linux-kernel - Re: [PATCH v4 10/14] drm/vblank: Add helper to get next vblank time

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Date:   Wed, 22 Feb 2023 07:48:14 -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>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org
Subject: Re: [PATCH v4 10/14] drm/vblank: Add helper to get next vblank time

On Wed, Feb 22, 2023 at 2:37 AM Luben Tuikov <luben.tuikov@....com> wrote:
>
> On 2023-02-18 16:15, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > Will be used in the next commit to set a deadline on fences that an
> > atomic update is waiting on.
> >
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > ---
> >  drivers/gpu/drm/drm_vblank.c | 32 ++++++++++++++++++++++++++++++++
> >  include/drm/drm_vblank.h     |  1 +
> >  2 files changed, 33 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> > index 2ff31717a3de..caf25ebb34c5 100644
> > --- a/drivers/gpu/drm/drm_vblank.c
> > +++ b/drivers/gpu/drm/drm_vblank.c
> > @@ -980,6 +980,38 @@ u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
> >  }
> >  EXPORT_SYMBOL(drm_crtc_vblank_count_and_time);
> >
> > +/**
> > + * drm_crtc_next_vblank_time - calculate the time of the next vblank
> > + * @crtc: the crtc for which to calculate next vblank time
> > + * @vblanktime: pointer to time to receive the next vblank timestamp.
> > + *
> > + * Calculate the expected time of the next vblank based on time of previous
> > + * vblank and frame duration
> > + */
> > +int drm_crtc_next_vblank_time(struct drm_crtc *crtc, ktime_t *vblanktime)
> > +{
> > +     unsigned int pipe = drm_crtc_index(crtc);
> > +     struct drm_vblank_crtc *vblank = &crtc->dev->vblank[pipe];
> > +     u64 count;
> > +
> > +     if (!vblank->framedur_ns)
> > +             return -EINVAL;
> > +
> > +     count = drm_vblank_count_and_time(crtc->dev, pipe, vblanktime);
> > +
> > +     /*
> > +      * If we don't get a valid count, then we probably also don't
> > +      * have a valid time:
> > +      */
> > +     if (!count)
> > +             return -EINVAL;
> > +
> > +     *vblanktime = ktime_add(*vblanktime, ns_to_ktime(vblank->framedur_ns));
>
> I'd rather this not do any arithmetic, i.e. add, and simply return the calculated
> remaining time, i.e. time left--so instead of add, it would simply assign
> the remaining time, and possibly rename the vblanktime to something like "time_to_vblank."
>

Note that since I sent the last iteration, I've renamed it to
drm_crtc_next_vblank_start().

I would prefer to keep the arithmetic, because I have another use for
this helper in drm/msm (for async/cursor updates, where we want to set
an hrtimer for start of vblank).  It is a bit off the topic of this
series, but I can include the patch when I repost.

BR,
-R

> Changing the top comment to "calculate the time remaining to the next vblank".
> --
> Regards,
> Luben
>
> > +
> > +     return 0;
> > +}
> > +EXPORT_SYMBOL(drm_crtc_next_vblank_time);
> > +
> >  static void send_vblank_event(struct drm_device *dev,
> >               struct drm_pending_vblank_event *e,
> >               u64 seq, ktime_t now)
> > diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
> > index 733a3e2d1d10..a63bc2c92f3c 100644
> > --- a/include/drm/drm_vblank.h
> > +++ b/include/drm/drm_vblank.h
> > @@ -230,6 +230,7 @@ bool drm_dev_has_vblank(const struct drm_device *dev);
> >  u64 drm_crtc_vblank_count(struct drm_crtc *crtc);
> >  u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
> >                                  ktime_t *vblanktime);
> > +int drm_crtc_next_vblank_time(struct drm_crtc *crtc, ktime_t *vblanktime);
> >  void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
> >                              struct drm_pending_vblank_event *e);
> >  void drm_crtc_arm_vblank_event(struct drm_crtc *crtc,
>