Re: [RFC 2/3] drm/atomic: Call dma_fence_boost() when we've missed a vblank

[Daniel Vetter <daniel@ffwll.ch>]

On Sun, May 30, 2021 at 07:33:57AM -0700, Rob Clark wrote:
> On Thu, May 20, 2021 at 9:29 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Wed, May 19, 2021 at 11:38:53AM -0700, Rob Clark wrote:
> > > From: Rob Clark <robdclark@chromium.org>
> > >
> > > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > > ---
> > >  drivers/gpu/drm/drm_atomic_helper.c | 11 +++++++++++
> > >  1 file changed, 11 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > > index 560aaecba31b..fe10fc2e7f86 100644
> > > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > > @@ -1435,11 +1435,15 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
> > >       int i, ret;
> > >
> > >       for_each_new_plane_in_state(state, plane, new_plane_state, i) {
> > > +             u64 vblank_count;
> > > +
> > >               if (!new_plane_state->fence)
> > >                       continue;
> > >
> > >               WARN_ON(!new_plane_state->fb);
> > >
> > > +             vblank_count = drm_crtc_vblank_count(new_plane_state->crtc);
> > > +
> > >               /*
> > >                * If waiting for fences pre-swap (ie: nonblock), userspace can
> > >                * still interrupt the operation. Instead of blocking until the
> > > @@ -1449,6 +1453,13 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
> > >               if (ret)
> > >                       return ret;
> > >
> > > +             /*
> > > +              * Check if we've missed a vblank while waiting, and if we have
> > > +              * signal the fence that it's signaler should be boosted
> > > +              */
> > > +             if (vblank_count != drm_crtc_vblank_count(new_plane_state->crtc))
> > > +                     dma_fence_boost(new_plane_state->fence);
> >
> > I think we should do a lot better here:
> > - maybe only bother doing this for single-crtc updates, and only if
> >   modeset isn't set. No one else cares about latency.
> >
> > - We should boost _right_ when we've missed the frame, so I think we
> >   should have a _timeout wait here that guesstimates when the vblank is
> >   over (might need to throw in a vblank wait if we missed) and then boost
> >   immediately. Not wait a bunch of frames (worst case) until we finally
> >   decide to boost.
> 
> I was thinking about this a bit more.. How about rather than calling
> some fence->op->boost() type thing when we are about to miss a vblank
> (IMO that is also already too late), we do something more like
> fence->ops->set_deadline() before we even wait?

Hm yeah that sounds like a clean idea.

Even more, why not add the deadline/waiter information to the callback
we're adding? That way drivers can inspect it whenever they feel like and
don't have to duplicate the tracking. And it's probably easier to
tune/adjust to the myriads of use-cases (flip target miss, userspace wait,
wakeup boost maybe too ...).

I like this direction a lot more than what we discussed with post-miss
hints thus far.

> It's probably a bit impossible for a gpu driver to really predict how
> long some rendering will take, but other cases like video decoder are
> somewhat more predictable.. the fence provider could predict given the
> remaining time until the deadline what clk rates are required to get
> you there.

Well if we do have a deadline the driver can note that in its scheduler
and arm a driver to kick the clocks. Or maybe use past history to do this
upfront.
-Daniel

> 
> BR,
> -R
> 
> 
> >
> > Otherwise I really like this, I think it's about the only real reason i915
> > isn't using atomic helpers.
> >
> > Also adding Matt B for this topic.
> > -Daniel
> >
> > > +
> > >               dma_fence_put(new_plane_state->fence);
> > >               new_plane_state->fence = NULL;
> > >       }
> > > --
> > > 2.30.2
> > >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch