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

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Message-ID: <CAF6AEGua7JfqDJE-pYCDspDAKErCf1cXiUQBfp2RHhANaB0yKA@mail.gmail.com>
Date:   Tue, 21 Feb 2023 15:20:53 -0800
From:   Rob Clark <robdclark@...il.com>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     Rob Clark <robdclark@...omium.org>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org,
        Pekka Paalanen <ppaalanen@...il.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org
Subject: Re: [Freedreno] [PATCH v4 10/14] drm/vblank: Add helper to get next
 vblank time

On Tue, Feb 21, 2023 at 2:46 PM Ville Syrjälä
<ville.syrjala@...ux.intel.com> wrote:
>
> On Tue, Feb 21, 2023 at 02:28:10PM -0800, Rob Clark wrote:
> > On Tue, Feb 21, 2023 at 1:48 PM Ville Syrjälä
> > <ville.syrjala@...ux.intel.com> wrote:
> > >
> > > 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@...ux.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@...il.com> wrote:
> > > > > > >
> > > > > > > > On Mon, Feb 20, 2023 at 1:08 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
> > > > > > > > >
> > > > > > > > > On Sat, 18 Feb 2023 13:15:53 -0800
> > > > > > > > > Rob Clark <robdclark@...il.com> 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
> > > > > > > > >
> > > > > > > > > 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.
> >
> > IIRC the only time we wouldn't have a fresh sample is if the screen
> > has been idle for some time?
>
> IIRC "some time" == 1 idle frame, for any driver that sets
> vblank_disable_immediate.
>

hmm, ok so it should be still good down to 30fps ;-)

I thought we calculated based on frame # and line # on hw that
supported that?  But it's been a while since looking at vblank code

BR,
-R

> > In which case, I think that doesn't
> > matter.
> >
> > BR,
> > -R
> >
> > >
> > > --
> > > Ville Syrjälä
> > > Intel
>
> --
> Ville Syrjälä
> Intel