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

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)

v7: Fix up old/new_crtc_state confusion for real (Pekka/Ville)
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
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@fooishbar.org>
Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Simon Ser <contact@emersion.fr>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925084651.3250104-1-daniel.vetter@ffwll.ch

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, new_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, new_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);
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL(drm_atomic_check_only);