drm: don't block fb changes for async plane updates

From Helen Koike
fbb7e114e6e690c46f170dedd6fd2fb22f241519 in linux 4.19.y/4.19.50
89a4aac0ab0e6f5eea10d7bf4869dd15c3de2cd4 in mainline linux

1 files changed, 8 insertions, 0 deletions

diff --git a/sys/dev/pci/drm/include/drm/drm_modeset_helper_vtables.h b/sys/dev/pci/drm/include/drm/drm_modeset_helper_vtables.h
index 61142aa0ab2..0eb3372d031 100644
--- a/sys/dev/pci/drm/include/drm/drm_modeset_helper_vtables.h
+++ b/sys/dev/pci/drm/include/drm/drm_modeset_helper_vtables.h
@@ -1174,6 +1174,14 @@ struct drm_plane_helper_funcs {
 	 * current one with the new plane configurations in the new
 	 * plane_state.
 	 *
+	 * Drivers should also swap the framebuffers between current plane
+	 * state (&drm_plane.state) and new_state.
+	 * This is required since cleanup for async commits is performed on
+	 * the new state, rather than old state like for traditional commits.
+	 * Since we want to give up the reference on the current (old) fb
+	 * instead of our brand new one, swap them in the driver during the
+	 * async commit.
+	 *
 	 * FIXME:
 	 *  - It only works for single plane updates
 	 *  - Async Pageflips are not supported yet