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

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Message-ID: <CAF6AEGuK0_GtgXS7REAN=u4YZ7x11FrAxVW4iQcqV7bJdJFv6g@mail.gmail.com>
Date:   Wed, 22 Feb 2023 07:44:42 -0800
From:   Rob Clark <robdclark@...il.com>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        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>,
        dri-devel@...ts.freedesktop.org,
        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 1:57 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
>
> On Tue, 21 Feb 2023 09:50:20 -0800
> Rob Clark <robdclark@...il.com> 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.
> >
> > 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.
>
> The vblank timestamp is defined to be the time of the first active
> pixel of the frame in the video signal. At least that's the one that
> UAPI carries (when not tearing?). It is not the start of vblank period.
>
> With VRR, the front porch before the first active pixel can be multiple
> milliseconds. The difference between 144 Hz and 60 Hz is 9.7 ms for
> example.

What we really want is the deadline for the hw to latch for the next
frame.. which as Ville pointed out is definitely before the end of
vblank.

Honestly this sort of feature is a lot more critical for the non-VRR
case, and VRR is kind of a minority edge case.  So I'd prefer not to
get too hung up on VRR.  If there is an easy way for the helpers to
detect VRR, I'd be perfectly fine not setting a deadline hint in that
case, and let someone who actually has a VRR display figure out how to
handle that case.

BR,
-R