[PATCH] drm/atomic: document and enforce rules around "spurious" EBUSY

[Daniel Vetter]

On Thu, Sep 24, 2020 at 9:41 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
>
> On Wed, 23 Sep 2020 22:01:25 +0200
> Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> > On Wed, Sep 23, 2020 at 9:17 PM Marius Vlad <marius.vlad at collabora.com> wrote:
> > >
> > > On Wed, Sep 23, 2020 at 05:18:52PM +0200, Daniel Vetter wrote:
> > > > When doing an atomic modeset with ALLOW_MODESET drivers are allowed to
> > > > pull in arbitrary other resources, including CRTCs (e.g. when
> > > > reconfiguring global resources).
>
> ...
>
> > > > @@ -1313,6 +1322,26 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> > > >               }
> > > >       }
> > > >
> > > > +     for_each_new_crtc_in_state(state, crtc, old_crtc_state, i)
> > > > +             affected_crtc |= drm_crtc_mask(crtc);
> > > > +
> > > > +     /*
> > > > +      * For commits that allow modesets drivers can add other CRTCs to the
> > > > +      * atomic commit, e.g. when they need to reallocate global resources.
> > > > +      * This can cause spurious EBUSY, which robs compositors of a very
> > > > +      * effective sanity check for their drawing loop. Therefor only allow
> > > > +      * drivers to add unrelated CRTC states for modeset commits.
> > > > +      *
> > > > +      * FIXME: Should add affected_crtc mask to the ATOMIC IOCTL as an output
> > > > +      * so compositors know what's going on.
> > > > +      */
> > > > +     if (affected_crtc != requested_crtc) {
> > > > +             DRM_DEBUG_ATOMIC("driver added CRTC to commit: requested 0x%x, affected 0x%0x\n",
> > > > +                              requested_crtc, affected_crtc);
> > > > +             WARN(!state->allow_modeset, "adding CRTC not allowed without modesets: requested 0x%x, affected 0x%0x\n",
> > > > +                  requested_crtc, affected_crtc);
> > > Previous patch had the warn on state->allow_modeset now is
> > > !state->allow_modeset. Is that correct?
> >
> > We need to fire a warning when allow_modeset is _not_ set. An earlier
> > version got that wrong, and yes that would have caused a _ton_ of
> > warnings on any fairly new intel platform.
> >
> > > I haven't followed the entire thread on this matter, but I guess the idea
> > > is that somehow the kernel would pass to userspace a CRTC mask of
> > > affected_crtc (somehow, we don't know how atm) and with it, userspace
> > > can then issue a new commit (this commit blocking) with those?
> >
> > Either that, or just use that to track all the in-flight drm events.
> > Userspace will get events for all the crtc, not just the one it asked
> > to update.
>
> Wait, does that happen already? Getting CRTC events for CRTCs userspace
> didn't include in the atomic commit?

Yeah I'm pretty sure. With the affected_crtc mask you could update
your internal book-keeping to catch these, which should also prevent
all the spurious EBUSY. But I'm not entirely sure, I just read the
code, haven't tested.

> That could explain why Weston freaks out in
> https://gitlab.freedesktop.org/wayland/weston/-/issues/435

Hm it's strange that you first get an EBUSY, and only on the next
modeset get the spurious event. You should get one already on the
first modeset.
-Daniel

>
>
> Thanks,
> pq
>
>
> > If that's easier to do by re-issuing the commit with the
> > full set of crtc, then I guess that's an option. But not required (I
> > think at least, would need to test that to make sure).
> > -Daniel
> >
> > > > +     }
> > > > +
> > > >       return 0;
> > > >  }
> > > >  EXPORT_SYMBOL(drm_atomic_check_only);



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