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

[Ville Syrjälä]

On Thu, Sep 24, 2020 at 01:10:56PM +0300, Pekka Paalanen wrote:
> On Thu, 24 Sep 2020 10:04:12 +0200
> Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> 
> > 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.
> 
> If that actually happens, how does userspace know whether the
> userdata argument with the event is valid or not?

At some point I was worried about the kernel potentially sending spurious
events, but IIRC I managed to convince myself that it shouldn't happen.
I think I came to the conclusion the events were populated before the
core calls into the driver. But maybe I misanalyzed it, or something
has since broken?

-- 
Ville Syrjälä
Intel