drm/atomic: Add __drm_atomic_helper_plane_reset

There are a lot of drivers that subclass drm_plane_state, all of them
duplicate the code that links together the plane with plane_state.

On top of that, drivers that enable core properties also have to
duplicate the code for initializing the properties to their default
values, which in all cases are the same as the defaults from core.

Change since v1:
- Make it consistent with the other helpers and require that both
  plane and state not be NULL, suggested by Boris Brezillon and
  Philipp Zabel.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-2-alexandru-cosmin.gheorghe@arm.com

drivers/gpu/drm/drm_atomic_helper.c

@@ -3552,6 +3552,28 @@ void drm_atomic_helper_crtc_destroy_state(struct drm_crtc *crtc,
 }
 EXPORT_SYMBOL(drm_atomic_helper_crtc_destroy_state);
 
+/**
+ * __drm_atomic_helper_plane_reset - resets planes state to default values
+ * @plane: plane object, must not be NULL
+ * @state: atomic plane state, must not be NULL
+ *
+ * Initializes plane state to default. This is useful for drivers that subclass
+ * the plane state.
+ */
+void __drm_atomic_helper_plane_reset(struct drm_plane *plane,
+				     struct drm_plane_state *state)
+{
+	state->plane = plane;
+	state->rotation = DRM_MODE_ROTATE_0;
+
+	/* Reset the alpha value to fully opaque if it matters */
+	if (plane->alpha_property)
+		state->alpha = plane->alpha_property->values[1];
+
+	plane->state = state;
+}
+EXPORT_SYMBOL(__drm_atomic_helper_plane_reset);
+
 /**
  * drm_atomic_helper_plane_reset - default &drm_plane_funcs.reset hook for planes
  * @plane: drm plane
@@ -3566,15 +3588,8 @@ void drm_atomic_helper_plane_reset(struct drm_plane *plane)
 
 	kfree(plane->state);
 	plane->state = kzalloc(sizeof(*plane->state), GFP_KERNEL);
-
-	if (plane->state) {
-		plane->state->plane = plane;
-		plane->state->rotation = DRM_MODE_ROTATE_0;
-
-		/* Reset the alpha value to fully opaque if it matters */
-		if (plane->alpha_property)
-			plane->state->alpha = plane->alpha_property->values[1];
-	}
+	if (plane->state)
+		__drm_atomic_helper_plane_reset(plane, plane->state);
 }
 EXPORT_SYMBOL(drm_atomic_helper_plane_reset);
 

include/drm/drm_atomic_helper.h

@@ -156,6 +156,8 @@ void __drm_atomic_helper_crtc_destroy_state(struct drm_crtc_state *state);
 void drm_atomic_helper_crtc_destroy_state(struct drm_crtc *crtc,
 					  struct drm_crtc_state *state);
 
+void __drm_atomic_helper_plane_reset(struct drm_plane *plane,
+				     struct drm_plane_state *state);
 void drm_atomic_helper_plane_reset(struct drm_plane *plane);
 void __drm_atomic_helper_plane_duplicate_state(struct drm_plane *plane,
 					       struct drm_plane_state *state);