The kerneldoc comment on dma_fence_init() and dma_fence_init64() describe the legacy reason to pass an external lock as a need to prevent multiple fences "from signaling out of order". However, this wording is a bit misleading: a shared spinlock does not (and cannot) prevent the signaler from signaling out of order. Signaling order is the driver's responsibility regardless of whether the lock is shared or per-fence.
What a shared lock actually provides is serialization of signaling and observation across fences in a given context, so that observers never see a later fence signaled while an earlier one is not.
Reword both comments to describe this more accurately.
Signed-off-by: Maíra Canal mcanal@igalia.com
---
v1 -> v2: https://lore.kernel.org/dri-devel/20260411185756.1887119-4-mcanal@igalia.com...
- Be more explicit about not allowing new users to use an external lock. - De-duplicate the explanation in dma_fence_init64() by pointing to the dma_fence_init() documentation. --- drivers/dma-buf/dma-fence.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 1c1eaecaf1b0..63b349ba9a34 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -1102,9 +1102,11 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, * context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). * - * It is strongly discouraged to provide an external lock because this couples - * lock and fence life time. This is only allowed for legacy use cases when - * multiple fences need to be prevented from signaling out of order. + * External locks are a relic from legacy use cases that needed a shared lock + * to serialize signaling and observation of fences within a context, so that + * observers never see a later fence signaled while an earlier one isn't. New + * users MUST NOT use external locks, as they force the issuer to outlive all + * fences that reference the lock. */ void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, @@ -1129,9 +1131,8 @@ EXPORT_SYMBOL(dma_fence_init); * Context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). * - * It is strongly discouraged to provide an external lock because this couples - * lock and fence life time. This is only allowed for legacy use cases when - * multiple fences need to be prevented from signaling out of order. + * New users MUST NOT use external locks. Check the documentation in + * dma_fence_init() to understand the motives behind the legacy use cases. */ void dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops,
On Sun, 2026-04-19 at 10:48 -0300, Maíra Canal wrote:
The kerneldoc comment on dma_fence_init() and dma_fence_init64() describe the legacy reason to pass an external lock as a need to prevent multiple fences "from signaling out of order". However, this wording is a bit misleading: a shared spinlock does not (and cannot) prevent the signaler from signaling out of order. Signaling order is the driver's responsibility regardless of whether the lock is shared or per-fence.
What a shared lock actually provides is serialization of signaling and observation across fences in a given context, so that observers never see a later fence signaled while an earlier one is not.
Reword both comments to describe this more accurately.
Signed-off-by: Maíra Canal mcanal@igalia.com
Reviewed-by: Philipp Stanner phasta@kernel.org
v1 -> v2: https://lore.kernel.org/dri-devel/20260411185756.1887119-4-mcanal@igalia.com...
- Be more explicit about not allowing new users to use an external lock.
- De-duplicate the explanation in dma_fence_init64() by pointing to the
dma_fence_init() documentation.
drivers/dma-buf/dma-fence.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 1c1eaecaf1b0..63b349ba9a34 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -1102,9 +1102,11 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, * context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). *
- It is strongly discouraged to provide an external lock because this couples
- lock and fence life time. This is only allowed for legacy use cases when
- multiple fences need to be prevented from signaling out of order.
- External locks are a relic from legacy use cases that needed a shared lock
- to serialize signaling and observation of fences within a context, so that
- observers never see a later fence signaled while an earlier one isn't. New
- users MUST NOT use external locks, as they force the issuer to outlive all
- fences that reference the lock.
*/ void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, @@ -1129,9 +1131,8 @@ EXPORT_SYMBOL(dma_fence_init); * Context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). *
- It is strongly discouraged to provide an external lock because this couples
- lock and fence life time. This is only allowed for legacy use cases when
- multiple fences need to be prevented from signaling out of order.
- New users MUST NOT use external locks. Check the documentation in
- dma_fence_init() to understand the motives behind the legacy use cases.
*/ void dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops,
Hi Christian,
When possible, could you take a look at this patch? If you are okay with it, I can apply it to drm-misc-next.
Best regards, - Maíra
On 20/04/26 06:18, Philipp Stanner wrote:
On Sun, 2026-04-19 at 10:48 -0300, Maíra Canal wrote:
The kerneldoc comment on dma_fence_init() and dma_fence_init64() describe the legacy reason to pass an external lock as a need to prevent multiple fences "from signaling out of order". However, this wording is a bit misleading: a shared spinlock does not (and cannot) prevent the signaler from signaling out of order. Signaling order is the driver's responsibility regardless of whether the lock is shared or per-fence.
What a shared lock actually provides is serialization of signaling and observation across fences in a given context, so that observers never see a later fence signaled while an earlier one is not.
Reword both comments to describe this more accurately.
Signed-off-by: Maíra Canal mcanal@igalia.com
Reviewed-by: Philipp Stanner phasta@kernel.org
v1 -> v2: https://lore.kernel.org/dri-devel/20260411185756.1887119-4-mcanal@igalia.com...
- Be more explicit about not allowing new users to use an external lock.
- De-duplicate the explanation in dma_fence_init64() by pointing to the
dma_fence_init() documentation.
drivers/dma-buf/dma-fence.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 1c1eaecaf1b0..63b349ba9a34 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -1102,9 +1102,11 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, * context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). *
- It is strongly discouraged to provide an external lock because this couples
- lock and fence life time. This is only allowed for legacy use cases when
- multiple fences need to be prevented from signaling out of order.
- External locks are a relic from legacy use cases that needed a shared lock
- to serialize signaling and observation of fences within a context, so that
- observers never see a later fence signaled while an earlier one isn't. New
- users MUST NOT use external locks, as they force the issuer to outlive all
- fences that reference the lock.
*/ void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, @@ -1129,9 +1131,8 @@ EXPORT_SYMBOL(dma_fence_init); * Context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). *
- It is strongly discouraged to provide an external lock because this couples
- lock and fence life time. This is only allowed for legacy use cases when
- multiple fences need to be prevented from signaling out of order.
- New users MUST NOT use external locks. Check the documentation in
- dma_fence_init() to understand the motives behind the legacy use cases.
*/ void dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops,
linaro-mm-sig@lists.linaro.org
-
Maíra Canal -
Philipp Stanner