net: dsa: introduce a dsa_switch_find function

Somewhat similar to dsa_tree_find, dsa_switch_find returns a dsa_switch
structure pointer by searching for its tree index and switch index (the
parameters from dsa,member). To be used, for example, by drivers who
implement .crosschip_bridge_join and need a reference to the other
switch indicated to by the tree_index and sw_index arguments.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/dsa.h

@@ -672,6 +672,7 @@ static inline bool dsa_can_decode(const struct sk_buff *skb,
 
 void dsa_unregister_switch(struct dsa_switch *ds);
 int dsa_register_switch(struct dsa_switch *ds);
+struct dsa_switch *dsa_switch_find(int tree_index, int sw_index);
 #ifdef CONFIG_PM_SLEEP
 int dsa_switch_suspend(struct dsa_switch *ds);
 int dsa_switch_resume(struct dsa_switch *ds);

net/dsa/dsa2.c

@@ -24,6 +24,27 @@ LIST_HEAD(dsa_tree_list);
 static const struct devlink_ops dsa_devlink_ops = {
 };
 
+struct dsa_switch *dsa_switch_find(int tree_index, int sw_index)
+{
+	struct dsa_switch_tree *dst;
+	struct dsa_port *dp;
+
+	list_for_each_entry(dst, &dsa_tree_list, list) {
+		if (dst->index != tree_index)
+			continue;
+
+		list_for_each_entry(dp, &dst->ports, list) {
+			if (dp->ds->index != sw_index)
+				continue;
+
+			return dp->ds;
+		}
+	}
+
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(dsa_switch_find);
+
 static struct dsa_switch_tree *dsa_tree_find(int index)
 {
 	struct dsa_switch_tree *dst;