[PATCH] drm: avoid spurious EBUSY due to nonblocking atomic modesets

[Daniel Vetter]

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. As a stop-gap
plug this problem by demoting nonblocking commits which might cause
issues by including CRTCs not in the original request to blocking
commits.

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).

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: stable at vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/drm_atomic.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index d5cefb1cb2a2..3c46a4ef7898 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -2018,15 +2018,43 @@ EXPORT_SYMBOL(drm_atomic_commit);
 int drm_atomic_nonblocking_commit(struct drm_atomic_state *state)
 {
 	struct drm_mode_config *config = &state->dev->mode_config;
-	int ret;
+	unsigned requested_crtc = 0;
+	unsigned affected_crtc = 0;
+	struct drm_crtc *crtc;
+	struct drm_crtc_state *crtc_state;
+	bool nonblocking = true;
+	int ret, i;
+
+	/*
+	 * For commits that allow modesets drivers can add other CRTCs to the
+	 * atomic commit, e.g. when they need to reallocate global resources.
+	 *
+	 * But when userspace also requests a nonblocking commit then userspace
+	 * cannot know that the commit affects other CRTCs, which can result in
+	 * spurious EBUSY failures. Until we have better uapi plug this by
+	 * demoting such commits to blocking mode.
+	 */
+	for_each_new_crtc_in_state(state, crtc, crtc_state, i)
+		requested_crtc |= drm_crtc_mask(crtc);
 
 	ret = drm_atomic_check_only(state);
 	if (ret)
 		return ret;
 
-	DRM_DEBUG_ATOMIC("committing %p nonblocking\n", state);
+	for_each_new_crtc_in_state(state, crtc, crtc_state, i)
+		affected_crtc |= drm_crtc_mask(crtc);
+
+	if (affected_crtc != requested_crtc) {
+		/* adding other CRTC is only allowed for modeset commits */
+		WARN_ON(!state->allow_modeset);
+
+		DRM_DEBUG_ATOMIC("demoting %p to blocking mode to avoid EBUSY\n", state);
+		nonblocking = false;
+	} else {
+		DRM_DEBUG_ATOMIC("committing %p nonblocking\n", state);
+	}
 
-	return config->funcs->atomic_commit(state->dev, state, true);
+	return config->funcs->atomic_commit(state->dev, state, nonblocking);
 }
 EXPORT_SYMBOL(drm_atomic_nonblocking_commit);
 
-- 
2.18.0