net: dsa: clear VLAN PVID flag for CPU port

When the bridge offloads a VLAN on a slave port, we also need to
program its dedicated CPU port as a member of the VLAN.

Drivers may handle the CPU port's membership as they want. For example,
Marvell as a special "Unmodified" mode to pass frames as is through
such ports.

Even though DSA expects the drivers to handle the CPU port membership,
it does not make sense to program user VLANs as PVID on the CPU port.
This patch clears this flag before programming the CPU port.
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -332,6 +332,12 @@ static int dsa_slave_vlan_add(struct net_device *dev,
 	if (err)
 		return err;
 
+	/* We need the dedicated CPU port to be a member of the VLAN as well.
+	 * Even though drivers often handle CPU membership in special ways,
+	 * it doesn't make sense to program a PVID, so clear this flag.
+	 */
+	vlan.flags &= ~BRIDGE_VLAN_INFO_PVID;
+
 	err = dsa_port_vlan_add(dp->cpu_dp, &vlan, trans);
 	if (err)
 		return err;