drm/vc4: Use wait_for_flip_done() instead of wait_for_vblanks()

drm_atomic_helper_wait_for_vblanks() assumes the CRTC will continuously
generate VBLANK events and the vblank counter will keep increasing.
While this work for a regular pipeline, it doesn't when you have the
CRTC is feeding the transposer block, because this block works in
oneshot mode, and, by the time we reach
drm_atomic_helper_wait_for_vblanks() the only VBLANK event might have
already been sent and the VBLANK counter will stay unchanged, thus
triggering a timeout.

Luckily, we can replace the drm_atomic_helper_wait_for_vblanks() call
by drm_atomic_helper_wait_for_flip_done() because the only thing we
want to check when calling drm_atomic_helper_wait_for_vblanks() from
vc4_atomic_complete_commit() is that new FBs are in use and the old
ones can be safely released.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-5-boris.brezillon@bootlin.com

drivers/gpu/drm/vc4/vc4_kms.c

@@ -153,18 +153,9 @@ vc4_atomic_complete_commit(struct drm_atomic_state *state)
 
 	drm_atomic_helper_commit_modeset_enables(dev, state);
 
-	/* Make sure that drm_atomic_helper_wait_for_vblanks()
-	 * actually waits for vblank.  If we're doing a full atomic
-	 * modeset (as opposed to a vc4_update_plane() short circuit),
-	 * then we need to wait for scanout to be done with our
-	 * display lists before we free it and potentially reallocate
-	 * and overwrite the dlist memory with a new modeset.
-	 */
-	state->legacy_cursor_update = false;
-
 	drm_atomic_helper_commit_hw_done(state);
 
-	drm_atomic_helper_wait_for_vblanks(dev, state);
+	drm_atomic_helper_wait_for_flip_done(dev, state);
 
 	drm_atomic_helper_cleanup_planes(dev, state);