drm/atomic-helpers: Recover full cursor plane behaviour

Cursor plane updates have historically been fully async and mutliple
updates batched together for the next vsync. And userspace relies upon
that. Since implementing a full queue of async atomic updates is a bit
of work lets just recover the cursor specific behaviour with a hint
flag and some hacks to drop the vblank wait.

v2: Fix kerneldoc, reported by Wu Fengguang.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -909,6 +909,11 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
 		if (!crtc->state->enable)
 			continue;
 
+		/* Legacy cursor ioctls are completely unsynced, and userspace
+		 * relies on that (by doing tons of cursor updates). */
+		if (old_state->legacy_cursor_update)
+			continue;
+
 		if (!framebuffer_changed(dev, old_state, crtc))
 			continue;
 
@@ -1335,6 +1340,9 @@ retry:
 	if (ret != 0)
 		goto fail;
 
+	if (plane == crtc->cursor)
+		state->legacy_cursor_update = true;
+
 	/* Driver takes ownership of state on successful commit. */
 	return 0;
 fail:
@@ -1410,6 +1418,9 @@ retry:
 	plane_state->src_h = 0;
 	plane_state->src_w = 0;
 
+	if (plane == plane->crtc->cursor)
+		state->legacy_cursor_update = true;
+
 	ret = drm_atomic_commit(state);
 	if (ret != 0)
 		goto fail;

include/drm/drm_crtc.h

@@ -909,6 +909,7 @@ struct drm_bridge {
  * struct struct drm_atomic_state - the global state object for atomic updates
  * @dev: parent DRM device
  * @allow_modeset: allow full modeset
+ * @legacy_cursor_update: hint to enforce legacy cursor ioctl semantics
  * @planes: pointer to array of plane pointers
  * @plane_states: pointer to array of plane states pointers
  * @crtcs: pointer to array of CRTC pointers
@@ -921,6 +922,7 @@ struct drm_bridge {
 struct drm_atomic_state {
 	struct drm_device *dev;
 	bool allow_modeset : 1;
+	bool legacy_cursor_update : 1;
 	struct drm_plane **planes;
 	struct drm_plane_state **plane_states;
 	struct drm_crtc **crtcs;