Revert "drm/atomic: Remove drm_atomic_helper_framebuffer_changed."

This reverts commit f2b0fee.

Author: ehristev

drivers/gpu/drm/drm_atomic_helper.c

@@ -1460,7 +1460,42 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
 EXPORT_SYMBOL(drm_atomic_helper_wait_for_fences);
 
 /**
- * drm_atomic_helper_wait_for_vblanks - wait for vblank on CRTCs
+ * drm_atomic_helper_framebuffer_changed - check if framebuffer has changed
+ * @dev: DRM device
+ * @old_state: atomic state object with old state structures
+ * @crtc: DRM crtc
+ *
+ * Checks whether the framebuffer used for this CRTC changes as a result of
+ * the atomic update.  This is useful for drivers which cannot use
+ * drm_atomic_helper_wait_for_vblanks() and need to reimplement its
+ * functionality.
+ *
+ * Returns:
+ * true if the framebuffer changed.
+ */
+bool drm_atomic_helper_framebuffer_changed(struct drm_device *dev,
+					   struct drm_atomic_state *old_state,
+					   struct drm_crtc *crtc)
+{
+	struct drm_plane *plane;
+	struct drm_plane_state *old_plane_state;
+	int i;
+
+	for_each_plane_in_state(old_state, plane, old_plane_state, i) {
+		if (plane->state->crtc != crtc &&
+		    old_plane_state->crtc != crtc)
+			continue;
+
+		if (plane->state->fb != old_plane_state->fb)
+			return true;
+	}
+
+	return false;
+}
+EXPORT_SYMBOL(drm_atomic_helper_framebuffer_changed);
+
+/**
+ * drm_atomic_helper_wait_for_vblanks - wait for vblank on crtcs
  * @dev: DRM device
  * @old_state: atomic state object with old state structures
  *

include/drm/drm_atomic_helper.h

@@ -63,6 +63,9 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
 int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
 					struct drm_atomic_state *state,
 					bool pre_swap);
+bool drm_atomic_helper_framebuffer_changed(struct drm_device *dev,
+					   struct drm_atomic_state *old_state,
+					   struct drm_crtc *crtc);
 
 void drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
 					struct drm_atomic_state *old_state);