On Wed, Sep 23, 2020 at 12:31 PM Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> wrote: > > On Tue, Sep 22, 2020 at 08:18:34PM +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. > > > > 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: stable@xxxxxxxxxxxxxxx > > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > > --- > > drivers/gpu/drm/drm_atomic.c | 27 +++++++++++++++++++++++++++ > > 1 file changed, 27 insertions(+) > > > > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c > > index 58527f151984..ef106e7153a6 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 +1322,24 @@ int drm_atomic_check_only(struct drm_atomic_state *state) > > } > > } > > > > + for_each_new_crtc_in_state(state, crtc, old_crtc_state, i) > > Inconsistent old vs. new. Will fix, but also doesn't matter since I don't care about the state, just that it's in there. > > + 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 > > + * this 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) { > > + /* adding other CRTC is only allowed for modeset commits */ > > + WARN_ON(!state->allow_modeset); > > + } > > I think this means pretty much all non-pageflip commits will > have to have allow_modeset==true on i915 or else we just can't > guarantee that we can anything (due to sagv and/or cdclk mainly). I guess not enough machines with multiple outputs in the shards. > Also a bit baffled that CI didn't hit this. I think it should be > totally possible to hit this now. To avoid that I guess we'd just > need to make intel_atomic_serialize_global_state() fail if it > has to add any new crtcs when allow_modeset==false. Hopefully > there aren't many other places that add crtcs to the state > without forcing a modeset on them. Oh we don't do that? That feels like a pretty bad bug ... Wacking random other crtc without allow_modeset is pretty nasty. -Daniel > > > + > > return 0; > > } > > EXPORT_SYMBOL(drm_atomic_check_only); > > -- > > 2.28.0 > > -- > Ville Syrjälä > Intel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
