net: dsa: Make mdio bus optional

The switch may want to instantiate its own MDIO bus. Only do it
centrally if the switch has not already created one, and the read op
is implemented.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa.c

@@ -340,8 +340,9 @@ static int dsa_switch_setup_one(struct dsa_switch *ds, struct device *parent)
 	if (ret < 0)
 		goto out;
 
+	if (!ds->slave_mii_bus && drv->phy_read) {
 		ds->slave_mii_bus = devm_mdiobus_alloc(parent);
-	if (ds->slave_mii_bus == NULL) {
+		if (!ds->slave_mii_bus) {
 			ret = -ENOMEM;
 			goto out;
 		}
@@ -350,7 +351,7 @@ static int dsa_switch_setup_one(struct dsa_switch *ds, struct device *parent)
 		ret = mdiobus_register(ds->slave_mii_bus);
 		if (ret < 0)
 			goto out;
-
+	}
 
 	/*
 	 * Create network devices for physical switch ports.
@@ -493,6 +494,7 @@ static void dsa_switch_destroy(struct dsa_switch *ds)
 		dsa_cpu_dsa_destroy(ds->ports[port].dn);
 	}
 
+	if (ds->slave_mii_bus && ds->drv->phy_read)
 		mdiobus_unregister(ds->slave_mii_bus);
 }