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(a)igalia.com>
---
v1 -> v2: https://lore.kernel.org/dri-devel/20260411185756.1887119-4-mcanal@igalia.co…
- 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,
--
2.53.0
