drm/display/dp_mst: Add drm_atomic_get_old_mst_topology_state()

Add a function to get the old MST topology state, required by a
follow-up i915 patch.

While at it clarify the code comment of
drm_atomic_get_new_mst_topology_state() and add _new prefix
to the new state pointer to remind about its difference from the old
state.

v2: Use old_/new_ prefixes for the state pointers. (Ville)

Cc: Lyude Paul <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Cc: [e-mail] # 6.1
Cc: [e-mail]
Reviewed-by: Ville Syrjälä <[e-mail]>
Reviewed-by: Lyude Paul <[e-mail]>
Acked-by: Lyude Paul <[e-mail]>
Acked-by: Daniel Vetter <[e-mail]>
Acked-by: Wayne Lin <[e-mail]>
Acked-by: Jani Nikula <[e-mail]>
Signed-off-by: Imre Deak <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/display/drm_dp_mst_topology.c

···
 EXPORT_SYMBOL(drm_atomic_get_mst_topology_state);
 
 /**
- * drm_atomic_get_new_mst_topology_state: get new MST topology state in atomic state, if any
+ * drm_atomic_get_old_mst_topology_state: get old MST topology state in atomic state, if any
  * @state: global atomic state
  * @mgr: MST topology manager, also the private object in this case
  *
- * This function wraps drm_atomic_get_priv_obj_state() passing in the MST atomic
+ * This function wraps drm_atomic_get_old_private_obj_state() passing in the MST atomic
  * state vtable so that the private object state returned is that of a MST
  * topology object.
  *
  * Returns:
  *
- * The MST topology state, or NULL if there's no topology state for this MST mgr
+ * The old MST topology state, or NULL if there's no topology state for this MST mgr
+ * in the global atomic state
+ */
+struct drm_dp_mst_topology_state *
+drm_atomic_get_old_mst_topology_state(struct drm_atomic_state *state,
+				      struct drm_dp_mst_topology_mgr *mgr)
+{
+	struct drm_private_state *old_priv_state =
+		drm_atomic_get_old_private_obj_state(state, &mgr->base);
+
+	return old_priv_state ? to_dp_mst_topology_state(old_priv_state) : NULL;
+}
+EXPORT_SYMBOL(drm_atomic_get_old_mst_topology_state);
+
+/**
+ * drm_atomic_get_new_mst_topology_state: get new MST topology state in atomic state, if any
+ * @state: global atomic state
+ * @mgr: MST topology manager, also the private object in this case
+ *
+ * This function wraps drm_atomic_get_new_private_obj_state() passing in the MST atomic
+ * state vtable so that the private object state returned is that of a MST
+ * topology object.
+ *
+ * Returns:
+ *
+ * The new MST topology state, or NULL if there's no topology state for this MST mgr
  * in the global atomic state
  */
 struct drm_dp_mst_topology_state *
 drm_atomic_get_new_mst_topology_state(struct drm_atomic_state *state,
 				      struct drm_dp_mst_topology_mgr *mgr)
 {
-	struct drm_private_state *priv_state =
+	struct drm_private_state *new_priv_state =
 		drm_atomic_get_new_private_obj_state(state, &mgr->base);
 
-	return priv_state ? to_dp_mst_topology_state(priv_state) : NULL;
+	return new_priv_state ? to_dp_mst_topology_state(new_priv_state) : NULL;
 }
 EXPORT_SYMBOL(drm_atomic_get_new_mst_topology_state);
 

include/drm/display/drm_dp_mst_helper.h

···
 drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
 				  struct drm_dp_mst_topology_mgr *mgr);
 struct drm_dp_mst_topology_state *
+drm_atomic_get_old_mst_topology_state(struct drm_atomic_state *state,
+				      struct drm_dp_mst_topology_mgr *mgr);
+struct drm_dp_mst_topology_state *
 drm_atomic_get_new_mst_topology_state(struct drm_atomic_state *state,
 				      struct drm_dp_mst_topology_mgr *mgr);
 struct drm_dp_mst_atomic_payload *