[PATCH 28/28] drm/atomic-helper: Reject legacy flips on a disabled pipe

Daniel Vetter
2015-12-04T08:46:47Z

We want this for consistency with existing page_flip semantics.

Since this spurred quite a discussion on IRC also document why we
reject even generation when the pipe is off: It's not that it's hard
to implement, but userspace has a track recording proofing that it's
way too easy to accidentally abuse and cause havoc. We want to make
sure userspace doesn't get away with that.

Cc: Daniel Stone <daniels@collabora.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
drivers/gpu/drm/drm_atomic.c | 9 +++++++++
drivers/gpu/drm/drm_atomic_helper.c | 8 ++++++++
2 files changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 7426d40017a0..06cdb52907da 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -1265,6 +1265,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
if (config->funcs->atomic_check)
ret = config->funcs->atomic_check(state->dev, state);

+ /*
+ * Reject event generation for when a CRTC is off and stays off. It
+ * wouldn't be hard to implement this, but userspace has a track record
+ * of happily burning through 100% cpu (or worse, crash) when the
+ * display pipe is suspended. To avoid all that fun just reject updates
+ * that ask for events since likely that indicates a bug in the
+ * compositors drawing loop. This is consistent with the vblank ioctl
+ * which also rejects service on a disabled pipe.
+ */
if (!state->allow_modeset) {
for_each_crtc_in_state(state, crtc, crtc_state, i) {
if (drm_atomic_crtc_needs_modeset(crtc_state)) {
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 110f3db8dd05..8e281a96c35f 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -2283,6 +2283,14 @@ retry:
goto fail;
drm_atomic_set_fb_for_plane(plane_state, fb);

+ state->allow_modeset = false;
+ if (!crtc_state->active) {
+ DRM_DEBUG_ATOMIC("[CRTC:%d] disabled, rejecting legacy flip\n",
+ crtc->base.id);
+ ret = -EINVAL;
+ goto fail;
+ }
+
ret = drm_atomic_async_commit(state);
if (ret != 0)
goto fail;
--
2.5.1