linux-kernel - Re: [PATCH 2/2] dma-buf/dma-fence: Unify return codes for signalled fences

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Message-ID: <44d30056-612f-4563-ad9f-92696c5a803a@ursulin.net>
Date: Tue, 25 Nov 2025 13:20:06 +0000
From: Tvrtko Ursulin <tursulin@...ulin.net>
To: Philipp Stanner <phasta@...nel.org>,
 Sumit Semwal <sumit.semwal@...aro.org>, Gustavo Padovan
 <gustavo@...ovan.org>, Christian König
 <christian.koenig@....com>, Daniel Almeida <daniel.almeida@...labora.com>
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dma-buf/dma-fence: Unify return codes for signalled
 fences


On 25/11/2025 10:44, Philipp Stanner wrote:
> Almost all users of dma_fence_signal() ignore the return code which
> would indicate that the fence was already signaled. The same return code
> by dma_fence_add_callback() cannot be ignored, however, because it's
> needed to detect races.
> 
> For an already signaled fence, dma_fence_signal() returns -EINVAL,
> whereas dma_fence_add_callback() returns -ENOENT.
> 
> Unify the error codes by having dma_fence_signal() return -ENOENT, too.
> 
> Signed-off-by: Philipp Stanner <phasta@...nel.org>
> ---
>   drivers/dma-buf/dma-fence.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 3a48896ded62..09d97624e647 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -373,7 +373,7 @@ int dma_fence_signal_timestamp_locked(struct dma_fence *fence,
>   	lockdep_assert_held(fence->lock);
>   
>   	if (unlikely(__dma_fence_is_signaled(fence)))
> -		return -EINVAL;
> +		return -ENOENT;
>   
>   	/* Stash the cb_list before replacing it with the timestamp */
>   	list_replace(&fence->cb_list, &cb_list);

Story checks out AFAICT - only two callers fetch the error, xe and kfd, 
and neither does anything with it. So I'd say it makes sense to unify 
the errno.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@...lia.com>

Regards,

Tvrtko

P.S. Just not sure of the 1/2 on which this one ends depending on. I 
*think* I suggested the helper in the context of some discussion long 
long time ago but what it was? And what about all the drivers which look 
at the signaled bit directly?