linux-kernel - Re: [PATCH 4/4] drm/nouveau: Check dma

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Message-ID: <45afaa00-e627-4a8c-b25e-2f74357c3430@amd.com>
Date: Thu, 8 May 2025 11:54:10 +0200
From: Christian König <christian.koenig@....com>
To: phasta@...nel.org, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Sumit Semwal <sumit.semwal@...aro.org>
Cc: dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH 4/4] drm/nouveau: Check dma_fence in canonical way

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@...nel.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);
>>
>