linux-kernel - Re: [PATCH v4 11/14] drm/atomic-helper: Set fence deadline for vblank

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Date:   Wed, 22 Feb 2023 05:46:43 -0500
From:   Luben Tuikov <luben.tuikov@....com>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     Rob Clark <robdclark@...omium.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        open list <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org
Subject: Re: [PATCH v4 11/14] drm/atomic-helper: Set fence deadline for vblank

On 2023-02-18 16:15, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.org>
> 
> For an atomic commit updating a single CRTC (ie. a pageflip) calculate
> the next vblank time, and inform the fence(s) of that deadline.
> 
> v2: Comment typo fix (danvet)
> 
> Signed-off-by: Rob Clark <robdclark@...omium.org>
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Rob Clark <robdclark@...omium.org>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 36 +++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index d579fd8f7cb8..35a4dc714920 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1511,6 +1511,40 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
>  }
>  EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
>  
> +/*
> + * For atomic updates which touch just a single CRTC, calculate the time of the
> + * next vblank, and inform all the fences of the deadline.
> + */
> +static void set_fence_deadline(struct drm_device *dev,
> +			       struct drm_atomic_state *state)
> +{
> +	struct drm_crtc *crtc, *wait_crtc = NULL;
> +	struct drm_crtc_state *new_crtc_state;
> +	struct drm_plane *plane;
> +	struct drm_plane_state *new_plane_state;
> +	ktime_t vbltime;

I've not looked at the latest language spec, but is AFAIR "vbltime"
would be uninitialized here. Has this changed?

> +	int i;
> +
> +	for_each_new_crtc_in_state (state, crtc, new_crtc_state, i) {
> +		if (wait_crtc)
> +			return;
> +		wait_crtc = crtc;
> +	}
> +
> +	/* If no CRTCs updated, then nothing to do: */
> +	if (!wait_crtc)
> +		return;
> +
> +	if (drm_crtc_next_vblank_time(wait_crtc, &vbltime))
> +		return;

We have a problem here in that we're adding the time remaining to the next
vblank event to an uninitialized local variable. As per my comment on patch 10,
I'd rather drm_crtc_next_vblank_time() yield the time remaining to the vblank event,
and we can do the arithmetic locally here in this function.
-- 
Regards,
Luben

> +
> +	for_each_new_plane_in_state (state, plane, new_plane_state, i) {
> +		if (!new_plane_state->fence)
> +			continue;
> +		dma_fence_set_deadline(new_plane_state->fence, vbltime);
> +	}
> +}
> +
>  /**
>   * drm_atomic_helper_wait_for_fences - wait for fences stashed in plane state
>   * @dev: DRM device
> @@ -1540,6 +1574,8 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
>  	struct drm_plane_state *new_plane_state;
>  	int i, ret;
>  
> +	set_fence_deadline(dev, state);
> +
>  	for_each_new_plane_in_state(state, plane, new_plane_state, i) {
>  		if (!new_plane_state->fence)
>  			continue;