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From:   Rob Clark <robdclark@gmail.com>
Date:   Tue, 21 Feb 2023 14:28:10 -0800
Message-ID: <CAF6AEGtkgDNZTr-NS_rUuEDxPy5aoMycPDn2RXFEXfAiA7=E9A@mail.gmail.com>
Subject: Re: [Freedreno] [PATCH v4 10/14] drm/vblank: Add helper to get next
 vblank time
To:     =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>
Cc:     Rob Clark <robdclark@chromium.org>,
        Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <ckoenig.leichtzumerken@gmail.com>,
        =?UTF-8?Q?Michel_D=C3=A4nzer?= <michel@daenzer.net>,
        open list <linux-kernel@vger.kernel.org>,
        dri-devel@lists.freedesktop.org,
        Pekka Paalanen <ppaalanen@gmail.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        freedreno@lists.freedesktop.org
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On Tue, Feb 21, 2023 at 1:48 PM Ville Syrj=C3=A4l=C3=A4
<ville.syrjala@linux.intel.com> wrote:
>
> On Tue, Feb 21, 2023 at 11:39:40PM +0200, Ville Syrj=C3=A4l=C3=A4 wrote:
> > On Tue, Feb 21, 2023 at 11:54:55AM -0800, Rob Clark wrote:
> > > On Tue, Feb 21, 2023 at 5:01 AM Ville Syrj=C3=A4l=C3=A4
> > > <ville.syrjala@linux.intel.com> 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@gmail.com> wrote:
> > > > >
> > > > > > On Mon, Feb 20, 2023 at 1:08 AM Pekka Paalanen <ppaalanen@gmail=
.com> wrote:
> > > > > > >
> > > > > > > On Sat, 18 Feb 2023 13:15:53 -0800
> > > > > > > Rob Clark <robdclark@gmail.com> wrote:
> > > > > > >
> > > > > > > > From: Rob Clark <robdclark@chromium.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@chromium.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(str=
uct drm_crtc *crtc,
> > > > > > > >  }
> > > > > > > >  EXPORT_SYMBOL(drm_crtc_vblank_count_and_time);
> > > > > > > >
> > > > > > > > +/**
> > > > > > > > + * drm_crtc_next_vblank_time - calculate the time of the n=
ext 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 maxi=
mum?
> > > > >
> > > > > 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 some=
thing like:
> > >
> > >   vsync_lines =3D vblank->hwmode.vsync_end - vblank->hwmode.vsync_sta=
rt;
> > >   vsyncdur =3D ns_to_ktime(vblank->linedur_ns * vsync_lines);
> > >   framedur =3D ns_to_ktime(vblank->framedur_ns);
> > >   *vblanktime =3D ktime_add(*vblanktime, ktime_sub(framedur, vsyncdur=
));
> >
> > Something like this should work:
> >  vblank_start =3D framedur_ns * crtc_vblank_start / crtc_vtotal
> >  deadline =3D 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.

IIRC the only time we wouldn't have a fresh sample is if the screen
has been idle for some time?  In which case, I think that doesn't
matter.

BR,
-R

>
> --
> Ville Syrj=C3=A4l=C3=A4
> Intel