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

[Ville Syrjälä]

On Tue, Feb 21, 2023 at 11:39:40PM +0200, Ville Syrjälä wrote:
> On Tue, Feb 21, 2023 at 11:54:55AM -0800, Rob Clark wrote:
> > 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.
> > >
> > > Also this does not calculate the the start of vblank, it
> > > calculates the start of active video.
> > 
> > AFAIU, vsync_end/vsync_start are in units of line, so I could do something like:
> > 
> >   vsync_lines = vblank->hwmode.vsync_end - vblank->hwmode.vsync_start;
> >   vsyncdur = ns_to_ktime(vblank->linedur_ns * vsync_lines);
> >   framedur = ns_to_ktime(vblank->framedur_ns);
> >   *vblanktime = ktime_add(*vblanktime, ktime_sub(framedur, vsyncdur));
> 
> Something like this should work:
>  vblank_start = framedur_ns * crtc_vblank_start / crtc_vtotal
>  deadline = vblanktime + vblank_start
> 
> That would be the expected time when the next start of vblank
> happens.

Except that drm_vblank_count_and_time() will give you the last
sampled timestamp, which may be long ago in the past. Would
need to add an _accurate version of that if we want to be
guaranteed a fresh sample.

-- 
Ville Syrjälä
Intel