[PATCH 1/2] drm: Fix dirtyfb stalls

[Rob Clark]

On Mon, May 10, 2021 at 10:44 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Mon, May 10, 2021 at 6:51 PM Rob Clark <robdclark at gmail.com> wrote:
> >
> > On Mon, May 10, 2021 at 9:14 AM Daniel Vetter <daniel at ffwll.ch> wrote:
> > >
> > > On Sat, May 08, 2021 at 12:56:38PM -0700, Rob Clark wrote:
> > > > From: Rob Clark <robdclark at chromium.org>
> > > >
> > > > drm_atomic_helper_dirtyfb() will end up stalling for vblank on "video
> > > > mode" type displays, which is pointless and unnecessary.  Add an
> > > > optional helper vfunc to determine if a plane is attached to a CRTC
> > > > that actually needs dirtyfb, and skip over them.
> > > >
> > > > Signed-off-by: Rob Clark <robdclark at chromium.org>
> > >
> > > So this is a bit annoying because the idea of all these "remap legacy uapi
> > > to atomic constructs" helpers is that they shouldn't need/use anything
> > > beyond what userspace also has available. So adding hacks for them feels
> > > really bad.
> >
> > I suppose the root problem is that userspace doesn't know if dirtyfb
> > (or similar) is actually required or is a no-op.
> >
> > But it is perhaps less of a problem because this essentially boils
> > down to "x11 vs wayland", and it seems like wayland compositors for
> > non-vsync'd rendering just pageflips and throws away extra frames from
> > the app?
>
> Yeah it's about not adequately batching up rendering and syncing with
> hw. bare metal x11 is just especially stupid about it :-)
>
> > > Also I feel like it's not entirely the right thing to do here either.
> > > We've had this problem already on the fbcon emulation side (which also
> > > shouldn't be able to peek behind the atomic kms uapi curtain), and the fix
> > > there was to have a worker which batches up all the updates and avoids any
> > > stalls in bad places.
> >
> > I'm not too worried about fbcon not being able to render faster than
> > vblank.  OTOH it is a pretty big problem for x11
>
> That's why we'd let the worker get ahead at most one dirtyfb. We do
> the same with fbcon, which trivially can get ahead of vblank otherwise
> (if sometimes flushes each character, so you have to pile them up into
> a single update if that's still pending).
>
> > > Since this is for frontbuffer rendering userspace only we can probably get
> > > away with assuming there's only a single fb, so the implementation becomes
> > > pretty simple:
> > >
> > > - 1 worker, and we keep track of a single pending fb
> > > - if there's already a dirty fb pending on a different fb, we stall for
> > >   the worker to start processing that one already (i.e. the fb we track is
> > >   reset to NULL)
> > > - if it's pending on the same fb we just toss away all the updates and go
> > >   with a full update, since merging the clip rects is too much work :-) I
> > >   think there's helpers so you could be slightly more clever and just have
> > >   an overall bounding box
> >
> > This doesn't really fix the problem, you still end up delaying sending
> > the next back-buffer to mesa
>
> With this the dirtyfb would never block. Also glorious frontbuffer
> tracking corruption is possible, but that's not the kernel's problem.
> So how would anything get held up in userspace.

the part about stalling if a dirtyfb is pending was what I was worried
about.. but I suppose you meant the worker stalling, rather than
userspace stalling (where I had interpreted it the other way around).
As soon as userspace needs to stall, you're losing again.

> > But we could re-work drm_framebuffer_funcs::dirty to operate on a
> > per-crtc basis and hoist the loop and check if dirtyfb is needed out
> > of drm_atomic_helper_dirtyfb()
>
> That's still using information that userspace doesn't have, which is a
> bit irky. We might as well go with your thing here then.

arguably, this is something we should expose to userspace.. for DSI
command-mode panels, you probably want to make a different decision
with regard to how many buffers in your flip-chain..

Possibly we should add/remove the fb_damage_clips property depending
on the display type (ie. video/pull vs cmd/push mode)?

BR,
-R

> -Daniel
>
> > BR,
> > -R
> >
> > >
> > > Could probably steal most of the implementation.
> > >
> > > This approach here feels a tad too much in the hacky area ...
> > >
> > > Thoughts?
> > > -Daniel
> > >
> > > > ---
> > > >  drivers/gpu/drm/drm_damage_helper.c      |  8 ++++++++
> > > >  include/drm/drm_modeset_helper_vtables.h | 14 ++++++++++++++
> > > >  2 files changed, 22 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/drm_damage_helper.c b/drivers/gpu/drm/drm_damage_helper.c
> > > > index 3a4126dc2520..a0bed1a2c2dc 100644
> > > > --- a/drivers/gpu/drm/drm_damage_helper.c
> > > > +++ b/drivers/gpu/drm/drm_damage_helper.c
> > > > @@ -211,6 +211,7 @@ int drm_atomic_helper_dirtyfb(struct drm_framebuffer *fb,
> > > >  retry:
> > > >       drm_for_each_plane(plane, fb->dev) {
> > > >               struct drm_plane_state *plane_state;
> > > > +             struct drm_crtc *crtc;
> > > >
> > > >               ret = drm_modeset_lock(&plane->mutex, state->acquire_ctx);
> > > >               if (ret)
> > > > @@ -221,6 +222,13 @@ int drm_atomic_helper_dirtyfb(struct drm_framebuffer *fb,
> > > >                       continue;
> > > >               }
> > > >
> > > > +             crtc = plane->state->crtc;
> > > > +             if (crtc->helper_private->needs_dirtyfb &&
> > > > +                             !crtc->helper_private->needs_dirtyfb(crtc)) {
> > > > +                     drm_modeset_unlock(&plane->mutex);
> > > > +                     continue;
> > > > +             }
> > > > +
> > > >               plane_state = drm_atomic_get_plane_state(state, plane);
> > > >               if (IS_ERR(plane_state)) {
> > > >                       ret = PTR_ERR(plane_state);
> > > > diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> > > > index eb706342861d..afa8ec5754e7 100644
> > > > --- a/include/drm/drm_modeset_helper_vtables.h
> > > > +++ b/include/drm/drm_modeset_helper_vtables.h
> > > > @@ -487,6 +487,20 @@ struct drm_crtc_helper_funcs {
> > > >                                    bool in_vblank_irq, int *vpos, int *hpos,
> > > >                                    ktime_t *stime, ktime_t *etime,
> > > >                                    const struct drm_display_mode *mode);
> > > > +
> > > > +     /**
> > > > +      * @needs_dirtyfb
> > > > +      *
> > > > +      * Optional callback used by damage helpers to determine if fb_damage_clips
> > > > +      * update is needed.
> > > > +      *
> > > > +      * Returns:
> > > > +      *
> > > > +      * True if fb_damage_clips update is needed to handle DIRTYFB, False
> > > > +      * otherwise.  If this callback is not implemented, then True is
> > > > +      * assumed.
> > > > +      */
> > > > +     bool (*needs_dirtyfb)(struct drm_crtc *crtc);
> > > >  };
> > > >
> > > >  /**
> > > > --
> > > > 2.30.2
> > > >
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
>
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch