net: dsa: mv88e6xxx: Workaround missing PHY ID on mv88e6390

The internal PHYs of the mv88e6390 do not have a model ID. Trap any
calls to the ID register, and if it is zero, return the ID for the
mv88e6390. The Marvell PHY driver can then bind to this ID.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -2930,6 +2930,14 @@ static int mv88e6xxx_mdio_read(struct mii_bus *bus, int phy, int reg)
 	err = chip->info->ops->phy_read(chip, bus, phy, reg, &val);
 	mutex_unlock(&chip->reg_lock);
 
+	if (reg == MII_PHYSID2) {
+		/* Some internal PHYS don't have a model number.  Use
+		 * the mv88e6390 family model number instead.
+		 */
+		if (!(val & 0x3f0))
+			val |= PORT_SWITCH_ID_PROD_NUM_6390;
+	}
+
 	return err ? err : val;
 }