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

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Date:   Wed, 22 Feb 2023 07:50:04 -0800
From:   Rob Clark <robdclark@...il.com>
To:     Luben Tuikov <luben.tuikov@....com>
Cc:     dri-devel@...ts.freedesktop.org,
        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 Wed, Feb 22, 2023 at 2:46 AM Luben Tuikov <luben.tuikov@....com> wrote:
>
> 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.

if drm_crtc_next_vblank_time() returns 0 then it has initialized
vbltime, so no problem here

BR,
-R

> --
> 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;
>