[PATCH v3] dma-fence: Clarify external lock use case in dma_fence_init() docs

[Maíra Canal <mcanal@igalia.com>]

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>

---

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.

v2 -> v3: https://lore.kernel.org/dri-devel/20260419134943.54833-2-mcanal@igalia.com/T/

- Apply Christian's suggestion with small readability improvements.

---
 drivers/dma-buf/dma-fence.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index b3bfa6943a8e..c7ea1e75d38a 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -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,
-- 
2.54.0