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

[Christian König <christian.koenig@amd.com>]

On 5/31/26 14:49, 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.
> 
> 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>

I'm going to push this to drm-misc-next.

Thanks,
Christian.

> 
> ---
> 
> 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,