drm/atomic: Add new reverse iterator over all plane state (V2)

Add reverse iterator for_each_oldnew_plane_in_state_reverse to
compliment the for_each_oldnew_plane_in_state way or reading plane
states.

The plane states are required to be read in reverse order for
amd drivers, cause the z order convention followed in linux is
opposite to how the planes are supposed to be presented to DC
engine, which is in common to both windows and linux.

V2: fix compile time errors due to -Werror flag.
Signed-off-by: Shirish S <shirish.s@amd.com>
Signed-off-by: Pratik Vishwakarma <Pratik.Vishwakarma@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1520392203-6885-1-git-send-email-shirish.s@amd.com

include/drm/drm_atomic.h

@@ -753,6 +753,28 @@ void drm_state_dump(struct drm_device *dev, struct drm_printer *p);
 			      (old_plane_state) = (__state)->planes[__i].old_state,\
 			      (new_plane_state) = (__state)->planes[__i].new_state, 1))
 
+/**
+ * for_each_oldnew_plane_in_state_reverse - iterate over all planes in an atomic
+ * update in reverse order
+ * @__state: &struct drm_atomic_state pointer
+ * @plane: &struct drm_plane iteration cursor
+ * @old_plane_state: &struct drm_plane_state iteration cursor for the old state
+ * @new_plane_state: &struct drm_plane_state iteration cursor for the new state
+ * @__i: int iteration cursor, for macro-internal use
+ *
+ * This iterates over all planes in an atomic update in reverse order,
+ * tracking both old and  new state. This is useful in places where the
+ * state delta needs to be considered, for example in atomic check functions.
+ */
+#define for_each_oldnew_plane_in_state_reverse(__state, plane, old_plane_state, new_plane_state, __i) \
+	for ((__i) = ((__state)->dev->mode_config.num_total_plane - 1);	\
+	     (__i) >= 0;						\
+	     (__i)--)							\
+		for_each_if ((__state)->planes[__i].ptr &&		\
+			     ((plane) = (__state)->planes[__i].ptr,	\
+			      (old_plane_state) = (__state)->planes[__i].old_state,\
+			      (new_plane_state) = (__state)->planes[__i].new_state, 1))
+
 /**
  * for_each_old_plane_in_state - iterate over all planes in an atomic update
  * @__state: &struct drm_atomic_state pointer