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

[Rob Clark]

On Tue, Feb 21, 2023 at 5:01 AM Ville Syrjälä
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 21, 2023 at 10:45:51AM +0200, Pekka Paalanen wrote:
> > On Mon, 20 Feb 2023 07:55:41 -0800
> > Rob Clark <robdclark@xxxxxxxxx> wrote:
> >
> > > On Mon, Feb 20, 2023 at 1:08 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > > >
> > > > On Sat, 18 Feb 2023 13:15:53 -0800
> > > > Rob Clark <robdclark@xxxxxxxxx> wrote:
> > > >
> > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > >
> > > > > 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@xxxxxxxxxxxx>
> > > > > ---
> > > > >  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
> > > >
> > > > Hi,
> > > >
> > > > for VRR this targets the highest frame rate possible for the current
> > > > VRR mode, right?
> > > >
> > >
> > > It is based on vblank->framedur_ns which is in turn based on
> > > mode->crtc_clock.  Presumably for VRR that ends up being a maximum?
> >
> > I don't know. :-)
>
> At least for i915 this will give you the maximum frame
> duration.

I suppose one could argue that maximum frame duration is the actual
deadline.  Anything less is just moar fps, but not going to involve
stalling until vblank N+1, AFAIU

> Also this does not calculate the the start of vblank, it
> calculates the start of active video.

Probably something like end of previous frame's video..  might not be
_exactly_ correct (because some buffering involved), but OTOH on the
GPU side, I expect the driver to set a timer for a few ms or so before
the deadline.  So there is some wiggle room.

BR,
-R

> >
> > You need a number of clock cycles in addition to the clock frequency,
> > and that could still be minimum, maximum, the last realized one, ...
> >
> > VRR works by adjusting the front porch length IIRC.
> >
> >
> > Thanks,
> > pq
> >
> > > BR,
> > > -R
> > >
> > >
> > > >
> > > > Thanks,
> > > > pq
> > > >
> > > > > + */
> > > > > +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));
> > > > > +
> > > > > +     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,
> > > >
> >
>
>
>
> --
> Ville Syrjälä
> Intel