Explicit synchronization is the future. At least, that seems to be what most userspace APIs are agreeing on at this point. However, most of our Linux APIs (both userspace and kernel UAPI) are currently built around implicit synchronization with dma-buf. While work is ongoing to change many of the userspace APIs and protocols to an explicit synchronization model, switching over piecemeal is difficult due to the number of potential components involved. On the kernel side, many drivers use dma-buf including GPU (3D/compute), display, v4l, and others. In userspace, we have X11, several Wayland compositors, 3D drivers, compute drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually manage the fences attached to a dma-buf. Alternatively, one can think of this as making dma-buf's implicit synchronization simply a carrier for an explicit fence. This is accomplished by adding two IOCTLs to dma-buf for importing and exporting a sync file to/from the dma-buf. This way a userspace component which is uses explicit synchronization, such as a Vulkan driver, can manually set the write fence on a buffer before handing it off to an implicitly synchronized component such as a Wayland compositor or video encoder. In this way, each of the different components can be upgraded to an explicit synchronization model one at a time as long as the userspace pieces connecting them are aware of it and import/export fences at the right times.

There is a potential race condition with this API if userspace is not careful. A typical use case for implicit synchronization is to wait for the dma-buf to be ready, use it, and then signal it for some other component. Because a sync_file cannot be created until it is guaranteed to complete in finite time, userspace can only signal the dma-buf after it has already submitted the work which uses it to the kernel and has received a sync_file back. There is no way to atomically submit a wait-use-signal operation. This is not, however, really a problem with this API so much as it is a problem with explicit synchronization itself. The way this is typically handled is to have very explicit ownership transfer points in the API or protocol which ensure that only one component is using it at any given time. Both X11 (via the PRESENT extension) and Wayland provide such ownership transfer points via explicit present and idle messages.

The decision was intentionally made in this patch to make the import and export operations IOCTLs on the dma-buf itself rather than as a DRM IOCTL. This makes it the import/export operation universal across all components which use dma-buf including GPU, display, v4l, and others. It also means that a userspace component can do the import/export without access to the DRM fd which may be tricky to get in cases where the client communicates with DRM via a userspace API such as OpenGL or Vulkan. At a future date we may choose to add direct import/export APIs to components such as drm_syncobj to avoid allocating a file descriptor and going through two ioctls. However, that seems to be something of a micro-optimization as import/export operations are likely to happen at a rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand): - Use a wrapper dma_fence_array of all fences including the new one when importing an exclusive fence.

Signed-off-by: Jason Ekstrand jason@jlekstrand.net --- drivers/dma-buf/dma-buf.c | 162 +++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 13 ++- 2 files changed, 173 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index d4097856c86b..162f90e8896b 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -20,6 +20,7 @@ #include <linux/debugfs.h> #include <linux/module.h> #include <linux/seq_file.h> +#include <linux/sync_file.h> #include <linux/poll.h> #include <linux/dma-resv.h> #include <linux/mm.h> @@ -348,6 +349,161 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) return ret; }

+/* This function takes a ref to add_fence on success. The caller still + * owns its ref and has to dma_fence_put it. + */ +static struct dma_fence *dma_buf_get_unified_fence(struct dma_buf *dmabuf, + struct dma_fence *add_fence) +{ + struct dma_fence **fences = NULL; + struct dma_fence_array *array; + unsigned i, num_fences = 0; + int ret; + + ret = dma_resv_get_fences_rcu(dmabuf->resv, NULL, + &num_fences, &fences); + if (ret) + return NULL; /* ret can only be 0 or -ENOMEM */ + + if (num_fences == 0) { + if (add_fence) { + return add_fence; + } else { + return dma_fence_get_stub(); + } + } else if (num_fences == 1 && (!add_fence || add_fence == fences[0])) { + struct dma_fence *fence = fences[0]; + kfree(fences); + return fence; + } + + if (add_fence) { + struct dma_fence **nfences; + size_t sz; + + /* Get a ref to add_fence so that we have a ref to every + * fence we are going to put in the array. + */ + dma_fence_get(add_fence); + + sz = (num_fences + 1) * sizeof(*fences); + nfences = krealloc(fences, sz, GFP_NOWAIT | __GFP_NOWARN); + if (!nfences) + goto err_put_fences; + + nfences[num_fences++] = add_fence; + } + + array = dma_fence_array_create(num_fences, fences, + dma_fence_context_alloc(1), + 1, false); + if (!array) + goto err_put_fences; + + /* The fence array now owns fences_arr and our references to each + * of the individual fences. We only own a reference to the one + * array fence. + */ + + return &array->base; + +err_put_fences: + for (i = 0; i < num_fences; i++) + dma_fence_put(fences[0]); + dma_fence_put(add_fence); + kfree(fences); + return NULL; +} + +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf, + const void __user *user_data) +{ + struct dma_buf_sync_file arg; + struct dma_fence *fence, *unified_fence; + int ret; + + if (copy_from_user(&arg, user_data, sizeof(arg))) + return -EFAULT; + + if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE) + return -EINVAL; + + fence = sync_file_get_fence(arg.fd); + if (!fence) + return -EINVAL; + + if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) { + dma_resv_lock(dmabuf->resv, NULL); + unified_fence = dma_buf_get_unified_fence(dmabuf, fence); + if (unified_fence) + dma_resv_add_excl_fence(dmabuf->resv, fence); + else + ret = -ENOMEM; + dma_resv_unlock(dmabuf->resv); + } else { + dma_resv_add_shared_fence(dmabuf->resv, fence); + } + + dma_fence_put(fence); + + return ret; +} + +static long dma_buf_signal_sync_file(struct dma_buf *dmabuf, + void __user *user_data) +{ + struct dma_buf_sync_file arg; + struct dma_fence *fence = NULL; + struct sync_file *sync_file; + int fd, ret; + + if (copy_from_user(&arg, user_data, sizeof(arg))) + return -EFAULT; + + if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE) + return -EINVAL; + + fd = get_unused_fd_flags(O_CLOEXEC); + if (fd < 0) + return fd; + + if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) { + /* We need to include both the exclusive fence and all of + * the shared fences in our fence. + */ + fence = dma_buf_get_unified_fence(dmabuf, NULL); + if (!fence) { + ret = -ENOMEM; + goto err_put_fd; + } + } else { + fence = dma_resv_get_excl_rcu(dmabuf->resv); + if (!fence) + fence = dma_fence_get_stub(); + } + + sync_file = sync_file_create(fence); + + dma_fence_put(fence); + + if (!sync_file) { + ret = -EINVAL; + goto err_put_fd; + } + + fd_install(fd, sync_file->file); + + arg.fd = fd; + if (copy_to_user(user_data, &arg, sizeof(arg))) + return -EFAULT; + + return 0; + +err_put_fd: + put_unused_fd(fd); + return ret; +} + static long dma_buf_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { @@ -390,6 +546,12 @@ static long dma_buf_ioctl(struct file *file, case DMA_BUF_SET_NAME: return dma_buf_set_name(dmabuf, (const char __user *)arg);

+ case DMA_BUF_IOCTL_WAIT_SYNC_FILE: + return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg); + + case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE: + return dma_buf_signal_sync_file(dmabuf, (void __user *)arg); + default: return -ENOTTY; } diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h index dbc7092e04b5..825b9a913c89 100644 --- a/include/uapi/linux/dma-buf.h +++ b/include/uapi/linux/dma-buf.h @@ -37,8 +37,17 @@ struct dma_buf_sync {

#define DMA_BUF_NAME_LEN 32

+struct dma_buf_sync_file { + __u32 flags; + __s32 fd; +}; + +#define DMA_BUF_SYNC_FILE_SYNC_WRITE (1 << 0) + #define DMA_BUF_BASE 'b' -#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) -#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) +#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync) +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE _IOW(DMA_BUF_BASE, 3, struct dma_buf_sync)

#endif

Vulkan WSI user of the new API:

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037

On Tue, Mar 3, 2020 at 1:03 PM Jason Ekstrand jason@jlekstrand.net wrote:

Explicit synchronization is the future. At least, that seems to be what most userspace APIs are agreeing on at this point. However, most of our Linux APIs (both userspace and kernel UAPI) are currently built around implicit synchronization with dma-buf. While work is ongoing to change many of the userspace APIs and protocols to an explicit synchronization model, switching over piecemeal is difficult due to the number of potential components involved. On the kernel side, many drivers use dma-buf including GPU (3D/compute), display, v4l, and others. In userspace, we have X11, several Wayland compositors, 3D drivers, compute drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually manage the fences attached to a dma-buf. Alternatively, one can think of this as making dma-buf's implicit synchronization simply a carrier for an explicit fence. This is accomplished by adding two IOCTLs to dma-buf for importing and exporting a sync file to/from the dma-buf. This way a userspace component which is uses explicit synchronization, such as a Vulkan driver, can manually set the write fence on a buffer before handing it off to an implicitly synchronized component such as a Wayland compositor or video encoder. In this way, each of the different components can be upgraded to an explicit synchronization model one at a time as long as the userspace pieces connecting them are aware of it and import/export fences at the right times.

There is a potential race condition with this API if userspace is not careful. A typical use case for implicit synchronization is to wait for the dma-buf to be ready, use it, and then signal it for some other component. Because a sync_file cannot be created until it is guaranteed to complete in finite time, userspace can only signal the dma-buf after it has already submitted the work which uses it to the kernel and has received a sync_file back. There is no way to atomically submit a wait-use-signal operation. This is not, however, really a problem with this API so much as it is a problem with explicit synchronization itself. The way this is typically handled is to have very explicit ownership transfer points in the API or protocol which ensure that only one component is using it at any given time. Both X11 (via the PRESENT extension) and Wayland provide such ownership transfer points via explicit present and idle messages.

The decision was intentionally made in this patch to make the import and export operations IOCTLs on the dma-buf itself rather than as a DRM IOCTL. This makes it the import/export operation universal across all components which use dma-buf including GPU, display, v4l, and others. It also means that a userspace component can do the import/export without access to the DRM fd which may be tricky to get in cases where the client communicates with DRM via a userspace API such as OpenGL or Vulkan. At a future date we may choose to add direct import/export APIs to components such as drm_syncobj to avoid allocating a file descriptor and going through two ioctls. However, that seems to be something of a micro-optimization as import/export operations are likely to happen at a rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):

• Use a wrapper dma_fence_array of all fences including the new one when importing an exclusive fence.

v3 (Jason Ekstrand):

• Lock around setting shared fences as well as exclusive
• Mark SIGNAL_SYNC_FILE as a read-write ioctl.
• Initialize ret to 0 in dma_buf_wait_sync_file

Signed-off-by: Jason Ekstrand jason@jlekstrand.net

drivers/dma-buf/dma-buf.c | 164 +++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 13 ++- 2 files changed, 175 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index d4097856c86b..2c4608bae3c2 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -20,6 +20,7 @@ #include <linux/debugfs.h> #include <linux/module.h> #include <linux/seq_file.h> +#include <linux/sync_file.h> #include <linux/poll.h> #include <linux/dma-resv.h> #include <linux/mm.h> @@ -348,6 +349,163 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) return ret; }

+/* This function takes a ref to add_fence on success. The caller still

• • owns its ref and has to dma_fence_put it.
• */

+static struct dma_fence *dma_buf_get_unified_fence(struct dma_buf *dmabuf,

•                                              struct dma_fence *add_fence)
  

+{

•   struct dma_fence **fences = NULL;
  

•   struct dma_fence_array *array;
  

•   unsigned i, num_fences = 0;
  

•   int ret;
  

•   ret = dma_resv_get_fences_rcu(dmabuf->resv, NULL,
  

•                                 &num_fences, &fences);
  

•   if (ret)
  

•           return NULL; /* ret can only be 0 or -ENOMEM */
  

•   if (num_fences == 0) {
  

•           if (add_fence) {
  

•                   return add_fence;
  

•           } else {
  

•                   return dma_fence_get_stub();
  

•           }
  

•   } else if (num_fences == 1 && (!add_fence || add_fence == fences[0])) {
  

•           struct dma_fence *fence = fences[0];
  

•           kfree(fences);
  

•           return fence;
  

•   }
  

•   if (add_fence) {
  

•           struct dma_fence **nfences;
  

•           size_t sz;
  

•           /* Get a ref to add_fence so that we have a ref to every
  

•            * fence we are going to put in the array.
  

•            */
  

•           dma_fence_get(add_fence);
  

•           sz = (num_fences + 1) * sizeof(*fences);
  

•           nfences = krealloc(fences, sz, GFP_NOWAIT | __GFP_NOWARN);
  

•           if (!nfences)
  

•                   goto err_put_fences;
  

•           nfences[num_fences++] = add_fence;
  

•   }
  

•   array = dma_fence_array_create(num_fences, fences,
  

•                                  dma_fence_context_alloc(1),
  

•                                  1, false);
  

•   if (!array)
  

•           goto err_put_fences;
  

•   /* The fence array now owns fences_arr and our references to each
  

•    * of the individual fences.  We only own a reference to the one
  

•    * array fence.
  

•    */
  

•   return &array->base;
  

+err_put_fences:

•   for (i = 0; i < num_fences; i++)
  

•           dma_fence_put(fences[0]);
  

•   dma_fence_put(add_fence);
  

•   kfree(fences);
  

•   return NULL;
  

+}

+static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,

•                              const void __user *user_data)
  

+{

•   struct dma_buf_sync_file arg;
  

•   struct dma_fence *fence, *unified_fence;
  

•   int ret = 0;
  

•   if (copy_from_user(&arg, user_data, sizeof(arg)))
  

•           return -EFAULT;
  

•   if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
  

•           return -EINVAL;
  

•   fence = sync_file_get_fence(arg.fd);
  

•   if (!fence)
  

•           return -EINVAL;
  

•   dma_resv_lock(dmabuf->resv, NULL);
  

•   if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
  

•           unified_fence = dma_buf_get_unified_fence(dmabuf, fence);
  

•           if (unified_fence)
  

•                   dma_resv_add_excl_fence(dmabuf->resv, fence);
  

•           else
  

•                   ret = -ENOMEM;
  

•   } else {
  

•           dma_resv_add_shared_fence(dmabuf->resv, fence);
  

•   }
  

•   dma_resv_unlock(dmabuf->resv);
  

•   dma_fence_put(fence);
  

•   return ret;
  

+}

+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,

•                                void __user *user_data)
  

+{

•   struct dma_buf_sync_file arg;
  

•   struct dma_fence *fence = NULL;
  

•   struct sync_file *sync_file;
  

•   int fd, ret;
  

•   if (copy_from_user(&arg, user_data, sizeof(arg)))
  

•           return -EFAULT;
  

•   if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
  

•           return -EINVAL;
  

•   fd = get_unused_fd_flags(O_CLOEXEC);
  

•   if (fd < 0)
  

•           return fd;
  

•   if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
  

•           /* We need to include both the exclusive fence and all of
  

•            * the shared fences in our fence.
  

•            */
  

•           fence = dma_buf_get_unified_fence(dmabuf, NULL);
  

•           if (!fence) {
  

•                   ret = -ENOMEM;
  

•                   goto err_put_fd;
  

•           }
  

•   } else {
  

•           fence = dma_resv_get_excl_rcu(dmabuf->resv);
  

•           if (!fence)
  

•                   fence = dma_fence_get_stub();
  

•   }
  

•   sync_file = sync_file_create(fence);
  

•   dma_fence_put(fence);
  

•   if (!sync_file) {
  

•           ret = -EINVAL;
  

•           goto err_put_fd;
  

•   }
  

•   fd_install(fd, sync_file->file);
  

•   arg.fd = fd;
  

•   if (copy_to_user(user_data, &arg, sizeof(arg)))
  

•           return -EFAULT;
  

•   return 0;
  

+err_put_fd:

•   put_unused_fd(fd);
  

•   return ret;
  

+}

static long dma_buf_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { @@ -390,6 +548,12 @@ static long dma_buf_ioctl(struct file *file, case DMA_BUF_SET_NAME: return dma_buf_set_name(dmabuf, (const char __user *)arg);

•   case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
  

•           return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
  

•   case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
  

•           return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
  

•   default:
            return -ENOTTY;
    }
  

diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h index dbc7092e04b5..86e07acca90c 100644 --- a/include/uapi/linux/dma-buf.h +++ b/include/uapi/linux/dma-buf.h @@ -37,8 +37,17 @@ struct dma_buf_sync {

#define DMA_BUF_NAME_LEN 32

+struct dma_buf_sync_file {

•   __u32 flags;
  

•   __s32 fd;
  

+};

+#define DMA_BUF_SYNC_FILE_SYNC_WRITE (1 << 0)

#define DMA_BUF_BASE 'b' -#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) -#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) +#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync) +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE _IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)

#endif

2.24.1

From: Christian König ckoenig.leichtzumerken@gmail.com

Add a helper function to get a single fence representing all fences in a dma_resv object.

This fence is either the only one in the object or all not signaled fences of the object in a flatted out dma_fence_array.

v2 (Jason Ekstrand): - Take reference of fences both for creating the dma_fence_array and in the case where we return one fence. - Handle the case where dma_resv_get_list() returns NULL

Signed-off-by: Jason Ekstrand jason@jlekstrand.net --- drivers/dma-buf/dma-resv.c | 118 +++++++++++++++++++++++++++++++++++++ include/linux/dma-resv.h | 3 + 2 files changed, 121 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index 4264e64788c4..66591d8ab7ef 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -33,6 +33,8 @@ */

#include <linux/dma-resv.h> +#include <linux/dma-fence-chain.h> +#include <linux/dma-fence-array.h> #include <linux/export.h> #include <linux/sched/mm.h>

@@ -47,6 +49,19 @@ * write-side updates. */

+/** + * dma_fence_deep_dive_for_each - deep dive into the fence containers + * @fence: resulting fence + * @chain: variable for a dma_fence_chain + * @index: index into a dma_fence_array + * @head: starting point + * + * Helper to deep dive into the fence containers for flattening them. + */ +#define dma_fence_deep_dive_for_each(fence, chain, index, head) \ + dma_fence_chain_for_each(chain, head) \ + dma_fence_array_for_each(fence, index, chain) + DEFINE_WD_CLASS(reservation_ww_class); EXPORT_SYMBOL(reservation_ww_class);

@@ -516,6 +531,109 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj, } EXPORT_SYMBOL_GPL(dma_resv_get_fences_rcu);

+/** + * dma_resv_get_singleton - get a single fence for the dma_resv object + * @obj: the reservation object + * @extra: extra fence to add to the resulting array + * @result: resulting dma_fence + * + * Get a single fence representing all unsignaled fences in the dma_resv object + * plus the given extra fence. If we got only one fence return a new + * reference to that, otherwise return a dma_fence_array object. + * + * RETURNS + * Returns -NOMEM if allocations fail, zero otherwise. + */ +int dma_resv_get_singleton(struct dma_resv *obj, struct dma_fence *extra, + struct dma_fence **result) +{ + struct dma_resv_list *fobj = dma_resv_get_list(obj); + struct dma_fence *excl = dma_resv_get_excl(obj); + struct dma_fence *fence, *chain, **fences; + struct dma_fence_array *array; + unsigned int num_fences, shared_count; + unsigned int i, j; + + num_fences = 0; + *result = NULL; + + dma_fence_deep_dive_for_each(fence, chain, i, extra) { + if (dma_fence_is_signaled(fence)) + continue; + + *result = fence; + ++num_fences; + } + + dma_fence_deep_dive_for_each(fence, chain, i, excl) { + if (dma_fence_is_signaled(fence)) + continue; + + *result = fence; + ++num_fences; + } + + shared_count = fobj ? fobj->shared_count : 0; + for (i = 0; i < shared_count; ++i) { + struct dma_fence *f; + + f = rcu_dereference_protected(fobj->shared[i], + dma_resv_held(obj)); + dma_fence_deep_dive_for_each(fence, chain, j, f) { + if (dma_fence_is_signaled(fence)) + continue; + + *result = fence; + ++num_fences; + } + } + + if (num_fences <= 1) { + *result = dma_fence_get(*result); + return 0; + } + + fences = kmalloc_array(num_fences, sizeof(struct dma_fence*), + GFP_KERNEL); + if (!fences) + return -ENOMEM; + + num_fences = 0; + + dma_fence_deep_dive_for_each(fence, chain, i, extra) + if (!dma_fence_is_signaled(fence)) + fences[num_fences++] = dma_fence_get(fence); + + dma_fence_deep_dive_for_each(fence, chain, i, excl) + if (!dma_fence_is_signaled(fence)) + fences[num_fences++] = dma_fence_get(fence); + + for (i = 0; i < shared_count; ++i) { + struct dma_fence *f; + + f = rcu_dereference_protected(fobj->shared[i], + dma_resv_held(obj)); + dma_fence_deep_dive_for_each(fence, chain, j, f) + if (!dma_fence_is_signaled(fence)) + fences[num_fences++] = dma_fence_get(fence); + } + + array = dma_fence_array_create(num_fences, fences, + dma_fence_context_alloc(1), + 1, false); + if (!array) + goto error_free; + + *result = &array->base; + return 0; + +error_free: + while (num_fences--) + dma_fence_put(fences[num_fences]); + kfree(fences); + return -ENOMEM; +} + /** * dma_resv_wait_timeout_rcu - Wait on reservation's objects * shared and/or exclusive fences. diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h index ee50d10f052b..d50e753e4550 100644 --- a/include/linux/dma-resv.h +++ b/include/linux/dma-resv.h @@ -287,6 +287,9 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,

int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src);

+int dma_resv_get_singleton(struct dma_resv *obj, struct dma_fence *extra, + struct dma_fence **result); + long dma_resv_wait_timeout_rcu(struct dma_resv *obj, bool wait_all, bool intr, unsigned long timeout);

Am 11.03.20 um 04:43 schrieb Jason Ekstrand:

Explicit synchronization is the future. At least, that seems to be what most userspace APIs are agreeing on at this point. However, most of our Linux APIs (both userspace and kernel UAPI) are currently built around implicit synchronization with dma-buf. While work is ongoing to change many of the userspace APIs and protocols to an explicit synchronization model, switching over piecemeal is difficult due to the number of potential components involved. On the kernel side, many drivers use dma-buf including GPU (3D/compute), display, v4l, and others. In userspace, we have X11, several Wayland compositors, 3D drivers, compute drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually manage the fences attached to a dma-buf. Alternatively, one can think of this as making dma-buf's implicit synchronization simply a carrier for an explicit fence. This is accomplished by adding two IOCTLs to dma-buf for importing and exporting a sync file to/from the dma-buf. This way a userspace component which is uses explicit synchronization, such as a Vulkan driver, can manually set the write fence on a buffer before handing it off to an implicitly synchronized component such as a Wayland compositor or video encoder. In this way, each of the different components can be upgraded to an explicit synchronization model one at a time as long as the userspace pieces connecting them are aware of it and import/export fences at the right times.

There is a potential race condition with this API if userspace is not careful. A typical use case for implicit synchronization is to wait for the dma-buf to be ready, use it, and then signal it for some other component. Because a sync_file cannot be created until it is guaranteed to complete in finite time, userspace can only signal the dma-buf after it has already submitted the work which uses it to the kernel and has received a sync_file back. There is no way to atomically submit a wait-use-signal operation. This is not, however, really a problem with this API so much as it is a problem with explicit synchronization itself. The way this is typically handled is to have very explicit ownership transfer points in the API or protocol which ensure that only one component is using it at any given time. Both X11 (via the PRESENT extension) and Wayland provide such ownership transfer points via explicit present and idle messages.

The decision was intentionally made in this patch to make the import and export operations IOCTLs on the dma-buf itself rather than as a DRM IOCTL. This makes it the import/export operation universal across all components which use dma-buf including GPU, display, v4l, and others. It also means that a userspace component can do the import/export without access to the DRM fd which may be tricky to get in cases where the client communicates with DRM via a userspace API such as OpenGL or Vulkan. At a future date we may choose to add direct import/export APIs to components such as drm_syncobj to avoid allocating a file descriptor and going through two ioctls. However, that seems to be something of a micro-optimization as import/export operations are likely to happen at a rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):

• Use a wrapper dma_fence_array of all fences including the new one when importing an exclusive fence.

v3 (Jason Ekstrand):

• Lock around setting shared fences as well as exclusive
• Mark SIGNAL_SYNC_FILE as a read-write ioctl.
• Initialize ret to 0 in dma_buf_wait_sync_file

v4 (Jason Ekstrand):

• Use the new dma_resv_get_singleton helper

Signed-off-by: Jason Ekstrand jason@jlekstrand.net

drivers/dma-buf/dma-buf.c | 96 ++++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 13 ++++- 2 files changed, 107 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index d4097856c86b..09973c689866 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -20,6 +20,7 @@ #include <linux/debugfs.h> #include <linux/module.h> #include <linux/seq_file.h> +#include <linux/sync_file.h> #include <linux/poll.h> #include <linux/dma-resv.h> #include <linux/mm.h> @@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) return ret; } +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,

• 		   const void __user *user_data)
  

+{

• struct dma_buf_sync_file arg;
• struct dma_fence *fence;
• int ret = 0;
• if (copy_from_user(&arg, user_data, sizeof(arg)))

• return -EFAULT;
  

• if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)

• return -EINVAL;
  

• fence = sync_file_get_fence(arg.fd);
• if (!fence)

• return -EINVAL;
  

• dma_resv_lock(dmabuf->resv, NULL);
• if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {

• struct dma_fence *singleton = NULL;
  

• ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
  

• if (!ret && singleton)
  

• 	dma_resv_add_excl_fence(dmabuf->resv, singleton);
  

• } else {

• dma_resv_add_shared_fence(dmabuf->resv, fence);
  

• }

You also need to create a singleton when adding a shared fences.

The problem is that shared fences must always signal after exclusive ones and you can't guarantee that for the fence you add here.

Regards, Christian.

• dma_resv_unlock(dmabuf->resv);
• dma_fence_put(fence);
• return ret;

+}

+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,

• 		     void __user *user_data)
  

+{

• struct dma_buf_sync_file arg;
• struct dma_fence *fence = NULL;
• struct sync_file *sync_file;
• int fd, ret;
• if (copy_from_user(&arg, user_data, sizeof(arg)))

• return -EFAULT;
  

• if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)

• return -EINVAL;
  

• fd = get_unused_fd_flags(O_CLOEXEC);
• if (fd < 0)

• return fd;
  

• if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {

• /* We need to include both the exclusive fence and all of
  

•  * the shared fences in our fence.
  

•  */
  

• ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
  

• if (ret)
  

• 	goto err_put_fd;
  

• } else {

• fence = dma_resv_get_excl_rcu(dmabuf->resv);
  

• }
• if (!fence)

• fence = dma_fence_get_stub();
  

• sync_file = sync_file_create(fence);
• dma_fence_put(fence);
• if (!sync_file) {

• ret = -EINVAL;
  

• goto err_put_fd;
  

• }
• fd_install(fd, sync_file->file);
• arg.fd = fd;
• if (copy_to_user(user_data, &arg, sizeof(arg)))

• return -EFAULT;
  

• return 0;

+err_put_fd:

• put_unused_fd(fd);
• return ret;

+}

• static long dma_buf_ioctl(struct file *file, unsigned int cmd, unsigned long arg) {

@@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file, case DMA_BUF_SET_NAME: return dma_buf_set_name(dmabuf, (const char __user *)arg);

• case DMA_BUF_IOCTL_WAIT_SYNC_FILE:

• return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
  

• case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:

• return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
  

• default: return -ENOTTY; }

diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h index dbc7092e04b5..86e07acca90c 100644 --- a/include/uapi/linux/dma-buf.h +++ b/include/uapi/linux/dma-buf.h @@ -37,8 +37,17 @@ struct dma_buf_sync { #define DMA_BUF_NAME_LEN 32 +struct dma_buf_sync_file {

• __u32 flags;
• __s32 fd;

+};

+#define DMA_BUF_SYNC_FILE_SYNC_WRITE (1 << 0)

• #define DMA_BUF_BASE 'b'

-#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) -#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) +#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync) +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE _IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync) #endif

Am 12.03.20 um 16:57 schrieb Jason Ekstrand:

On Wed, Mar 11, 2020 at 8:18 AM Christian König christian.koenig@amd.com wrote:

...

Am 11.03.20 um 04:43 schrieb Jason Ekstrand:

...

Explicit synchronization is the future. At least, that seems to be what most userspace APIs are agreeing on at this point. However, most of our Linux APIs (both userspace and kernel UAPI) are currently built around implicit synchronization with dma-buf. While work is ongoing to change many of the userspace APIs and protocols to an explicit synchronization model, switching over piecemeal is difficult due to the number of potential components involved. On the kernel side, many drivers use dma-buf including GPU (3D/compute), display, v4l, and others. In userspace, we have X11, several Wayland compositors, 3D drivers, compute drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually manage the fences attached to a dma-buf. Alternatively, one can think of this as making dma-buf's implicit synchronization simply a carrier for an explicit fence. This is accomplished by adding two IOCTLs to dma-buf for importing and exporting a sync file to/from the dma-buf. This way a userspace component which is uses explicit synchronization, such as a Vulkan driver, can manually set the write fence on a buffer before handing it off to an implicitly synchronized component such as a Wayland compositor or video encoder. In this way, each of the different components can be upgraded to an explicit synchronization model one at a time as long as the userspace pieces connecting them are aware of it and import/export fences at the right times.

There is a potential race condition with this API if userspace is not careful. A typical use case for implicit synchronization is to wait for the dma-buf to be ready, use it, and then signal it for some other component. Because a sync_file cannot be created until it is guaranteed to complete in finite time, userspace can only signal the dma-buf after it has already submitted the work which uses it to the kernel and has received a sync_file back. There is no way to atomically submit a wait-use-signal operation. This is not, however, really a problem with this API so much as it is a problem with explicit synchronization itself. The way this is typically handled is to have very explicit ownership transfer points in the API or protocol which ensure that only one component is using it at any given time. Both X11 (via the PRESENT extension) and Wayland provide such ownership transfer points via explicit present and idle messages.

The decision was intentionally made in this patch to make the import and export operations IOCTLs on the dma-buf itself rather than as a DRM IOCTL. This makes it the import/export operation universal across all components which use dma-buf including GPU, display, v4l, and others. It also means that a userspace component can do the import/export without access to the DRM fd which may be tricky to get in cases where the client communicates with DRM via a userspace API such as OpenGL or Vulkan. At a future date we may choose to add direct import/export APIs to components such as drm_syncobj to avoid allocating a file descriptor and going through two ioctls. However, that seems to be something of a micro-optimization as import/export operations are likely to happen at a rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):

• Use a wrapper dma_fence_array of all fences including the new one when importing an exclusive fence.

v3 (Jason Ekstrand):

• Lock around setting shared fences as well as exclusive
• Mark SIGNAL_SYNC_FILE as a read-write ioctl.
• Initialize ret to 0 in dma_buf_wait_sync_file

v4 (Jason Ekstrand):

• Use the new dma_resv_get_singleton helper

Signed-off-by: Jason Ekstrand jason@jlekstrand.net

drivers/dma-buf/dma-buf.c | 96 ++++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 13 ++++- 2 files changed, 107 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index d4097856c86b..09973c689866 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -20,6 +20,7 @@ #include <linux/debugfs.h> #include <linux/module.h> #include <linux/seq_file.h> +#include <linux/sync_file.h> #include <linux/poll.h> #include <linux/dma-resv.h> #include <linux/mm.h> @@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) return ret; }

+static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,

•                            const void __user *user_data)
  

+{

• struct dma_buf_sync_file arg;
  

• struct dma_fence *fence;
  

• int ret = 0;
  

• if (copy_from_user(&arg, user_data, sizeof(arg)))
  

•         return -EFAULT;
  

• if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
  

•         return -EINVAL;
  

• fence = sync_file_get_fence(arg.fd);
  

• if (!fence)
  

•         return -EINVAL;
  

• dma_resv_lock(dmabuf->resv, NULL);
  

• if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
  

•         struct dma_fence *singleton = NULL;
  

•         ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
  

•         if (!ret && singleton)
  

•                 dma_resv_add_excl_fence(dmabuf->resv, singleton);
  

• } else {
  

•         dma_resv_add_shared_fence(dmabuf->resv, fence);
  

• }
  

You also need to create a singleton when adding a shared fences.

The problem is that shared fences must always signal after exclusive ones and you can't guarantee that for the fence you add here.

I'm beginning to think that I should just drop the flags and always wait on all fences and always take what's currently the "write" path. Otherwise, something's going to get it wrong somewhere. Thoughts?

If that is sufficient for your use case then that is certainly the more defensive (e.g. less dangerous) approach.

Also, Michelle (added to CC) commented on IRC today that amdgpu does something with implicit sync fences where it sorts out the fences which affect one queue vs. others. He thought that stuffing fences in the dma-buf in this way might cause that to not work. Thoughts?

Yes that is correct. What amdgpu does is it ignores all fences from the same process.

E.g. when A submits IBs 1, 2 and 3 and then B submits IB 4 then 4 waits for 1,2,3, but 1,2,3 can run parallel to each other.

And yes adding anything as explicit sync would break that, but I don't think that this is much of a problem.

Regards, Christian.

--Jason

...

Regards, Christian.

...

• dma_resv_unlock(dmabuf->resv);
  

• dma_fence_put(fence);
  

• return ret;
  

+}

+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,

•                              void __user *user_data)
  

+{

• struct dma_buf_sync_file arg;
  

• struct dma_fence *fence = NULL;
  

• struct sync_file *sync_file;
  

• int fd, ret;
  

• if (copy_from_user(&arg, user_data, sizeof(arg)))
  

•         return -EFAULT;
  

• if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
  

•         return -EINVAL;
  

• fd = get_unused_fd_flags(O_CLOEXEC);
  

• if (fd < 0)
  

•         return fd;
  

• if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
  

•         /* We need to include both the exclusive fence and all of
  

•          * the shared fences in our fence.
  

•          */
  

•         ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
  

•         if (ret)
  

•                 goto err_put_fd;
  

• } else {
  

•         fence = dma_resv_get_excl_rcu(dmabuf->resv);
  

• }
  

• if (!fence)
  

•         fence = dma_fence_get_stub();
  

• sync_file = sync_file_create(fence);
  

• dma_fence_put(fence);
  

• if (!sync_file) {
  

•         ret = -EINVAL;
  

•         goto err_put_fd;
  

• }
  

• fd_install(fd, sync_file->file);
  

• arg.fd = fd;
  

• if (copy_to_user(user_data, &arg, sizeof(arg)))
  

•         return -EFAULT;
  

• return 0;
  

+err_put_fd:

• put_unused_fd(fd);
  

• return ret;
  

+}

• static long dma_buf_ioctl(struct file *file, unsigned int cmd, unsigned long arg) {

@@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file, case DMA_BUF_SET_NAME: return dma_buf_set_name(dmabuf, (const char __user *)arg);

• case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
  

•         return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
  

• case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
  

•         return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
  

•  default:
           return -ENOTTY;
   }
  

diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h index dbc7092e04b5..86e07acca90c 100644 --- a/include/uapi/linux/dma-buf.h +++ b/include/uapi/linux/dma-buf.h @@ -37,8 +37,17 @@ struct dma_buf_sync {

#define DMA_BUF_NAME_LEN 32

+struct dma_buf_sync_file {

• __u32 flags;
  

• __s32 fd;
  

+};

+#define DMA_BUF_SYNC_FILE_SYNC_WRITE (1 << 0)

• #define DMA_BUF_BASE 'b'

-#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) -#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync) +#define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *) +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync) +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE _IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)

#endif