drm/dp_mst: Add helper to determine if an MST port is downstream of another port

Add drm_dp_mst_port_downstream_of_parent() required by the i915
driver in a follow-up patch to resolve a BW overallocation of MST
streams going through a given MST port.

Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-4-imre.deak@intel.com

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -5126,6 +5126,58 @@ static bool drm_dp_mst_port_downstream_of_branch(struct drm_dp_mst_port *port,
 	return false;
 }
 
+static bool
+drm_dp_mst_port_downstream_of_parent_locked(struct drm_dp_mst_topology_mgr *mgr,
+					    struct drm_dp_mst_port *port,
+					    struct drm_dp_mst_port *parent)
+{
+	if (!mgr->mst_primary)
+		return false;
+
+	port = drm_dp_mst_topology_get_port_validated_locked(mgr->mst_primary,
+							     port);
+	if (!port)
+		return false;
+
+	if (!parent)
+		return true;
+
+	parent = drm_dp_mst_topology_get_port_validated_locked(mgr->mst_primary,
+							       parent);
+	if (!parent)
+		return false;
+
+	if (!parent->mstb)
+		return false;
+
+	return drm_dp_mst_port_downstream_of_branch(port, parent->mstb);
+}
+
+/**
+ * drm_dp_mst_port_downstream_of_parent - check if a port is downstream of a parent port
+ * @mgr: MST topology manager
+ * @port: the port being looked up
+ * @parent: the parent port
+ *
+ * The function returns %true if @port is downstream of @parent. If @parent is
+ * %NULL - denoting the root port - the function returns %true if @port is in
+ * @mgr's topology.
+ */
+bool
+drm_dp_mst_port_downstream_of_parent(struct drm_dp_mst_topology_mgr *mgr,
+				     struct drm_dp_mst_port *port,
+				     struct drm_dp_mst_port *parent)
+{
+	bool ret;
+
+	mutex_lock(&mgr->lock);
+	ret = drm_dp_mst_port_downstream_of_parent_locked(mgr, port, parent);
+	mutex_unlock(&mgr->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL(drm_dp_mst_port_downstream_of_parent);
+
 static int
 drm_dp_mst_atomic_check_port_bw_limit(struct drm_dp_mst_port *port,
 				      struct drm_dp_mst_topology_state *state);

include/drm/display/drm_dp_mst_helper.h

@@ -892,6 +892,9 @@ drm_atomic_get_new_mst_topology_state(struct drm_atomic_state *state,
 struct drm_dp_mst_atomic_payload *
 drm_atomic_get_mst_payload_state(struct drm_dp_mst_topology_state *state,
 				 struct drm_dp_mst_port *port);
+bool drm_dp_mst_port_downstream_of_parent(struct drm_dp_mst_topology_mgr *mgr,
+					  struct drm_dp_mst_port *port,
+					  struct drm_dp_mst_port *parent);
 int __must_check
 drm_dp_atomic_find_time_slots(struct drm_atomic_state *state,
 			      struct drm_dp_mst_topology_mgr *mgr,