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@xxxxxxxxxxxxx> > Cc: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxxxx> > Cc: Simon Ser <contact@xxxxxxxxxxx> > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > --- > 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? 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? > + } > + > return 0; > } > EXPORT_SYMBOL(drm_atomic_check_only); > -- > 2.28.0 > > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > https://lists.freedesktop.org/mailman/listinfo/dri-devel
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