Re: [RFC 2/3] drm/atomic: Call dma_fence_boost() when we've missed a vblank

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, May 19, 2021 at 11:38:53AM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 560aaecba31b..fe10fc2e7f86 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1435,11 +1435,15 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
>  	int i, ret;
>  
>  	for_each_new_plane_in_state(state, plane, new_plane_state, i) {
> +		u64 vblank_count;
> +
>  		if (!new_plane_state->fence)
>  			continue;
>  
>  		WARN_ON(!new_plane_state->fb);
>  
> +		vblank_count = drm_crtc_vblank_count(new_plane_state->crtc);
> +
>  		/*
>  		 * If waiting for fences pre-swap (ie: nonblock), userspace can
>  		 * still interrupt the operation. Instead of blocking until the
> @@ -1449,6 +1453,13 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
>  		if (ret)
>  			return ret;
>  
> +		/*
> +		 * Check if we've missed a vblank while waiting, and if we have
> +		 * signal the fence that it's signaler should be boosted
> +		 */
> +		if (vblank_count != drm_crtc_vblank_count(new_plane_state->crtc))
> +			dma_fence_boost(new_plane_state->fence);

I think we should do a lot better here:
- maybe only bother doing this for single-crtc updates, and only if
  modeset isn't set. No one else cares about latency.

- We should boost _right_ when we've missed the frame, so I think we
  should have a _timeout wait here that guesstimates when the vblank is
  over (might need to throw in a vblank wait if we missed) and then boost
  immediately. Not wait a bunch of frames (worst case) until we finally
  decide to boost.

Otherwise I really like this, I think it's about the only real reason i915
isn't using atomic helpers.

Also adding Matt B for this topic.
-Daniel

> +
>  		dma_fence_put(new_plane_state->fence);
>  		new_plane_state->fence = NULL;
>  	}
> -- 
> 2.30.2
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch