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dma-fence: Clarify external lock use case in dma_fence_init() docs

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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.

Reword both comments to better describe the legacy use cases where a
shared lock was needed.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20260531125115.1136036-1-mcanal@igalia.com
Signed-off-by: Christian König <christian.koenig@amd.com>
This commit is contained in:
Maíra Canal 2026-05-31 09:49:59 -03:00 committed by Christian König
parent 21fcb222f0
commit e84b07e0a3
1 changed files with 8 additions and 6 deletions
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14
drivers/dma-buf/dma-fence.c
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@ -1102,9 +1102,12 @@ __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 of legacy use cases that needed a shared lock
* to serialize signaling when no out-of-order signaling was possible through
* &dma_fence_ops.signaled. Drivers have abandoned this concept since the
* introduction of the callback, but the external lock is still around. 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 +1132,8 @@ 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,