Re: [PATCH 4/4] drm/nouveau: Check dma_fence in canonical way

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

On 5/8/25 11:13, Philipp Stanner wrote:
> On Mon, 2025-04-28 at 16:45 +0200, Christian König wrote:
>> On 4/24/25 15:02, Philipp Stanner wrote:
>>> In nouveau_fence_done(), a fence is checked for being signaled by
>>> manually evaluating the base fence's bits. This can be done in a
>>> canonical manner through dma_fence_is_signaled().
>>>
>>> Replace the bit-check with dma_fence_is_signaled().
>>>
>>> Signed-off-by: Philipp Stanner <phasta@kernel.org>
>>
>>
>> I think the bit check was used here as fast path optimization because
>> we later call dma_fence_is_signaled() anyway.
> 
> That fast path optimization effectively saves one JMP instruction to
> the function.


What I meant was that we might completely drop that optimization. It looks like overkill and potentially hides bugs.

Regards,
Christian.

> 
> I'm increasingly of the opinion that we shall work towards all DRM
> users only ever using infrastructure through officially documented API
> functions, without touching internal data structures.
> 
>> Feel free to add my acked-by, but honestly what nouveau does here
>> looks rather suspicious to me.
> 
> :)
> 
> 
> P.
> 
>>
>> Regards,
>> Christian.
>>
>>> ---
>>>  drivers/gpu/drm/nouveau/nouveau_fence.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> b/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> index fb9811938c82..d5654e26d5bc 100644
>>> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> @@ -253,7 +253,7 @@ nouveau_fence_done(struct nouveau_fence *fence)
>>>  	struct nouveau_channel *chan;
>>>  	unsigned long flags;
>>>  
>>> -	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence-
>>>> base.flags))
>>> +	if (dma_fence_is_signaled(&fence->base))
>>>  		return true;
>>>  
>>>  	spin_lock_irqsave(&fctx->lock, flags);
>>
>