net: dsa: mt7530: set CPU port to fallback mode

Currently, setting a bridge's self PVID to other value and deleting
the default VID 1 renders untagged ports of that VLAN unable to talk to
the CPU port:

	bridge vlan add dev br0 vid 2 pvid untagged self
	bridge vlan del dev br0 vid 1 self
	bridge vlan add dev sw0p0 vid 2 pvid untagged
	bridge vlan del dev sw0p0 vid 1
	# br0 cannot send untagged frames out of sw0p0 anymore

That is because the CPU port is set to security mode and its PVID is
still 1, and untagged frames are dropped due to VLAN member violation.

Set the CPU port to fallback mode so untagged frames can pass through.

Fixes: 83163f7d ("net: dsa: mediatek: add VLAN support for MT7530")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mt7530.c

@@ -810,10 +810,15 @@ mt7530_port_set_vlan_aware(struct dsa_switch *ds, int port)
 		   PCR_MATRIX_MASK, PCR_MATRIX(MT7530_ALL_MEMBERS));
 
 	/* Trapped into security mode allows packet forwarding through VLAN
-	 * table lookup.
+	 * table lookup. CPU port is set to fallback mode to let untagged
+	 * frames pass through.
 	 */
-	mt7530_rmw(priv, MT7530_PCR_P(port), PCR_PORT_VLAN_MASK,
-		   MT7530_PORT_SECURITY_MODE);
+	if (dsa_is_cpu_port(ds, port))
+		mt7530_rmw(priv, MT7530_PCR_P(port), PCR_PORT_VLAN_MASK,
+			   MT7530_PORT_FALLBACK_MODE);
+	else
+		mt7530_rmw(priv, MT7530_PCR_P(port), PCR_PORT_VLAN_MASK,
+			   MT7530_PORT_SECURITY_MODE);
 
 	/* Set the port as a user port which is to be able to recognize VID
 	 * from incoming packets before fetching entry within the VLAN table.

drivers/net/dsa/mt7530.h

@@ -152,6 +152,12 @@ enum mt7530_port_mode {
 	/* Port Matrix Mode: Frames are forwarded by the PCR_MATRIX members. */
 	MT7530_PORT_MATRIX_MODE = PORT_VLAN(0),
 
+	/* Fallback Mode: Forward received frames with ingress ports that do
+	 * not belong to the VLAN member. Frames whose VID is not listed on
+	 * the VLAN table are forwarded by the PCR_MATRIX members.
+	 */
+	MT7530_PORT_FALLBACK_MODE = PORT_VLAN(1),
+
 	/* Security Mode: Discard any frame due to ingress membership
 	 * violation or VID missed on the VLAN table.
 	 */