[PATCH] drm/amdgpu: restore wait-free fastpath for GEM_WAIT_IDLE

[Lucas Stach]

Hi Christian,

Am Montag, dem 27.01.2025 um 17:14 +0100 schrieb Christian König:
> Am 27.01.25 um 17:02 schrieb Lucas Stach:
> > This effectively reverts 0fea2ed61e7f ("drm/amdgpu: Remove call to
> > reservation_object_test_signaled_rcu before wait"), as the premise of
> > that commit is wrong. dma_resv_wait_timeout() without a timeout will
> > not turn into a wait-free dma_resv_test_signaled, but will wait a
> > jiffy for unsignaled fences, which is not at all what userspace expects
> > when it calls GEM_WAIT_IDLE with a timeout of 0.
> 
> Marek pinged me about that strange behavior as well. That sounds like a 
> separate bug in dma_resv_wait_timeout() to me.
> 
> I don't see why the function should be waiting with a timeout of 0 and 
> we have multiple cases where that is assumed.
> 
> What should happen is that dma_resv_wait_timeout() should return 1 when 
> all fences are signaled even when the timeout is 0.
> 
> My educated guess is that this behavior is somehow broken and instead we 
> wait for at least 1 jiffies.
> 
> This here is probably just the tip of the iceberg.
> 
dma_resv_wait_timeout() explicitly sets timeout to a single jiffy when
it is entered with timeout == 0. This behavior was introduced with your
commit 06a66b5c77ed ("reservation: revert "wait only with non-zero
timeout specified (v3)" v2"), which seems to fix a real bug.

Between the two behaviors I think it is wrong for callers of
dma_resv_wait_timeout() to assume that the call is necessarily wait-
free in case of timeout == 0. If you want wait-free but are able to
deal with false positive unsignaled returns use dma_resv_test_signaled,
otherwise use dma_resv_wait_timeout to get accurate answers with a
possible wait.

Regards,
Lucas

> Regards,
> Christian.
> 
> > 
> > Signed-off-by: Lucas Stach <l.stach at pengutronix.de>
> > ---
> > This is most likely the correct kernel-side solution for the unexpected
> > slowdown worked around with in userspace with this Mesa series:
> > https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32877
> > ---
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 9 +++++++--
> >   1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > index 1a5df8b94661..75b5d5149911 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > @@ -567,8 +567,13 @@ int amdgpu_gem_wait_idle_ioctl(struct drm_device *dev, void *data,
> >   		return -ENOENT;
> >   
> >   	robj = gem_to_amdgpu_bo(gobj);
> > -	ret = dma_resv_wait_timeout(robj->tbo.base.resv, DMA_RESV_USAGE_READ,
> > -				    true, timeout);
> > +	if (timeout == 0)
> > +		ret = dma_resv_test_signaled(robj->tbo.base.resv,
> > +					     DMA_RESV_USAGE_READ);
> > +	else
> > +		ret = dma_resv_wait_timeout(robj->tbo.base.resv,
> > +					    DMA_RESV_USAGE_READ,
> > +					    true, timeout);
> >   
> >   	/* ret == 0 means not signaled,
> >   	 * ret > 0 means signaled
>