[Intel-gfx] [PATCH 4/5] drm/atomic-helper: Reject legacy flips on a disabled pipe

[Daniel Vetter]

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.

v2: Somehow thought we do reject events already, but that code only
existed in my imagination ... Also suggestions from Thierry.

Cc: Daniel Stone <daniels at collabora.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Thierry Reding <thierry.reding at gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_atomic.c        | 16 ++++++++++++++++
 drivers/gpu/drm/drm_atomic_helper.c |  9 +++++++++
 include/drm/drm_crtc.h              |  3 ++-
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 07ab75e22b2b..029377513b1d 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -508,6 +508,22 @@ static int drm_atomic_crtc_check(struct drm_crtc *crtc,
 		return -EINVAL;
 	}
 
+	/*
+	 * 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
+	 * compositor's drawing loop. This is consistent with the vblank IOCTL
+	 * and legacy page_flip IOCTL which also reject service on a disabled
+	 * pipe.
+	 */
+	if (state->event && !state->active && !crtc->state->active) {
+		DRM_DEBUG_ATOMIC("[CRTC:%d] requesting event but off\n",
+				 crtc->base.id);
+		return -EINVAL;
+	}
+
 	return 0;
 }
 
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 76c124b2a775..9eb367cad790 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -2282,6 +2282,15 @@ retry:
 		goto fail;
 	drm_atomic_set_fb_for_plane(plane_state, fb);
 
+	/* Make sure we don't accidentally do a full modeset. */
+	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;
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 6fe14a773def..6da847a6cb2f 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -548,7 +548,8 @@ struct drm_crtc_funcs {
 	 * ->page_flip() operation is already pending the callback should return
 	 * -EBUSY. Pageflips on a disabled CRTC (either by setting a NULL mode
 	 * or just runtime disabled through DPMS respectively the new atomic
-	 * "ACTIVE" state) should result in an -EINVAL error code.
+	 * "ACTIVE" state) should result in an -EINVAL error code. Note that
+	 * drm_atomic_helper_page_flip() checks this already for atomic drivers.
 	 */
 	int (*page_flip)(struct drm_crtc *crtc,
 			 struct drm_framebuffer *fb,
-- 
2.5.1