net: bridge: mst: Add helper to query a port's MST state

This is useful for switchdev drivers who are offloading MST states
into hardware. As an example, a driver may wish to flush the FDB for a
port when it transitions from forwarding to blocking - which means
that the previous state must be discoverable.
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/if_bridge.h

@@ -121,6 +121,7 @@ int br_vlan_get_info_rcu(const struct net_device *dev, u16 vid,
 			 struct bridge_vlan_info *p_vinfo);
 bool br_mst_enabled(const struct net_device *dev);
 int br_mst_get_info(const struct net_device *dev, u16 msti, unsigned long *vids);
+int br_mst_get_state(const struct net_device *dev, u16 msti, u8 *state);
 #else
 static inline bool br_vlan_enabled(const struct net_device *dev)
 {
@@ -164,6 +165,11 @@ static inline int br_mst_get_info(const struct net_device *dev, u16 msti,
 {
 	return -EINVAL;
 }
+static inline int br_mst_get_state(const struct net_device *dev, u16 msti,
+				   u8 *state)
+{
+	return -EINVAL;
+}
 #endif
 
 #if IS_ENABLED(CONFIG_BRIDGE)

net/bridge/br_mst.c

@@ -48,6 +48,31 @@ int br_mst_get_info(const struct net_device *dev, u16 msti, unsigned long *vids)
 }
 EXPORT_SYMBOL_GPL(br_mst_get_info);
 
+int br_mst_get_state(const struct net_device *dev, u16 msti, u8 *state)
+{
+	const struct net_bridge_port *p = NULL;
+	const struct net_bridge_vlan_group *vg;
+	const struct net_bridge_vlan *v;
+
+	ASSERT_RTNL();
+
+	p = br_port_get_check_rtnl(dev);
+	if (!p || !br_opt_get(p->br, BROPT_MST_ENABLED))
+		return -EINVAL;
+
+	vg = nbp_vlan_group(p);
+
+	list_for_each_entry(v, &vg->vlan_list, vlist) {
+		if (v->brvlan->msti == msti) {
+			*state = v->state;
+			return 0;
+		}
+	}
+
+	return -ENOENT;
+}
+EXPORT_SYMBOL_GPL(br_mst_get_state);
+
 static void br_mst_vlan_set_state(struct net_bridge_port *p, struct net_bridge_vlan *v,
 				  u8 state)
 {