + + rcu_read_lock(); + restart = dma_resv_fences_list(cursor->obj) != cursor->fences; + do { + if (restart) + dma_resv_iter_restart_unlocked(cursor); + dma_resv_iter_walk_unlocked(cursor); + restart = true; + } while (dma_resv_fences_list(cursor->obj) != cursor->fences); + rcu_read_unlock(); + + } while (cursor->fence && dma_fence_is_signaled(cursor->fence));
return cursor->fence; } @@ -491,7 +501,7 @@ struct dma_fence *dma_resv_iter_next(struct dma_resv_iter *cursor)
dma_resv_list_entry(cursor->fences, cursor->index++, cursor->obj, &fence, &cursor->fence_usage); - } while (cursor->fence_usage > cursor->usage); + } while (!dma_resv_iter_match_usage(cursor));
return fence; } @@ -663,6 +673,9 @@ long dma_resv_wait_timeout(struct dma_resv *obj, enum dma_resv_usage usage, struct dma_resv_iter cursor; struct dma_fence *fence;
+ if (usage == DMA_RESV_USAGE_USER) + lockdep_assert_none_held_once(); + dma_resv_iter_begin(&cursor, obj, usage); dma_resv_for_each_fence_unlocked(&cursor, fence) {
@@ -678,7 +691,6 @@ long dma_resv_wait_timeout(struct dma_resv *obj, enum dma_resv_usage usage, } EXPORT_SYMBOL_GPL(dma_resv_wait_timeout);
- /** * dma_resv_test_signaled - Test if a reservation object's fences have been * signaled. @@ -717,7 +729,9 @@ EXPORT_SYMBOL_GPL(dma_resv_test_signaled); */ void dma_resv_describe(struct dma_resv *obj, struct seq_file *seq) { - static const char *usage[] = { "kernel", "write", "read", "bookkeep" }; + static const char *usage[] = { + "kernel", "write", "read", "bookkeep", "user" + }; struct dma_resv_iter cursor; struct dma_fence *fence;
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h index c8ccbc94d5d2..81a9ca32cc69 100644 --- a/include/linux/dma-resv.h +++ b/include/linux/dma-resv.h @@ -42,7 +42,6 @@ #include <linux/ww_mutex.h> #include <linux/dma-fence.h> #include <linux/slab.h> -#include <linux/seqlock.h> #include <linux/rcupdate.h>
extern struct ww_class reservation_ww_class; @@ -57,11 +56,15 @@ struct dma_resv_list; * * An important fact is that there is the order KERNEL<WRITE<READ<BOOKKEEP and * when the dma_resv object is asked for fences for one use case the fences - * for the lower use case are returned as well. + * for the lower use case are returned as well. The exception are USER fences + * which only return USER fences and nothing else. * * For example when asking for WRITE fences then the KERNEL fences are returned * as well. Similar when asked for READ fences then both WRITE and KERNEL * fences are returned as well. + * + * But when asked for USER fences only USER fences are returned and not WRITE + * nor any other fences. */ enum dma_resv_usage { /** @@ -103,7 +106,18 @@ enum dma_resv_usage { * The most common case are preemption fences as well as page table * updates and their TLB flushes. */ - DMA_RESV_USAGE_BOOKKEEP + DMA_RESV_USAGE_BOOKKEEP, + + /** + * @DMA_RESV_USAGE_USER: Special usage for user fences. + * + * This must only be used with fences which have DMA_FENCE_FLAG_USER + * set so that memory mangement completely ignores those fences. + * + * A warning is raised if a fence with DMA_FENCE_FLAG USER is added with + * any other usage than DMA_RESV_USAGE_USER. + */ + DMA_RESV_USAGE_USER };
/** @@ -221,6 +235,9 @@ static inline void dma_resv_iter_begin(struct dma_resv_iter *cursor, struct dma_resv *obj, enum dma_resv_usage usage) { + if (usage == DMA_RESV_USAGE_USER) + lockdep_assert_not_held(&(obj)->lock.base); + cursor->obj = obj; cursor->usage = usage; cursor->fence = NULL;
Just make sure that we forward the user fence status to the output whenever any of the inputs are an user fence.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-fence-unwrap.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c index 87ee2efced10..dd3c43aba8f1 100644 --- a/drivers/dma-buf/dma-fence-unwrap.c +++ b/drivers/dma-buf/dma-fence-unwrap.c @@ -66,17 +66,19 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, { struct dma_fence_array *result; struct dma_fence *tmp, **array; + bool user_fence = false; unsigned int i, count;
count = 0; for (i = 0; i < num_fences; ++i) { + user_fence |= test_bit(DMA_FENCE_FLAG_USER, &fences[i]->flags); dma_fence_unwrap_for_each(tmp, &iter[i], fences[i]) if (!dma_fence_is_signaled(tmp)) ++count; }
if (count == 0) - return dma_fence_get_stub(false); + return dma_fence_get_stub(user_fence);
if (count > INT_MAX) return NULL; @@ -129,11 +131,12 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, } while (tmp);
if (count == 0) { - tmp = dma_fence_get_stub(false); + tmp = dma_fence_get_stub(user_fence); goto return_tmp; }
- if (count == 1) { + if (count == 1 && + test_bit(DMA_FENCE_FLAG_USER, &array[0]->flags) == user_fence) { tmp = array[0]; goto return_tmp; } @@ -145,6 +148,8 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, tmp = NULL; goto return_tmp; } + if (user_fence) + set_bit(DMA_FENCE_FLAG_USER, &result->base.flags); return &result->base;
return_tmp:
Allows to filter user fences from a dma_fence_chain and wait for user fences in containers.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-fence-unwrap.c | 47 ++++++++++++++++++++++++++++++ include/linux/dma-fence-unwrap.h | 3 ++ 2 files changed, 50 insertions(+)
diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c index dd3c43aba8f1..4e9eda0b65c5 100644 --- a/drivers/dma-buf/dma-fence-unwrap.c +++ b/drivers/dma-buf/dma-fence-unwrap.c @@ -157,3 +157,50 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, return tmp; } EXPORT_SYMBOL_GPL(__dma_fence_merge); + +/** + * dma_fence_filter_user - filter user fences + * @fence: entry point into the chain + * + * Check @fence and if it's a dma_fence_chain advance it until it doesn't depend + * on any user fence any more. + * + * Returns the advanced fence or NULL if no none user fence could be found. + */ +struct dma_fence *dma_fence_filter_user(struct dma_fence *fence) +{ + + while (fence && test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + fence = dma_fence_chain_walk(fence); + + return fence; +} +EXPORT_SYMBOL(dma_fence_filter_user); + +/** + * dma_fence_wait_user - wait for all user fences to signal + * @fence: entry point + * @timeout: timeout for the wait + * + * Unwrap the potential container in @fence and wait for all the user fences to + * signal. + * Returns: Either negative error code or the remaining timeout. + */ +long dma_fence_wait_user(struct dma_fence *fence, unsigned long timeout) +{ + struct dma_fence_unwrap iter; + long res; + + dma_fence_unwrap_for_each(fence, &iter, fence) { + if (!test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) + continue; + + res = dma_fence_wait(fence, timeout); + if (res <= 0) + return res; + if (timeout) + timeout = res; + } + return timeout; +} +EXPORT_SYMBOL(dma_fence_wait_user); diff --git a/include/linux/dma-fence-unwrap.h b/include/linux/dma-fence-unwrap.h index 7c0fab318301..2a786f5a7819 100644 --- a/include/linux/dma-fence-unwrap.h +++ b/include/linux/dma-fence-unwrap.h @@ -72,4 +72,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, __dma_fence_merge(ARRAY_SIZE(__f), __f, __c); \ })
+struct dma_fence *dma_fence_filter_user(struct dma_fence *fence); +long dma_fence_wait_user(struct dma_fence *fence, unsigned long timeout); + #endif
Just also wait for user fences to signal when we wait for write fences.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-buf.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 79795857be3e..5558f4e555f8 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -225,6 +225,16 @@ static bool dma_buf_poll_add_cb(struct dma_resv *resv, bool write, dma_fence_put(fence); }
+ if (!write) + return false; + + dma_resv_for_each_fence(&cursor, resv, DMA_RESV_USAGE_USER, fence) { + dma_fence_get(fence); + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb); + if (!r) + return true; + dma_fence_put(fence); + } return false; }
Keep user fences separate from normal fences.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/sync_file.c | 82 +++++++++++++++++++++++++++++++++---- include/linux/sync_file.h | 4 +- 2 files changed, 76 insertions(+), 10 deletions(-)
diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c index fe149d7e3ce2..630472d79dc1 100644 --- a/drivers/dma-buf/sync_file.c +++ b/drivers/dma-buf/sync_file.c @@ -70,7 +70,13 @@ struct sync_file *sync_file_create(struct dma_fence *fence) if (!sync_file) return NULL;
- sync_file->fence = dma_fence_get(fence); + if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags)) { + sync_file->fence = dma_fence_get_stub(false); + sync_file->user_fence = dma_fence_get(fence); + } else { + sync_file->fence = dma_fence_get(fence); + sync_file->user_fence = dma_fence_get_stub(true); + }
return sync_file; } @@ -116,6 +122,28 @@ struct dma_fence *sync_file_get_fence(int fd) } EXPORT_SYMBOL(sync_file_get_fence);
+/** + * sync_file_get_user_fence - get user fence related to the sync_file fd + * @fd: sync_file fd to get the fence from + * + * Ensures @fd references a valid sync_file and returns an user fence reference + * which represents all fence in the sync_file. On error NULL is returned. + */ +struct dma_fence *sync_file_get_user_fence(int fd) +{ + struct sync_file *sync_file; + struct dma_fence *fence; + + sync_file = sync_file_fdget(fd); + if (!sync_file) + return NULL; + + fence = dma_fence_merge(sync_file->fence, sync_file->user_fence); + fput(sync_file->file); + return fence; +} +EXPORT_SYMBOL(sync_file_get_user_fence); + /** * sync_file_get_name - get the name of the sync_file * @sync_file: sync_file to get the fence from @@ -136,6 +164,9 @@ char *sync_file_get_name(struct sync_file *sync_file, char *buf, int len) } else { struct dma_fence *fence = sync_file->fence;
+ if (dma_fence_is_signaled(fence)) + fence = sync_file->user_fence; + snprintf(buf, len, "%s-%s%llu-%lld", fence->ops->get_driver_name(fence), fence->ops->get_timeline_name(fence), @@ -159,21 +190,32 @@ char *sync_file_get_name(struct sync_file *sync_file, char *buf, int len) static struct sync_file *sync_file_merge(const char *name, struct sync_file *a, struct sync_file *b) { + struct dma_fence *fence, *user_fence; struct sync_file *sync_file; - struct dma_fence *fence;
sync_file = sync_file_alloc(); if (!sync_file) return NULL;
fence = dma_fence_merge(a->fence, b->fence); - if (!fence) { - fput(sync_file->file); - return NULL; - } + if (!fence) + goto error_fput; + + user_fence = dma_fence_merge(a->user_fence, b->user_fence); + if (!user_fence) + goto error_put_fence; + sync_file->fence = fence; + sync_file->user_fence = user_fence; strlcpy(sync_file->user_name, name, sizeof(sync_file->user_name)); return sync_file; + +error_put_fence: + dma_fence_put(fence); + +error_fput: + fput(sync_file->file); + return NULL; }
static int sync_file_release(struct inode *inode, struct file *file) @@ -183,6 +225,7 @@ static int sync_file_release(struct inode *inode, struct file *file) if (test_bit(POLL_ENABLED, &sync_file->flags)) dma_fence_remove_callback(sync_file->fence, &sync_file->cb); dma_fence_put(sync_file->fence); + dma_fence_put(sync_file->user_fence); kfree(sync_file);
return 0; @@ -191,17 +234,25 @@ static int sync_file_release(struct inode *inode, struct file *file) static __poll_t sync_file_poll(struct file *file, poll_table *wait) { struct sync_file *sync_file = file->private_data; + int ret;
poll_wait(file, &sync_file->wq, wait);
if (list_empty(&sync_file->cb.node) && !test_and_set_bit(POLL_ENABLED, &sync_file->flags)) { - if (dma_fence_add_callback(sync_file->fence, &sync_file->cb, - fence_check_cb_func) < 0) + ret = dma_fence_add_callback(sync_file->fence, &sync_file->cb, + fence_check_cb_func); + if (ret) { + ret = dma_fence_add_callback(sync_file->user_fence, + &sync_file->cb, + fence_check_cb_func); + } + if (ret) wake_up_all(&sync_file->wq); }
- return dma_fence_is_signaled(sync_file->fence) ? EPOLLIN : 0; + return (dma_fence_is_signaled(sync_file->fence) && + dma_fence_is_signaled(sync_file->user_fence)) ? EPOLLIN : 0; }
static long sync_file_ioctl_merge(struct sync_file *sync_file, @@ -299,6 +350,8 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file, num_fences = 0; dma_fence_unwrap_for_each(fence, &iter, sync_file->fence) ++num_fences; + dma_fence_unwrap_for_each(fence, &iter, sync_file->user_fence) + ++num_fences;
/* * Passing num_fences = 0 means that userspace doesn't want to @@ -307,7 +360,12 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file, * info->num_fences. */ if (!info.num_fences) { + int status; + info.status = dma_fence_get_status(sync_file->fence); + status = dma_fence_get_status(sync_file->user_fence); + if (!info.status) + info.status = status; goto no_fences; } else { info.status = 1; @@ -328,6 +386,12 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file, status = sync_fill_fence_info(fence, &fence_info[num_fences++]); info.status = info.status <= 0 ? info.status : status; } + dma_fence_unwrap_for_each(fence, &iter, sync_file->user_fence) { + int status; + + status = sync_fill_fence_info(fence, &fence_info[num_fences++]); + info.status = info.status <= 0 ? info.status : status; + }
if (copy_to_user(u64_to_user_ptr(info.sync_fence_info), fence_info, size)) { diff --git a/include/linux/sync_file.h b/include/linux/sync_file.h index 790ca021203a..14aff1a4ee75 100644 --- a/include/linux/sync_file.h +++ b/include/linux/sync_file.h @@ -50,13 +50,15 @@ struct sync_file { unsigned long flags;
struct dma_fence *fence; - struct dma_fence_cb cb; + struct dma_fence *user_fence; + struct dma_fence_cb cb; };
#define POLL_ENABLED 0
struct sync_file *sync_file_create(struct dma_fence *fence); struct dma_fence *sync_file_get_fence(int fd); +struct dma_fence *sync_file_get_user_fence(int fd); char *sync_file_get_name(struct sync_file *sync_file, char *buf, int len);
#endif /* _LINUX_SYNC_H */
Add a new driver flag indicating support for user fences.
This flag is then used when creating out fences for atomic mode setting, indicating that the mode set might depend on an user fence.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/drm_atomic_uapi.c | 8 ++++++++ include/drm/drm_drv.h | 7 +++++++ 2 files changed, 15 insertions(+)
diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c index 434f3d4cb8a2..e2112c10569b 100644 --- a/drivers/gpu/drm/drm_atomic_uapi.c +++ b/drivers/gpu/drm/drm_atomic_uapi.c @@ -1111,6 +1111,7 @@ static int prepare_signaling(struct drm_device *dev, struct drm_out_fence_state **fence_state, unsigned int *num_fences) { + bool use_user_fence = drm_core_check_feature(dev, DRIVER_USER_FENCE); struct drm_crtc *crtc; struct drm_crtc_state *crtc_state; struct drm_connector *conn; @@ -1120,6 +1121,7 @@ static int prepare_signaling(struct drm_device *dev, if (arg->flags & DRM_MODE_ATOMIC_TEST_ONLY) return 0;
+ for_each_new_crtc_in_state(state, crtc, crtc_state, i) { s32 __user *fence_ptr;
@@ -1168,6 +1170,9 @@ static int prepare_signaling(struct drm_device *dev, if (!fence) return -ENOMEM;
+ if (use_user_fence) + set_bit(DMA_FENCE_FLAG_USER, &fence->flags); + ret = setup_out_fence(&f[(*num_fences)++], fence); if (ret) { dma_fence_put(fence); @@ -1208,6 +1213,9 @@ static int prepare_signaling(struct drm_device *dev, if (!fence) return -ENOMEM;
+ if (use_user_fence) + set_bit(DMA_FENCE_FLAG_USER, &fence->flags); + ret = setup_out_fence(&f[(*num_fences)++], fence); if (ret) { dma_fence_put(fence); diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h index f6159acb8856..b2b8ea8d4a9e 100644 --- a/include/drm/drm_drv.h +++ b/include/drm/drm_drv.h @@ -94,6 +94,13 @@ enum drm_driver_feature { * synchronization of command submission. */ DRIVER_SYNCOBJ_TIMELINE = BIT(6), + /** + * @DRIVER_USER_FENCE: + * + * Drivers supports user fences and waiting for the before command + * submission. + */ + DRIVER_USER_FENCE = BIT(7),
/* IMPORTANT: Below are all the legacy flags, add new ones above. */
When the DRIVER_USER_FENCE flag is set we grab the user fence from the sync file instead of the normal one.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/drm_atomic_uapi.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c index e2112c10569b..d1b13657e2ae 100644 --- a/drivers/gpu/drm/drm_atomic_uapi.c +++ b/drivers/gpu/drm/drm_atomic_uapi.c @@ -517,7 +517,10 @@ static int drm_atomic_plane_set_property(struct drm_plane *plane, if (U642I64(val) == -1) return 0;
- state->fence = sync_file_get_fence(val); + if (drm_core_check_feature(dev, DRIVER_USER_FENCE)) + state->fence = sync_file_get_user_fence(val); + else + state->fence = sync_file_get_fence(val); if (!state->fence) return -EINVAL;
Just handle them the same way as fences with the DMA_RESV_USAGE_WRITE flag when the DRIVER_USER_FENCE flag is set.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/drm_gem_atomic_helper.c | 68 ++++++++++++++++--------- 1 file changed, 45 insertions(+), 23 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem_atomic_helper.c b/drivers/gpu/drm/drm_gem_atomic_helper.c index a5026f617739..75d04333ff2e 100644 --- a/drivers/gpu/drm/drm_gem_atomic_helper.c +++ b/drivers/gpu/drm/drm_gem_atomic_helper.c @@ -5,6 +5,7 @@
#include <drm/drm_atomic_state_helper.h> #include <drm/drm_atomic_uapi.h> +#include <drm/drm_drv.h> #include <drm/drm_gem.h> #include <drm/drm_gem_atomic_helper.h> #include <drm/drm_gem_framebuffer_helper.h> @@ -121,6 +122,40 @@ * Plane Helpers */
+static int chain_fb_fences(struct drm_framebuffer *fb, + enum dma_resv_usage usage, + struct dma_fence **fence) +{ + size_t i; + int ret; + + for (i = 0; i < fb->format->num_planes; ++i) { + struct drm_gem_object *obj = drm_gem_fb_get_obj(fb, i); + struct dma_fence *new; + + if (WARN_ON_ONCE(!obj)) + continue; + + ret = dma_resv_get_singleton(obj->resv, usage, &new); + if (ret) + return ret; + + if (new && *fence) { + struct dma_fence_chain *chain = dma_fence_chain_alloc(); + + if (!chain) + return -ENOMEM; + + dma_fence_chain_init(chain, *fence, new, 1); + *fence = &chain->base; + + } else if (new) { + *fence = new; + } + } + return 0; +} + /** * drm_gem_plane_helper_prepare_fb() - Prepare a GEM backed framebuffer * @plane: Plane @@ -143,8 +178,6 @@ int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, struct drm_plane_state *state) { struct dma_fence *fence = dma_fence_get(state->fence); - enum dma_resv_usage usage; - size_t i; int ret;
if (!state->fb) @@ -163,32 +196,21 @@ int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, * obeys both implicit and explicit fences for plane updates, then it * will break all the benefits of explicit fencing. */ - usage = fence ? DMA_RESV_USAGE_KERNEL : DMA_RESV_USAGE_WRITE; - - for (i = 0; i < state->fb->format->num_planes; ++i) { - struct drm_gem_object *obj = drm_gem_fb_get_obj(state->fb, i); - struct dma_fence *new; - - if (WARN_ON_ONCE(!obj)) - continue; - - ret = dma_resv_get_singleton(obj->resv, usage, &new); + if (fence) { + ret = chain_fb_fences(state->fb, DMA_RESV_USAGE_KERNEL, &fence); if (ret) goto error;
- if (new && fence) { - struct dma_fence_chain *chain = dma_fence_chain_alloc(); + } else { + ret = chain_fb_fences(state->fb, DMA_RESV_USAGE_WRITE, &fence); + if (ret) + goto error;
- if (!chain) { - ret = -ENOMEM; + if (drm_core_check_feature(plane->dev, DRIVER_USER_FENCE)) { + ret = chain_fb_fences(state->fb, DMA_RESV_USAGE_USER, + &fence); + if (ret) goto error; - } - - dma_fence_chain_init(chain, fence, new, 1); - fence = &chain->base; - - } else if (new) { - fence = new; } }
For now just filter or wait for user fences.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/drm_syncobj.c | 15 ++++++++++++--- include/drm/drm_syncobj.h | 25 +++++++++++++++++++++---- 2 files changed, 33 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c index 5a961ea90a35..8d25a2dd107b 100644 --- a/drivers/gpu/drm/drm_syncobj.c +++ b/drivers/gpu/drm/drm_syncobj.c @@ -386,7 +386,7 @@ int drm_syncobj_find_fence(struct drm_file *file_private, struct drm_syncobj *syncobj = drm_syncobj_find(file_private, handle); struct syncobj_wait_entry wait; u64 timeout = nsecs_to_jiffies64(DRM_SYNCOBJ_WAIT_FOR_SUBMIT_TIMEOUT); - int ret; + long ret;
if (!syncobj) return -ENOENT; @@ -398,10 +398,19 @@ int drm_syncobj_find_fence(struct drm_file *file_private, if (flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT) { might_sleep(); lockdep_assert_none_held_once(); + *fence = drm_syncobj_user_fence_get(syncobj); + if (*fence) { + ret = dma_fence_wait_user(*fence, timeout); + if (ret < 0) + return ret; + if (ret == 0) + return -ETIME; + timeout = ret; + } + } else { + *fence = drm_syncobj_fence_get(syncobj); }
- *fence = drm_syncobj_fence_get(syncobj); - if (*fence) { ret = dma_fence_chain_find_seqno(fence, point); if (!ret) { diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h index 6cf7243a1dc5..acc3979815eb 100644 --- a/include/drm/drm_syncobj.h +++ b/include/drm/drm_syncobj.h @@ -28,6 +28,7 @@
#include <linux/dma-fence.h> #include <linux/dma-fence-chain.h> +#include <linux/dma-fence-unwrap.h>
struct drm_file;
@@ -89,18 +90,18 @@ drm_syncobj_put(struct drm_syncobj *obj) }
/** - * drm_syncobj_fence_get - get a reference to a fence in a sync object + * drm_syncobj_user_fence_get - get an user fence from a sync object * @syncobj: sync object. * * This acquires additional reference to &drm_syncobj.fence contained in @obj, - * if not NULL. It is illegal to call this without already holding a reference. - * No locks required. + * if not NULL. It is illegal to call this without holding a reference to the + * syncobj. No locks required. * * Returns: * Either the fence of @obj or NULL if there's none. */ static inline struct dma_fence * -drm_syncobj_fence_get(struct drm_syncobj *syncobj) +drm_syncobj_user_fence_get(struct drm_syncobj *syncobj) { struct dma_fence *fence;
@@ -111,6 +112,22 @@ drm_syncobj_fence_get(struct drm_syncobj *syncobj) return fence; }
+/** + * drm_syncobj_fence_get - get a reference to a fence in a sync object + * @syncobj: sync object. + * + * Same functionality as drm_syncobj_user_fence_get(), but user fences are + * filtered out. + * + * Returns: + * Either the fence of @obj or NULL if there's none. + */ +static inline struct dma_fence * +drm_syncobj_fence_get(struct drm_syncobj *syncobj) +{ + return dma_fence_filter_user(drm_syncobj_user_fence_get(syncobj)); +} + struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private, u32 handle); void drm_syncobj_add_point(struct drm_syncobj *syncobj,
This gives us the standard atomic implicit and explicit fencing rules.
Signed-off-by: Christian König christian.koenig@amd.com Cc: Harry Wentland harry.wentland@amd.com Cc: Nicholas Kazlauskas nicholas.kazlauskas@amd.com Cc: Roman Li Roman.Li@amd.com Cc: Qingqing Zhuo qingqing.zhuo@amd.com Cc: Jude Shih shenshih@amd.com Cc: Wayne Lin Wayne.Lin@amd.com Cc: Rodrigo Siqueira Rodrigo.Siqueira@amd.com --- .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 23 ++++++++----------- 1 file changed, 10 insertions(+), 13 deletions(-)
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c index 2ade82cfb1ac..c5b2417adcc6 100644 --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c @@ -83,6 +83,7 @@ #include <drm/drm_edid.h> #include <drm/drm_vblank.h> #include <drm/drm_audio_component.h> +#include <drm/drm_gem_atomic_helper.h>
#if defined(CONFIG_DRM_AMD_DC_DCN) #include "ivsrcid/dcn/irqsrcs_dcn_1_0.h" @@ -7627,6 +7628,10 @@ static int dm_plane_helper_prepare_fb(struct drm_plane *plane, goto error_unpin; }
+ r = drm_gem_plane_helper_prepare_fb(plane, new_state); + if (unlikely(r != 0)) + goto error_unpin; + amdgpu_bo_unreserve(rbo);
afb->address = amdgpu_bo_gpu_offset(rbo); @@ -9160,7 +9165,6 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state, struct dm_crtc_state *dm_old_crtc_state = to_dm_crtc_state(drm_atomic_get_old_crtc_state(state, pcrtc)); int planes_count = 0, vpos, hpos; - long r; unsigned long flags; struct amdgpu_bo *abo; uint32_t target_vblank, last_flip_vblank; @@ -9173,6 +9177,7 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state, struct dc_flip_addrs flip_addrs[MAX_SURFACES]; struct dc_stream_update stream_update; } *bundle; + int r;
bundle = kzalloc(sizeof(*bundle), GFP_KERNEL);
@@ -9181,6 +9186,10 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state, goto cleanup; }
+ r = drm_atomic_helper_wait_for_fences(dev, state, false); + if (unlikely(r)) + DRM_ERROR("Waiting for fences timed out!"); + /* * Disable the cursor first if we're disabling all the planes. * It'll remain on the screen after the planes are re-enabled @@ -9235,18 +9244,6 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state, }
abo = gem_to_amdgpu_bo(fb->obj[0]); - - /* - * Wait for all fences on this FB. Do limited wait to avoid - * deadlock during GPU reset when this fence will not signal - * but we hold reservation lock for the BO. - */ - r = dma_resv_wait_timeout(abo->tbo.base.resv, - DMA_RESV_USAGE_WRITE, false, - msecs_to_jiffies(5000)); - if (unlikely(r <= 0)) - DRM_ERROR("Waiting for fences timed out!"); - fill_dc_plane_info_and_addr( dm->adev, new_plane_state, afb->tiling_flags,
Just some hack to test the functionality, not a real implementation of the interface.
Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-resv.c | 3 +- drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 28 ++++++++++++++++--- drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 2 +- .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 9 ++++-- 5 files changed, 34 insertions(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index da667c21ad55..e18efb21c452 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -286,7 +286,8 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence, /* Drivers should not add containers here, instead add each fence * individually. */ - WARN_ON(dma_fence_is_container(fence)); + //WARN_ON(dma_fence_is_container(fence)); +
/* User fences must be added using DMA_RESV_USAGE_USER */ WARN_ON(test_bit(DMA_FENCE_FLAG_USER, &fence->flags) != diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c index 95eeab527ca9..299ab8e50c42 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c @@ -453,6 +453,7 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p, struct amdgpu_vm *vm = &fpriv->vm; struct amdgpu_bo_list_entry *e; struct drm_gem_object *obj; + long timeout = HZ / 10; struct amdgpu_bo *gds; struct amdgpu_bo *gws; struct amdgpu_bo *oa; @@ -476,6 +477,17 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p, return r; }
+ amdgpu_bo_list_for_each_entry(e, p->bo_list) { + struct dma_resv *resv = e->bo->tbo.base.resv; + + timeout = dma_resv_wait_timeout(resv, DMA_RESV_USAGE_USER, + true, timeout); + if (unlikely(timeout < 0)) + return timeout; + if (unlikely(timeout == 0)) + return -ETIME; + } + /* Get userptr backing pages. If pages are updated after registered * in amdgpu_gem_userptr_ioctl(), amdgpu_cs_list_validate() will do * amdgpu_ttm_backend_bind() to flush and invalidate new pages @@ -516,7 +528,7 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p, return r;
amdgpu_bo_list_for_each_entry(e, p->bo_list) { - r = drm_exec_prepare_obj(&p->exec, &e->bo->tbo.base, 2); + r = drm_exec_prepare_obj(&p->exec, &e->bo->tbo.base, 3); drm_exec_break_on_contention(&p->exec); if (unlikely(r)) return r; @@ -527,7 +539,7 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p,
if (p->uf_bo) { r = drm_exec_prepare_obj(&p->exec, &p->uf_bo->tbo.base, - 2); + 3); drm_exec_continue_on_contention(&p->exec); if (unlikely(r)) return r; @@ -1160,6 +1172,7 @@ static int amdgpu_cs_submit(struct amdgpu_cs_parser *p, struct drm_sched_entity *entity = p->entity; struct amdgpu_bo_list_entry *e; struct drm_gem_object *gobj; + struct dma_fence *dummy; struct amdgpu_job *job; unsigned long index; uint64_t seq; @@ -1191,6 +1204,11 @@ static int amdgpu_cs_submit(struct amdgpu_cs_parser *p, }
p->fence = dma_fence_get(&job->base.s_fence->finished); + dummy = dma_fence_merge(p->fence, dma_fence_get_stub(true)); + if (!dummy) { + r = -ENOMEM; + goto error_abort; + }
amdgpu_ctx_add_fence(p->ctx, entity, p->fence, &seq); amdgpu_cs_post_dependencies(p); @@ -1214,11 +1232,13 @@ static int amdgpu_cs_submit(struct amdgpu_cs_parser *p,
drm_exec_for_each_duplicate_object(&p->exec, index, gobj) { ttm_bo_move_to_lru_tail_unlocked(&gem_to_amdgpu_bo(gobj)->tbo); - dma_resv_add_fence(gobj->resv, p->fence, DMA_RESV_USAGE_WRITE); + dma_resv_add_fence(gobj->resv, p->fence, DMA_RESV_USAGE_KERNEL); + dma_resv_add_fence(gobj->resv, dummy, DMA_RESV_USAGE_USER); } drm_exec_for_each_locked_object(&p->exec, index, gobj) { ttm_bo_move_to_lru_tail_unlocked(&gem_to_amdgpu_bo(gobj)->tbo); - dma_resv_add_fence(gobj->resv, p->fence, DMA_RESV_USAGE_WRITE); + dma_resv_add_fence(gobj->resv, p->fence, DMA_RESV_USAGE_KERNEL); + dma_resv_add_fence(gobj->resv, dummy, DMA_RESV_USAGE_USER); }
mutex_unlock(&p->adev->notifier_lock); diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c index b03663f42cc9..bd334f5fd64f 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c @@ -2655,7 +2655,7 @@ static const struct drm_driver amdgpu_kms_driver = { DRIVER_ATOMIC | DRIVER_GEM | DRIVER_RENDER | DRIVER_MODESET | DRIVER_SYNCOBJ | - DRIVER_SYNCOBJ_TIMELINE, + DRIVER_SYNCOBJ_TIMELINE | DRIVER_USER_FENCE, .open = amdgpu_driver_open_kms, .postclose = amdgpu_driver_postclose_kms, .lastclose = amdgpu_driver_lastclose_kms, diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c index e5c8e72a9485..6705287887e9 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c @@ -628,7 +628,7 @@ static void amdgpu_vm_pt_next_dfs(struct amdgpu_device *adev, */ int amdgpu_vm_lock_pd(struct amdgpu_vm *vm, struct drm_exec *exec) { - return drm_exec_prepare_obj(exec, &vm->root.bo->tbo.base, 4); + return drm_exec_prepare_obj(exec, &vm->root.bo->tbo.base, 5); }
/** diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c index c5b2417adcc6..2e0f059b9d12 100644 --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c @@ -7627,12 +7627,11 @@ static int dm_plane_helper_prepare_fb(struct drm_plane *plane, DRM_ERROR("%p bind failed\n", rbo); goto error_unpin; } + amdgpu_bo_unreserve(rbo);
r = drm_gem_plane_helper_prepare_fb(plane, new_state); if (unlikely(r != 0)) - goto error_unpin; - - amdgpu_bo_unreserve(rbo); + goto error_reserve;
afb->address = amdgpu_bo_gpu_offset(rbo);
@@ -7665,6 +7664,10 @@ static int dm_plane_helper_prepare_fb(struct drm_plane *plane,
return 0;
+error_reserve: + if (WARN_ON(amdgpu_bo_reserve(rbo, true))) + return r; + error_unpin: amdgpu_bo_unpin(rbo);
On Mon, May 02, 2022 at 06:37:07PM +0200, Christian König wrote:
Hello everyone,
it's a well known problem that the DMA-buf subsystem mixed synchronization and memory management requirements into the same dma_fence and dma_resv objects. Because of this dma_fence objects need to guarantee that they complete within a finite amount of time or otherwise the system can easily deadlock.
One of the few good things about this problem is that it is really good understood by now.
Daniel and others came up with some documentation: https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_b...
And Jason did an excellent presentation about that problem on last years LPC: https://lpc.events/event/11/contributions/1115/
Based on that we had been able to reject new implementations of infinite/user DMA fences and mitigate the effect of the few existing ones.
The still remaining down side is that we don't have a way of using user fences as dependency in both the explicit (sync_file, drm_syncobj) as well as the implicit (dma_resv) synchronization objects, resulting in numerous problems and limitations for things like HMM, user queues etc....
This patch set here now tries to tackle this problem by untangling the synchronization from the memory management. What it does *not* try to do is to fix the existing kernel fences, because I think we now can all agree on that this isn't really possible.
To archive this goal what I do in this patch set is to add some parallel infrastructure to cleanly separate normal kernel dma_fence objects from indefinite/user fences:
- It introduce a DMA_FENCE_FLAG_USER define (after renaming some
existing driver defines). To note that a certain dma_fence is an user fence and *must* be ignore by memory management and never used as dependency for normal none user dma_fence objects.
- The dma_fence_array and dma_fence_chain containers are modified so
that they are marked as user fences whenever any of their contained fences are an user fence.
- The dma_resv object gets a new DMA_RESV_USAGE_USER flag which must be
used with indefinite/user fences and separates those into it's own synchronization domain.
- The existing dma_buf_poll_add_cb() function is modified so that
indefinite/user fences are included in the polling.
- The sync_file synchronization object is modified so that we
essentially have two fence streams instead of just one.
- The drm_syncobj is modified in a similar way. User fences are just
ignored unless the driver explicitly states support to wait for them.
- The DRM subsystem gains a new DRIVER_USER_FENCE flag which drivers
can use to indicate the need for user fences. If user fences are used the atomic mode setting starts to support user fences as IN/OUT fences.
- Lockdep is used at various critical locations to ensure that nobody
ever tries to mix user fences with non user fences.
The general approach is to just ignore user fences unless a driver stated explicitely support for them.
On top of all of this I've hacked amdgpu so that we add the resulting CS fence only as kernel dependency to the dma_resv object and an additional wrapped up with a dma_fence_array and a stub user fence.
The result is that the newly added atomic modeset functions now correctly wait for the user fence to complete before doing the flip. And dependent CS don't pipeline any more, but rather block on the CPU before submitting work.
After tons of debugging and testing everything now seems to not go up in flames immediately and even lockdep is happy with the annotations.
I'm perfectly aware that this is probably by far the most controversial patch set I've ever created and I really wish we wouldn't need it. But we certainly have the requirement for this and I don't see much other chance to get that working in an UAPI compatible way.
Thoughts/comments?
I think you need to type up the goal or exact problem statement you're trying to solve first. What you typed up is a solution along the lines of "try to stuff userspace memory fences into dma_fence and see how horrible it all is", and that's certainly an interesting experiment, but what are you trying to solve with it?
Like if the issue is to enable opencl or whatever, then that's no problem (rocm on amdkfd is a thing, same maybe without the kfd part can be done anywhere else). If the goal is to enable userspace memory fences for vk, then we really don't need these everywhere, but really only in drm_syncobj (and maybe sync_file).
If the goal is specifically atomic kms, then there's an entire can of worms there that I really don't want to think about, but it exists: We have dma_fence as out-fences from atomic commit, and that's already massively broken since most drivers allocate some memory or at least take locks which can allocate memory in their commit path. Like i2c. Putting a userspace memory fence as in-fence in there makes that problem substantially worse, since at least in theory you're just not allowed to might_faul in atomic_commit_tail.
If the goal is to keep the uapi perfectly compatible then your patch set doesn't look like a solution, since as soon as another driver is involved which doesn't understand userspace memory fences it all falls apart. So works great for a quick demo with amd+amd sharing, but not much further. And I don't think it's feasible to just rev the entire ecosystem, since that kinda defeats the point of keeping uapi stable - if we rev everything we might as well also rev the uapi and make this a bit more incremental again :-)
There's probably more to ponder here ...
I'm not sure what exactly the problem statement is that matches your solution here though, so that seems to be missing. -Daniel
Am 04.05.22 um 12:08 schrieb Daniel Vetter:
On Mon, May 02, 2022 at 06:37:07PM +0200, Christian König wrote:
Hello everyone,
it's a well known problem that the DMA-buf subsystem mixed synchronization and memory management requirements into the same dma_fence and dma_resv objects. Because of this dma_fence objects need to guarantee that they complete within a finite amount of time or otherwise the system can easily deadlock.
One of the few good things about this problem is that it is really good understood by now.
Daniel and others came up with some documentation: https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_b...
And Jason did an excellent presentation about that problem on last years LPC: https://lpc.events/event/11/contributions/1115/
Based on that we had been able to reject new implementations of infinite/user DMA fences and mitigate the effect of the few existing ones.
The still remaining down side is that we don't have a way of using user fences as dependency in both the explicit (sync_file, drm_syncobj) as well as the implicit (dma_resv) synchronization objects, resulting in numerous problems and limitations for things like HMM, user queues etc....
This patch set here now tries to tackle this problem by untangling the synchronization from the memory management. What it does *not* try to do is to fix the existing kernel fences, because I think we now can all agree on that this isn't really possible.
To archive this goal what I do in this patch set is to add some parallel infrastructure to cleanly separate normal kernel dma_fence objects from indefinite/user fences:
- It introduce a DMA_FENCE_FLAG_USER define (after renaming some
existing driver defines). To note that a certain dma_fence is an user fence and *must* be ignore by memory management and never used as dependency for normal none user dma_fence objects.
- The dma_fence_array and dma_fence_chain containers are modified so
that they are marked as user fences whenever any of their contained fences are an user fence.
- The dma_resv object gets a new DMA_RESV_USAGE_USER flag which must be
used with indefinite/user fences and separates those into it's own synchronization domain.
- The existing dma_buf_poll_add_cb() function is modified so that
indefinite/user fences are included in the polling.
- The sync_file synchronization object is modified so that we
essentially have two fence streams instead of just one.
- The drm_syncobj is modified in a similar way. User fences are just
ignored unless the driver explicitly states support to wait for them.
- The DRM subsystem gains a new DRIVER_USER_FENCE flag which drivers
can use to indicate the need for user fences. If user fences are used the atomic mode setting starts to support user fences as IN/OUT fences.
- Lockdep is used at various critical locations to ensure that nobody
ever tries to mix user fences with non user fences.
The general approach is to just ignore user fences unless a driver stated explicitely support for them.
On top of all of this I've hacked amdgpu so that we add the resulting CS fence only as kernel dependency to the dma_resv object and an additional wrapped up with a dma_fence_array and a stub user fence.
The result is that the newly added atomic modeset functions now correctly wait for the user fence to complete before doing the flip. And dependent CS don't pipeline any more, but rather block on the CPU before submitting work.
After tons of debugging and testing everything now seems to not go up in flames immediately and even lockdep is happy with the annotations.
I'm perfectly aware that this is probably by far the most controversial patch set I've ever created and I really wish we wouldn't need it. But we certainly have the requirement for this and I don't see much other chance to get that working in an UAPI compatible way.
Thoughts/comments?
I think you need to type up the goal or exact problem statement you're trying to solve first. What you typed up is a solution along the lines of "try to stuff userspace memory fences into dma_fence and see how horrible it all is", and that's certainly an interesting experiment, but what are you trying to solve with it?
Well, good point. I explained to much how it works, but now why.
In general I would describe the goal as: Providing a standard kernel infrastructure for user fences.
Like if the issue is to enable opencl or whatever, then that's no problem (rocm on amdkfd is a thing, same maybe without the kfd part can be done anywhere else). If the goal is to enable userspace memory fences for vk, then we really don't need these everywhere, but really only in drm_syncobj (and maybe sync_file).
Yes, having an in kernel representation for vk user space fences is one of the goals.
And I was going back and forth if I should rather come up with a new structure for this or use the existing dma_fence with a flag as well.
I've decided to go down the later router because we have quite a lot of existing functionality which can be re-used. But if you have a good argument that it would be more defensive to come up with something completely new, I'm perfectly fine with that as well.
If the goal is specifically atomic kms, then there's an entire can of worms there that I really don't want to think about, but it exists: We have dma_fence as out-fences from atomic commit, and that's already massively broken since most drivers allocate some memory or at least take locks which can allocate memory in their commit path. Like i2c. Putting a userspace memory fence as in-fence in there makes that problem substantially worse, since at least in theory you're just not allowed to might_faul in atomic_commit_tail.
Yes, that's unfortunately one of the goals as well and yes I completely agree on the can of worms. But I think I've solved that.
What I do in the patch set is to enforce that the out fence is an user fence when the driver supports user in fences as well.
Since user fences doesn't have the memory management dependency drivers can actually allocate memory or call I2C functions which takes locks which have memory allocation dependencies.
Or do I miss some other reason why you can't fault or allocate memory in atomic_commit_tail? At least lockdep seems to be happy about that now.
If the goal is to keep the uapi perfectly compatible then your patch set doesn't look like a solution, since as soon as another driver is involved which doesn't understand userspace memory fences it all falls apart. So works great for a quick demo with amd+amd sharing, but not much further. And I don't think it's feasible to just rev the entire ecosystem, since that kinda defeats the point of keeping uapi stable - if we rev everything we might as well also rev the uapi and make this a bit more incremental again :-)
Yes, unfortunately the uapi needs to stay compatible as well and yes that means we need to deploy this to all drivers involved.
We at least need to be able to provide a stack on new hardware with (for example) Ubuntu 18.04 without replacing all the userspace components.
What we can replace is the OpenGL stack and if necessary libdrm, but not (for example) the X server and most likely not the DDX in some cases.
The same applies with surfaceflinger and to some extend Wayland as well.
Regards, Christian.
There's probably more to ponder here ...
I'm not sure what exactly the problem statement is that matches your solution here though, so that seems to be missing. -Daniel
On Mon, May 09, 2022 at 08:56:41AM +0200, Christian König wrote:
Am 04.05.22 um 12:08 schrieb Daniel Vetter:
On Mon, May 02, 2022 at 06:37:07PM +0200, Christian König wrote:
Hello everyone,
it's a well known problem that the DMA-buf subsystem mixed synchronization and memory management requirements into the same dma_fence and dma_resv objects. Because of this dma_fence objects need to guarantee that they complete within a finite amount of time or otherwise the system can easily deadlock.
One of the few good things about this problem is that it is really good understood by now.
Daniel and others came up with some documentation: https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_b...
And Jason did an excellent presentation about that problem on last years LPC: https://lpc.events/event/11/contributions/1115/
Based on that we had been able to reject new implementations of infinite/user DMA fences and mitigate the effect of the few existing ones.
The still remaining down side is that we don't have a way of using user fences as dependency in both the explicit (sync_file, drm_syncobj) as well as the implicit (dma_resv) synchronization objects, resulting in numerous problems and limitations for things like HMM, user queues etc....
This patch set here now tries to tackle this problem by untangling the synchronization from the memory management. What it does *not* try to do is to fix the existing kernel fences, because I think we now can all agree on that this isn't really possible.
To archive this goal what I do in this patch set is to add some parallel infrastructure to cleanly separate normal kernel dma_fence objects from indefinite/user fences:
- It introduce a DMA_FENCE_FLAG_USER define (after renaming some
existing driver defines). To note that a certain dma_fence is an user fence and *must* be ignore by memory management and never used as dependency for normal none user dma_fence objects.
- The dma_fence_array and dma_fence_chain containers are modified so
that they are marked as user fences whenever any of their contained fences are an user fence.
- The dma_resv object gets a new DMA_RESV_USAGE_USER flag which must be
used with indefinite/user fences and separates those into it's own synchronization domain.
- The existing dma_buf_poll_add_cb() function is modified so that
indefinite/user fences are included in the polling.
- The sync_file synchronization object is modified so that we
essentially have two fence streams instead of just one.
- The drm_syncobj is modified in a similar way. User fences are just
ignored unless the driver explicitly states support to wait for them.
- The DRM subsystem gains a new DRIVER_USER_FENCE flag which drivers
can use to indicate the need for user fences. If user fences are used the atomic mode setting starts to support user fences as IN/OUT fences.
- Lockdep is used at various critical locations to ensure that nobody
ever tries to mix user fences with non user fences.
The general approach is to just ignore user fences unless a driver stated explicitely support for them.
On top of all of this I've hacked amdgpu so that we add the resulting CS fence only as kernel dependency to the dma_resv object and an additional wrapped up with a dma_fence_array and a stub user fence.
The result is that the newly added atomic modeset functions now correctly wait for the user fence to complete before doing the flip. And dependent CS don't pipeline any more, but rather block on the CPU before submitting work.
After tons of debugging and testing everything now seems to not go up in flames immediately and even lockdep is happy with the annotations.
I'm perfectly aware that this is probably by far the most controversial patch set I've ever created and I really wish we wouldn't need it. But we certainly have the requirement for this and I don't see much other chance to get that working in an UAPI compatible way.
Thoughts/comments?
I think you need to type up the goal or exact problem statement you're trying to solve first. What you typed up is a solution along the lines of "try to stuff userspace memory fences into dma_fence and see how horrible it all is", and that's certainly an interesting experiment, but what are you trying to solve with it?
Well, good point. I explained to much how it works, but now why.
In general I would describe the goal as: Providing a standard kernel infrastructure for user fences.
So on that goal the part I fully agree on is that drm_syncobj can (and should imo) be able to contain userspace memory fences. The uapi semantics and everything is already fully set up to support that, but maybe with reduced performance: Non-aware userspace (or when you don't trust the supplier of the umf) needs to block when looking up the fence, and the dma_fence returned will always be signalled already. But that's just a mild performance issue (and vk drivers paper over that already with threading) and not a correctness issue.
Like if the issue is to enable opencl or whatever, then that's no problem (rocm on amdkfd is a thing, same maybe without the kfd part can be done anywhere else). If the goal is to enable userspace memory fences for vk, then we really don't need these everywhere, but really only in drm_syncobj (and maybe sync_file).
Yes, having an in kernel representation for vk user space fences is one of the goals.
And I was going back and forth if I should rather come up with a new structure for this or use the existing dma_fence with a flag as well.
I've decided to go down the later router because we have quite a lot of existing functionality which can be re-used. But if you have a good argument that it would be more defensive to come up with something completely new, I'm perfectly fine with that as well.
Yeah so stuffing that into dma_fence already freaks me out a bit. It is quite fundamentally a different thing, and it would be really nice to make that very apparent at the type level too.
E.g. to make sure you never ever end up with an umf fence in mmu notifier invalidate callback. You can enforce that with runtime checks too, but imo compile time fail is better than runtime fail.
If the goal is specifically atomic kms, then there's an entire can of worms there that I really don't want to think about, but it exists: We have dma_fence as out-fences from atomic commit, and that's already massively broken since most drivers allocate some memory or at least take locks which can allocate memory in their commit path. Like i2c. Putting a userspace memory fence as in-fence in there makes that problem substantially worse, since at least in theory you're just not allowed to might_faul in atomic_commit_tail.
Yes, that's unfortunately one of the goals as well and yes I completely agree on the can of worms. But I think I've solved that.
What I do in the patch set is to enforce that the out fence is an user fence when the driver supports user in fences as well.
Since user fences doesn't have the memory management dependency drivers can actually allocate memory or call I2C functions which takes locks which have memory allocation dependencies.
Or do I miss some other reason why you can't fault or allocate memory in atomic_commit_tail? At least lockdep seems to be happy about that now.
The problem is a bit that this breaks the uapi already. At least if the goal is to have this all be perfectly transparent for userspace - as you as you have multi-gpu setups going on at least.
If the goal is to keep the uapi perfectly compatible then your patch set doesn't look like a solution, since as soon as another driver is involved which doesn't understand userspace memory fences it all falls apart. So works great for a quick demo with amd+amd sharing, but not much further. And I don't think it's feasible to just rev the entire ecosystem, since that kinda defeats the point of keeping uapi stable - if we rev everything we might as well also rev the uapi and make this a bit more incremental again :-)
Yes, unfortunately the uapi needs to stay compatible as well and yes that means we need to deploy this to all drivers involved.
We at least need to be able to provide a stack on new hardware with (for example) Ubuntu 18.04 without replacing all the userspace components.
What we can replace is the OpenGL stack and if necessary libdrm, but not (for example) the X server and most likely not the DDX in some cases.
The same applies with surfaceflinger and to some extend Wayland as well.
So for perfect uapi compat for existing compositor I really don't think stuffing umf into the kernel is the right approach. Too many little corners that break:
- the in/out fence mismatch every - cross gpu with different userspace that doesn't understand umf and then just ignores them - compositors which currently assume implicit sync finishes eventually, and with umf that gets complicated at best - same with sync_file, the uapi atm does not have a concept of future fence
So you can kinda make this work, but it falls apart all over the place. And I also don't think smashing umf into all these old concepts helps us in any way to get towards a desktop which is umf-native.
My take is still that for backwards compat the simplest way is if a umf-native driver simply provides dma-fence backwards compat as an opt-in, which userspace chooses when it's necessary. There's really only two things you need for that to work:
- a timeout of some sort on the dma_fence, which might or might not kill the entire context. This is entirey up to how your userspace does or does not implement stuff like arb robustness or vk_error_device_lost
- pre-pinned memory management to block out the all the inversions. This is a bit more nasty, but since we do have all the code for this already it really shouldn't be too tricky to make that happen for the fancy new umf world.
You do not need a kernel scheduler or anything like that at all, you can do full userspace direct submit to hw and all that fun. Maybe do a drm/sched frontend (and then your submit code does exactly what userspace would do too).
Importantly the things you really don't need:
- special hw support, even if the only mode your hw supports is with page faults and all that: You can make sure all the pages are present upfront, and then simply kill the entire context is a page fault happens.
- special fw scheduler support: Once the memory management inversions are taken care of with pre-pinning under dma_fences, then the only other thing you need is a timeout for the dma_fence to signal. And maybe some kind of guaranteed ordering if you want to use a dma_fence timeline since that one can't go backwards.
Trying to shoehorn umf into all the old concepts like implicit sync or sync_file which really don't support umf works for a demo, but imo just isn't solid enough for shipping everywhere.
And long term I really don't think we ever want umf anywhere else than drm_syncobj, at least for a 100% umf-native stack.
So maybe this all goes back to the old discussion with had, where you argued for the need for special fw and hw and all that to make the old dma_fence stuff work. Why is that needed? I still don't get that part ... -Daniel
Am 09.05.22 um 16:10 schrieb Daniel Vetter:
On Mon, May 09, 2022 at 08:56:41AM +0200, Christian König wrote:
Am 04.05.22 um 12:08 schrieb Daniel Vetter:
On Mon, May 02, 2022 at 06:37:07PM +0200, Christian König wrote:
Hello everyone,
it's a well known problem that the DMA-buf subsystem mixed synchronization and memory management requirements into the same dma_fence and dma_resv objects. Because of this dma_fence objects need to guarantee that they complete within a finite amount of time or otherwise the system can easily deadlock.
One of the few good things about this problem is that it is really good understood by now.
Daniel and others came up with some documentation: https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_b...
And Jason did an excellent presentation about that problem on last years LPC: https://lpc.events/event/11/contributions/1115/
Based on that we had been able to reject new implementations of infinite/user DMA fences and mitigate the effect of the few existing ones.
The still remaining down side is that we don't have a way of using user fences as dependency in both the explicit (sync_file, drm_syncobj) as well as the implicit (dma_resv) synchronization objects, resulting in numerous problems and limitations for things like HMM, user queues etc....
This patch set here now tries to tackle this problem by untangling the synchronization from the memory management. What it does *not* try to do is to fix the existing kernel fences, because I think we now can all agree on that this isn't really possible.
To archive this goal what I do in this patch set is to add some parallel infrastructure to cleanly separate normal kernel dma_fence objects from indefinite/user fences:
- It introduce a DMA_FENCE_FLAG_USER define (after renaming some
existing driver defines). To note that a certain dma_fence is an user fence and *must* be ignore by memory management and never used as dependency for normal none user dma_fence objects.
- The dma_fence_array and dma_fence_chain containers are modified so
that they are marked as user fences whenever any of their contained fences are an user fence.
- The dma_resv object gets a new DMA_RESV_USAGE_USER flag which must be
used with indefinite/user fences and separates those into it's own synchronization domain.
- The existing dma_buf_poll_add_cb() function is modified so that
indefinite/user fences are included in the polling.
- The sync_file synchronization object is modified so that we
essentially have two fence streams instead of just one.
- The drm_syncobj is modified in a similar way. User fences are just
ignored unless the driver explicitly states support to wait for them.
- The DRM subsystem gains a new DRIVER_USER_FENCE flag which drivers
can use to indicate the need for user fences. If user fences are used the atomic mode setting starts to support user fences as IN/OUT fences.
- Lockdep is used at various critical locations to ensure that nobody
ever tries to mix user fences with non user fences.
The general approach is to just ignore user fences unless a driver stated explicitely support for them.
On top of all of this I've hacked amdgpu so that we add the resulting CS fence only as kernel dependency to the dma_resv object and an additional wrapped up with a dma_fence_array and a stub user fence.
The result is that the newly added atomic modeset functions now correctly wait for the user fence to complete before doing the flip. And dependent CS don't pipeline any more, but rather block on the CPU before submitting work.
After tons of debugging and testing everything now seems to not go up in flames immediately and even lockdep is happy with the annotations.
I'm perfectly aware that this is probably by far the most controversial patch set I've ever created and I really wish we wouldn't need it. But we certainly have the requirement for this and I don't see much other chance to get that working in an UAPI compatible way.
Thoughts/comments?
I think you need to type up the goal or exact problem statement you're trying to solve first. What you typed up is a solution along the lines of "try to stuff userspace memory fences into dma_fence and see how horrible it all is", and that's certainly an interesting experiment, but what are you trying to solve with it?
Well, good point. I explained to much how it works, but now why.
In general I would describe the goal as: Providing a standard kernel infrastructure for user fences.
So on that goal the part I fully agree on is that drm_syncobj can (and should imo) be able to contain userspace memory fences. The uapi semantics and everything is already fully set up to support that, but maybe with reduced performance: Non-aware userspace (or when you don't trust the supplier of the umf) needs to block when looking up the fence, and the dma_fence returned will always be signalled already. But that's just a mild performance issue (and vk drivers paper over that already with threading) and not a correctness issue.
Exactly that, yes.
Like if the issue is to enable opencl or whatever, then that's no problem (rocm on amdkfd is a thing, same maybe without the kfd part can be done anywhere else). If the goal is to enable userspace memory fences for vk, then we really don't need these everywhere, but really only in drm_syncobj (and maybe sync_file).
Yes, having an in kernel representation for vk user space fences is one of the goals.
And I was going back and forth if I should rather come up with a new structure for this or use the existing dma_fence with a flag as well.
I've decided to go down the later router because we have quite a lot of existing functionality which can be re-used. But if you have a good argument that it would be more defensive to come up with something completely new, I'm perfectly fine with that as well.
Yeah so stuffing that into dma_fence already freaks me out a bit. It is quite fundamentally a different thing, and it would be really nice to make that very apparent at the type level too.
E.g. to make sure you never ever end up with an umf fence in mmu notifier invalidate callback. You can enforce that with runtime checks too, but imo compile time fail is better than runtime fail.
Well, I see arguments for both sides.
There is certainly the danger that we have an umf wait in the mmu notifier, but then lockdep will scream "bloody hell" immediately.
On the other hand when I make this a separate structure we need to maintain containers for both variants, especially a chain implementation for drm_syncobj. And here I don't have lockdep to keep an eye that nobody does anything strange.
It's only a gut feeling with no clear evidence for one side. If you insists on a separate structure I will go down that route.
If the goal is specifically atomic kms, then there's an entire can of worms there that I really don't want to think about, but it exists: We have dma_fence as out-fences from atomic commit, and that's already massively broken since most drivers allocate some memory or at least take locks which can allocate memory in their commit path. Like i2c. Putting a userspace memory fence as in-fence in there makes that problem substantially worse, since at least in theory you're just not allowed to might_faul in atomic_commit_tail.
Yes, that's unfortunately one of the goals as well and yes I completely agree on the can of worms. But I think I've solved that.
What I do in the patch set is to enforce that the out fence is an user fence when the driver supports user in fences as well.
Since user fences doesn't have the memory management dependency drivers can actually allocate memory or call I2C functions which takes locks which have memory allocation dependencies.
Or do I miss some other reason why you can't fault or allocate memory in atomic_commit_tail? At least lockdep seems to be happy about that now.
The problem is a bit that this breaks the uapi already. At least if the goal is to have this all be perfectly transparent for userspace - as you as you have multi-gpu setups going on at least.
Question here is why do you think there is an UAPI break? We currently wait in a work item already, so where exactly is the problem?
If the goal is to keep the uapi perfectly compatible then your patch set doesn't look like a solution, since as soon as another driver is involved which doesn't understand userspace memory fences it all falls apart. So works great for a quick demo with amd+amd sharing, but not much further. And I don't think it's feasible to just rev the entire ecosystem, since that kinda defeats the point of keeping uapi stable - if we rev everything we might as well also rev the uapi and make this a bit more incremental again :-)
Yes, unfortunately the uapi needs to stay compatible as well and yes that means we need to deploy this to all drivers involved.
We at least need to be able to provide a stack on new hardware with (for example) Ubuntu 18.04 without replacing all the userspace components.
What we can replace is the OpenGL stack and if necessary libdrm, but not (for example) the X server and most likely not the DDX in some cases.
The same applies with surfaceflinger and to some extend Wayland as well.
So for perfect uapi compat for existing compositor I really don't think stuffing umf into the kernel is the right approach. Too many little corners that break:
- the in/out fence mismatch every
On that part I need further explanation, cause I hoped that this is solved.
- cross gpu with different userspace that doesn't understand umf and then just ignores them
Well by stuffing umf into the kernel the whole thing becomes transparent for userspace.
So it won't matter that you have a new amdgpu stack which wants to use umf and an older i915 stack which knows nothing about umfs.
The kernel will block on command submission as soon as a buffer is used by i915. And as you said above as well that might cause performance trouble, but is not a correctness problem.
- compositors which currently assume implicit sync finishes eventually, and with umf that gets complicated at best
But for userspace compositors there is no difference between an umf which times out and a dma_fence which timesout? Or am I missing something here?
- same with sync_file, the uapi atm does not have a concept of future fence
So you can kinda make this work, but it falls apart all over the place. And I also don't think smashing umf into all these old concepts helps us in any way to get towards a desktop which is umf-native.
Yeah, but having an umf compatibility memory management doesn't help either to get away from pre-pinned pages.
My take is still that for backwards compat the simplest way is if a umf-native driver simply provides dma-fence backwards compat as an opt-in, which userspace chooses when it's necessary. There's really only two things you need for that to work:
a timeout of some sort on the dma_fence, which might or might not kill the entire context. This is entirey up to how your userspace does or does not implement stuff like arb robustness or vk_error_device_lost
pre-pinned memory management to block out the all the inversions. This is a bit more nasty, but since we do have all the code for this already it really shouldn't be too tricky to make that happen for the fancy new umf world.
Well, exactly that's what we want to get away from.
You do not need a kernel scheduler or anything like that at all, you can do full userspace direct submit to hw and all that fun. Maybe do a drm/sched frontend (and then your submit code does exactly what userspace would do too).
Importantly the things you really don't need:
- special hw support, even if the only mode your hw supports is with page faults and all that: You can make sure all the pages are present upfront, and then simply kill the entire context is a page fault happens.
Well, that's only like 90% correct.
You can make that work without special hardware support, but from the experience with ROCm and very extensive talks with out hardware folks we have seriously problems making sure that the hw can't access freed up memory any more.
Except for the solution of never freeing up memory the only other possibility is to wait between 1 and 6 seconds until a shoot down made sure that there is really nobody accessing old page tables entries any more.
In the case of an user space queue with hardware scheduler support and HMM the memory would just still be referenced until userspace cooperatively inserted a barrier, but that again breaks some dma_fence assumptions as far as I can see.
- special fw scheduler support: Once the memory management inversions are taken care of with pre-pinning under dma_fences, then the only other thing you need is a timeout for the dma_fence to signal. And maybe some kind of guaranteed ordering if you want to use a dma_fence timeline since that one can't go backwards.
Yeah, that not going backward thing turned out to be massively more tricky than I thought initially as well.
Alex, Marek and I worked quite hard on relaying those requirements to our internal teams, but I'm still not quite sure if that will turn out working or not.
Trying to shoehorn umf into all the old concepts like implicit sync or sync_file which really don't support umf works for a demo, but imo just isn't solid enough for shipping everywhere.
And long term I really don't think we ever want umf anywhere else than drm_syncobj, at least for a 100% umf-native stack.
Ok then I will concentrate on drm_syncobj for now.
What about in driver backward compatibility? E.g. blocking wait in the multimedia driver CS IOCTL until umf signals?
Thanks, Christian.
So maybe this all goes back to the old discussion with had, where you argued for the need for special fw and hw and all that to make the old dma_fence stuff work. Why is that needed? I still don't get that part ... -Daniel
Apologies I'm constantly behind on m-l discussions :-/
On Tue, May 17, 2022 at 12:28:17PM +0200, Christian König wrote:
Am 09.05.22 um 16:10 schrieb Daniel Vetter:
On Mon, May 09, 2022 at 08:56:41AM +0200, Christian König wrote:
Am 04.05.22 um 12:08 schrieb Daniel Vetter:
On Mon, May 02, 2022 at 06:37:07PM +0200, Christian König wrote:
Hello everyone,
it's a well known problem that the DMA-buf subsystem mixed synchronization and memory management requirements into the same dma_fence and dma_resv objects. Because of this dma_fence objects need to guarantee that they complete within a finite amount of time or otherwise the system can easily deadlock.
One of the few good things about this problem is that it is really good understood by now.
Daniel and others came up with some documentation: https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_b...
And Jason did an excellent presentation about that problem on last years LPC: https://lpc.events/event/11/contributions/1115/
Based on that we had been able to reject new implementations of infinite/user DMA fences and mitigate the effect of the few existing ones.
The still remaining down side is that we don't have a way of using user fences as dependency in both the explicit (sync_file, drm_syncobj) as well as the implicit (dma_resv) synchronization objects, resulting in numerous problems and limitations for things like HMM, user queues etc....
This patch set here now tries to tackle this problem by untangling the synchronization from the memory management. What it does *not* try to do is to fix the existing kernel fences, because I think we now can all agree on that this isn't really possible.
To archive this goal what I do in this patch set is to add some parallel infrastructure to cleanly separate normal kernel dma_fence objects from indefinite/user fences:
- It introduce a DMA_FENCE_FLAG_USER define (after renaming some
existing driver defines). To note that a certain dma_fence is an user fence and *must* be ignore by memory management and never used as dependency for normal none user dma_fence objects.
- The dma_fence_array and dma_fence_chain containers are modified so
that they are marked as user fences whenever any of their contained fences are an user fence.
- The dma_resv object gets a new DMA_RESV_USAGE_USER flag which must be
used with indefinite/user fences and separates those into it's own synchronization domain.
- The existing dma_buf_poll_add_cb() function is modified so that
indefinite/user fences are included in the polling.
- The sync_file synchronization object is modified so that we
essentially have two fence streams instead of just one.
- The drm_syncobj is modified in a similar way. User fences are just
ignored unless the driver explicitly states support to wait for them.
- The DRM subsystem gains a new DRIVER_USER_FENCE flag which drivers
can use to indicate the need for user fences. If user fences are used the atomic mode setting starts to support user fences as IN/OUT fences.
- Lockdep is used at various critical locations to ensure that nobody
ever tries to mix user fences with non user fences.
The general approach is to just ignore user fences unless a driver stated explicitely support for them.
On top of all of this I've hacked amdgpu so that we add the resulting CS fence only as kernel dependency to the dma_resv object and an additional wrapped up with a dma_fence_array and a stub user fence.
The result is that the newly added atomic modeset functions now correctly wait for the user fence to complete before doing the flip. And dependent CS don't pipeline any more, but rather block on the CPU before submitting work.
After tons of debugging and testing everything now seems to not go up in flames immediately and even lockdep is happy with the annotations.
I'm perfectly aware that this is probably by far the most controversial patch set I've ever created and I really wish we wouldn't need it. But we certainly have the requirement for this and I don't see much other chance to get that working in an UAPI compatible way.
Thoughts/comments?
I think you need to type up the goal or exact problem statement you're trying to solve first. What you typed up is a solution along the lines of "try to stuff userspace memory fences into dma_fence and see how horrible it all is", and that's certainly an interesting experiment, but what are you trying to solve with it?
Well, good point. I explained to much how it works, but now why.
In general I would describe the goal as: Providing a standard kernel infrastructure for user fences.
So on that goal the part I fully agree on is that drm_syncobj can (and should imo) be able to contain userspace memory fences. The uapi semantics and everything is already fully set up to support that, but maybe with reduced performance: Non-aware userspace (or when you don't trust the supplier of the umf) needs to block when looking up the fence, and the dma_fence returned will always be signalled already. But that's just a mild performance issue (and vk drivers paper over that already with threading) and not a correctness issue.
Exactly that, yes.
Like if the issue is to enable opencl or whatever, then that's no problem (rocm on amdkfd is a thing, same maybe without the kfd part can be done anywhere else). If the goal is to enable userspace memory fences for vk, then we really don't need these everywhere, but really only in drm_syncobj (and maybe sync_file).
Yes, having an in kernel representation for vk user space fences is one of the goals.
And I was going back and forth if I should rather come up with a new structure for this or use the existing dma_fence with a flag as well.
I've decided to go down the later router because we have quite a lot of existing functionality which can be re-used. But if you have a good argument that it would be more defensive to come up with something completely new, I'm perfectly fine with that as well.
Yeah so stuffing that into dma_fence already freaks me out a bit. It is quite fundamentally a different thing, and it would be really nice to make that very apparent at the type level too.
E.g. to make sure you never ever end up with an umf fence in mmu notifier invalidate callback. You can enforce that with runtime checks too, but imo compile time fail is better than runtime fail.
Well, I see arguments for both sides.
There is certainly the danger that we have an umf wait in the mmu notifier, but then lockdep will scream "bloody hell" immediately.
On the other hand when I make this a separate structure we need to maintain containers for both variants, especially a chain implementation for drm_syncobj. And here I don't have lockdep to keep an eye that nobody does anything strange.
It's only a gut feeling with no clear evidence for one side. If you insists on a separate structure I will go down that route.
If the goal is specifically atomic kms, then there's an entire can of worms there that I really don't want to think about, but it exists: We have dma_fence as out-fences from atomic commit, and that's already massively broken since most drivers allocate some memory or at least take locks which can allocate memory in their commit path. Like i2c. Putting a userspace memory fence as in-fence in there makes that problem substantially worse, since at least in theory you're just not allowed to might_faul in atomic_commit_tail.
Yes, that's unfortunately one of the goals as well and yes I completely agree on the can of worms. But I think I've solved that.
What I do in the patch set is to enforce that the out fence is an user fence when the driver supports user in fences as well.
Since user fences doesn't have the memory management dependency drivers can actually allocate memory or call I2C functions which takes locks which have memory allocation dependencies.
Or do I miss some other reason why you can't fault or allocate memory in atomic_commit_tail? At least lockdep seems to be happy about that now.
The problem is a bit that this breaks the uapi already. At least if the goal is to have this all be perfectly transparent for userspace - as you as you have multi-gpu setups going on at least.
Question here is why do you think there is an UAPI break? We currently wait in a work item already, so where exactly is the problem?
It's a bit washy, but dma_fence and hence implicit sync is supposed to finish in finite time. umf just doesn't.
Ofc in reality you can still flood your compositor and they're not very robust, but with umf it's trivial to just hang your compositor forever and nothing happens. There was a least quite a bit of screaming from compositor folks 2 years ago when we've gone through the i915-gem fun of "hey we redefined dma_fence to have indefinite fence semantcis, cool isn't it". And this is kinda similar - you don't hang the kernel with all the lockdep stuff, meaning ^C can get you out of any hang. But userspace is still dead, and the user is still potentially rather unhappy.
So either we force a timeout in the kernel or in userspace or somewhere, and at that point I'm not entirely sure this is really worth it.
Same holds for sync_file. drm_syncobj is different and does have the entire "this might never signal" concept encoded in it's uapi already, so that's fine.
If the goal is to keep the uapi perfectly compatible then your patch set doesn't look like a solution, since as soon as another driver is involved which doesn't understand userspace memory fences it all falls apart. So works great for a quick demo with amd+amd sharing, but not much further. And I don't think it's feasible to just rev the entire ecosystem, since that kinda defeats the point of keeping uapi stable - if we rev everything we might as well also rev the uapi and make this a bit more incremental again :-)
Yes, unfortunately the uapi needs to stay compatible as well and yes that means we need to deploy this to all drivers involved.
We at least need to be able to provide a stack on new hardware with (for example) Ubuntu 18.04 without replacing all the userspace components.
What we can replace is the OpenGL stack and if necessary libdrm, but not (for example) the X server and most likely not the DDX in some cases.
The same applies with surfaceflinger and to some extend Wayland as well.
So for perfect uapi compat for existing compositor I really don't think stuffing umf into the kernel is the right approach. Too many little corners that break:
- the in/out fence mismatch every
On that part I need further explanation, cause I hoped that this is solved.
The thing is even if you force the out fence to be an umf too if the in-fence is, they're chained. So as soon as you have an unsignalled in fence all subsequent out fences need to be forced to be umf semantics. Possible, but definitely messy.
- cross gpu with different userspace that doesn't understand umf and then just ignores them
Well by stuffing umf into the kernel the whole thing becomes transparent for userspace.
So it won't matter that you have a new amdgpu stack which wants to use umf and an older i915 stack which knows nothing about umfs.
The kernel will block on command submission as soon as a buffer is used by i915. And as you said above as well that might cause performance trouble, but is not a correctness problem.
The problem is that you still have to adjust all drivers. Which yes eventually we have to do, but if the compat code is just in the driver which has switched to umf already, then the cost is a lot more localized. And with the dma_fence trick below it's all good.
- compositors which currently assume implicit sync finishes eventually, and with umf that gets complicated at best
But for userspace compositors there is no difference between an umf which times out and a dma_fence which timesout? Or am I missing something here?
- same with sync_file, the uapi atm does not have a concept of future fence
So you can kinda make this work, but it falls apart all over the place. And I also don't think smashing umf into all these old concepts helps us in any way to get towards a desktop which is umf-native.
Yeah, but having an umf compatibility memory management doesn't help either to get away from pre-pinned pages.
I think it's really two worlds:
a) dma_fence syncing (sync_file, implicit sync in dma_resv, drm_syncobj with dma_fence) and pre-pinned everything memory
b) umf mode, which opens the door for page faults and clever tricks and avoids the need to pre-pin everything
It sucks, but I really haven't found a way to mix these. And I feel like that it's pretty dangerous to pretend that we can do it - it freaks me out at least :-)
My take is still that for backwards compat the simplest way is if a umf-native driver simply provides dma-fence backwards compat as an opt-in, which userspace chooses when it's necessary. There's really only two things you need for that to work:
a timeout of some sort on the dma_fence, which might or might not kill the entire context. This is entirey up to how your userspace does or does not implement stuff like arb robustness or vk_error_device_lost
pre-pinned memory management to block out the all the inversions. This is a bit more nasty, but since we do have all the code for this already it really shouldn't be too tricky to make that happen for the fancy new umf world.
Well, exactly that's what we want to get away from.
You do not need a kernel scheduler or anything like that at all, you can do full userspace direct submit to hw and all that fun. Maybe do a drm/sched frontend (and then your submit code does exactly what userspace would do too).
Importantly the things you really don't need:
- special hw support, even if the only mode your hw supports is with page faults and all that: You can make sure all the pages are present upfront, and then simply kill the entire context is a page fault happens.
Well, that's only like 90% correct.
You can make that work without special hardware support, but from the experience with ROCm and very extensive talks with out hardware folks we have seriously problems making sure that the hw can't access freed up memory any more.
Except for the solution of never freeing up memory the only other possibility is to wait between 1 and 6 seconds until a shoot down made sure that there is really nobody accessing old page tables entries any more.
In the case of an user space queue with hardware scheduler support and HMM the memory would just still be referenced until userspace cooperatively inserted a barrier, but that again breaks some dma_fence assumptions as far as I can see.
So two things:
- Don't use legacy semantics for compute. They're completely different worlds, and i915 plan is to have completely different mode between legacy and compute mode creation, because they really don't mesh, at context creation time. So kinda like amdgpu renderD* vs amdkfd, except all int he renderD* node.
You _never_ want to run ROCm or level0 or cuda with legacy mode, because it sucks too much. The trouble is a bit vulkan since if you run it headless, you want to run in compute mode, but if want wayland/x11 winsys support, then you need legacy support. But we've come up with some supremely nasty tricks for that too in the discussion at plumbers last year.
The other trouble is interop. I think the best path here is the vk approach of "you only get drm_syncobj, no implicit sync ever". But that might not be workable for rocm/opencl/whatever. I'm not sure what's the best option there, if absolutely everything breaks I guess we could look into adding a umf implicit sync slot to dma_resv, but I really think we should exhaust all other options first before we step into that dragon's lair :-/
If we do go with the umf implicit sync I think we should really do it as a timeline thing, i.e. you attach the umf container once, the other side gets it once, an after that point the only thing that keeps happening is that writers increment the syncpt seqno, and readers just sample that every time they use the buffer. So essentially:
struct dma_buf_umf_implict_sync { struct drm_syncobj *sync; atomic_t seqno; };
Plus a few ioctls to increment the seqno (and receive the value) and read the current seqno. That still means interop isn't perfectly transparent, but maybe we could at least reuse some of the codepaths we have for drm_syncobj in both kernel and userspace.
Maybe we could make it backwards compat but just blocking rendering until the umf signals if a buffer is shared with any importer that doesn't understand umf implicit sync.
But that's all rather horrible imo.
- For memory management I think the right long term approach is to punt this all to userspace, for both legacy and compute mode, with exactly the same semantics as mmap()/munmap() on the cpu side. i915 calls this vm_bind (the rfc is floating around somewhere), but essentially i915.ko will execute vm_bind and vm_unbind directly, completely decoupled from any in-flight command submission (we just punch out the ptes and force a tlb flush and done). If userspace unmaps memory it still uses, it gets to keep all the pieces. This is how vk and all the new stuff works. It does mean there's more work for gl and that kind of stuff, but this really shouldn't be the kernel's problem.
Aside: We're aiming for the same for gpu context. If userspace closes a gpu context we just preempt and destroy it right away, userspace gets to keep he pieces if they wanted to let it finish first.
This is exactly like on the cpu side where userspace can create/destroy threads and mmap/munmap as it sees fit, and it just blows up if it gets it all wrong.
Ofc if you're hw is busted and you can't do reasonable efficient tlb flushing (even with pipeling and batching) then you're screwed, so I'm hoping your 1-6 seconds of waiting isn't that. If it is then you kinda have an unfixable security bug :-/ But iirc amdkfd already has batched up tlb flushes as dma_fence in USAGE_OTHER now, so this should all work.
Anything else, i.e. userpsace fails to insert the required cache flushes or barriers or anything else before it calls gpu_unamp (or vm_unbind or whatever you want to call it) is a userspace bug.
Ofc if you preempt a context for memory eviction due to memory pressure then the kernel has to do all the cache flushing and everything. Same for evicting pages due to memory pressure in the hmm use case. If you're preemption takes a few seconds I think there's bigger problems no matter what :-)
Btw on vm_bind it'd be great if you or someone from amd's rocm/kfd side can take a look and drop your input. I expect that we'll have the same discussion with nouveau/nvidia and maybe also some of the armsoc chips. So it would be great if we can collect upstream wisdom on this topic a bit.
Neither of these are a reason to stuff umf into implicit sync or sync file imo.
- special fw scheduler support: Once the memory management inversions are taken care of with pre-pinning under dma_fences, then the only other thing you need is a timeout for the dma_fence to signal. And maybe some kind of guaranteed ordering if you want to use a dma_fence timeline since that one can't go backwards.
Yeah, that not going backward thing turned out to be massively more tricky than I thought initially as well.
Alex, Marek and I worked quite hard on relaying those requirements to our internal teams, but I'm still not quite sure if that will turn out working or not.
I really don't think you need to teach this to your fw scheduler, that sounds backwards. At least for i915+guc we don't have any plans to inflict this on our fw scheduler (otoh our fw scheduler sucks at resolving dependencies, but that's a different topic - currently it just busy-spins if a context is stuck, there's some plans to preempt contexts instead and round-robin to the next one):
All our fw scheduler is doing is scheduling gpu context and round-robing them with preemption if they use up their time slice before they go idle again. Everything else is meaning and semantics we put on top either in the kernel (for dma_fence based sync) or in userspace (for direct submit userspace galore, which currently is done in the most hilarious way in a downstream hack since the hw support isn't there yet). Hilarious here means we submit a batchbuffer to the kernel which jumps back to it's start, and then use conditional rendering CS commands to latch in the next real userspace batch as userspace queues them up. There's a ring of these to keep it all nicely pipelined and the gpu busy. It's terrible, but for certain niche use-cases it gets the job done :-)
Trying to shoehorn umf into all the old concepts like implicit sync or sync_file which really don't support umf works for a demo, but imo just isn't solid enough for shipping everywhere.
And long term I really don't think we ever want umf anywhere else than drm_syncobj, at least for a 100% umf-native stack.
Ok then I will concentrate on drm_syncobj for now.
What about in driver backward compatibility? E.g. blocking wait in the multimedia driver CS IOCTL until umf signals?
Yeah I think for that we can keep on using drm_syncobj exactly as-is, i.e. if it's a umf you just block until signalled when the driver tries to look up the dma_fence. Ofc userspace can be a bit more clever (and they all should do so) by doing that waiting in a userspace thread, and eventually we want them to get at the umf behind it all if they trust the other side, so that the wait can happen on the gpu.
But that's kinda details, in general I think drm_syncobj should be ready for cross-driver/process/whatever umf sharing.
Cheers, Daniel
Thanks, Christian.
So maybe this all goes back to the old discussion with had, where you argued for the need for special fw and hw and all that to make the old dma_fence stuff work. Why is that needed? I still don't get that part ... -Daniel
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