[Intel-gfx] [PATCH] drm/atomic: document and enforce rules around "spurious" EBUSY
Daniel Vetter
daniel.vetter at ffwll.ch
Wed Sep 23 20:01:25 UTC 2020
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).
> >
> > But in nonblocking mode userspace has then no idea this happened,
> > which can lead to spurious EBUSY calls, both:
> > - when that other CRTC is currently busy doing a page_flip the
> > ALLOW_MODESET commit can fail with an EBUSY
> > - on the other CRTC a normal atomic flip can fail with EBUSY because
> > of the additional commit inserted by the kernel without userspace's
> > knowledge
> >
> > For blocking commits this isn't a problem, because everyone else will
> > just block until all the CRTC are reconfigured. Only thing userspace
> > can notice is the dropped frames without any reason for why frames got
> > dropped.
> >
> > Consensus is that we need new uapi to handle this properly, but no one
> > has any idea what exactly the new uapi should look like. Since this
> > has been shipping for years already compositors need to deal no matter
> > what, so as a first step just try to enforce this across drivers
> > better with some checks.
> >
> > v2: Add comments and a WARN_ON to enforce this only when allowed - we
> > don't want to silently convert page flips into blocking plane updates
> > just because the driver is buggy.
> >
> > v3: Fix inverted WARN_ON (Pekka).
> >
> > v4: Drop the uapi changes, only add a WARN_ON for now to enforce some
> > rules for drivers.
> >
> > v5: Make the WARNING more informative (Daniel)
> >
> > v6: Add unconditional debug output for compositor hackers to figure
> > out what's going on when they get an EBUSY (Daniel)
> >
> > References: https://lists.freedesktop.org/archives/dri-devel/2018-July/182281.html
> > Bugzilla: https://gitlab.freedesktop.org/wayland/weston/issues/24#note_9568
> > Cc: Daniel Stone <daniel at fooishbar.org>
> > Cc: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> > Cc: Simon Ser <contact at emersion.fr>
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> > drivers/gpu/drm/drm_atomic.c | 29 +++++++++++++++++++++++++++++
> > 1 file changed, 29 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> > index 58527f151984..f1a912e80846 100644
> > --- a/drivers/gpu/drm/drm_atomic.c
> > +++ b/drivers/gpu/drm/drm_atomic.c
> > @@ -281,6 +281,10 @@ EXPORT_SYMBOL(__drm_atomic_state_free);
> > * needed. It will also grab the relevant CRTC lock to make sure that the state
> > * is consistent.
> > *
> > + * WARNING: Drivers may only add new CRTC states to a @state if
> > + * drm_atomic_state.allow_modeset is set, or if it's a driver-internal commit
> > + * not created by userspace through an IOCTL call.
> > + *
> > * Returns:
> > *
> > * Either the allocated state or the error code encoded into the pointer. When
> > @@ -1262,10 +1266,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> > struct drm_crtc_state *new_crtc_state;
> > struct drm_connector *conn;
> > struct drm_connector_state *conn_state;
> > + unsigned requested_crtc = 0;
> > + unsigned affected_crtc = 0;
> > int i, ret = 0;
> >
> > DRM_DEBUG_ATOMIC("checking %p\n", state);
> >
> > + for_each_new_crtc_in_state(state, crtc, old_crtc_state, i)
> > + requested_crtc |= drm_crtc_mask(crtc);
> > +
> > for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
> > ret = drm_atomic_plane_check(old_plane_state, new_plane_state);
> > if (ret) {
> > @@ -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. 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);
> > --
> > 2.28.0
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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