net: dsa: microchip: ksz8795: Use software untagging on CPU port

On the CPU port, we can support both tagged and untagged VLANs at the
same time by doing any necessary untagging in software rather than
hardware.  To enable that, keep the CPU port's Remove Tag flag cleared
and set the dsa_switch::untag_bridge_pvid flag.

Fixes: e66f840c08 ("net: dsa: ksz: Add Microchip KSZ8795 DSA driver")
Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/microchip/ksz8795.c

@@ -1150,8 +1150,10 @@ static int ksz8_port_vlan_add(struct dsa_switch *ds, int port,
 	/* If a VLAN is added with untagged flag different from the
 	 * port's Remove Tag flag, we need to change the latter.
 	 * Ignore VID 0, which is always untagged.
+	 * Ignore CPU port, which will always be tagged.
 	 */
-	if (untagged != p->remove_tag && vlan->vid != 0) {
+	if (untagged != p->remove_tag && vlan->vid != 0 &&
+	    port != dev->cpu_port) {
 		unsigned int vid;
 
 		/* Reject attempts to add a VLAN that requires the
@@ -1751,6 +1753,11 @@ static int ksz8_switch_init(struct ksz_device *dev)
 	/* set the real number of ports */
 	dev->ds->num_ports = dev->port_cnt;
 
+	/* We rely on software untagging on the CPU port, so that we
+	 * can support both tagged and untagged VLANs
+	 */
+	dev->ds->untag_bridge_pvid = true;
+
 	return 0;
 }