net: dsa: sja1105: use 4095 as the private VLAN for untagged traffic

One thing became visible when writing the blamed commit, and that was
that STP and PTP frames injected by net/dsa/tag_sja1105.c using the
deferred xmit mechanism are always classified to the pvid of the CPU
port, regardless of whatever VLAN there might be in these packets.

So a decision needed to be taken regarding the mechanism through which
we should ensure that delivery of STP and PTP traffic is possible when
we are in a VLAN awareness mode that involves tag_8021q. This is because
tag_8021q is not concerned with managing the pvid of the CPU port, since
as far as tag_8021q is concerned, no traffic should be sent as untagged
from the CPU port. So we end up not actually having a pvid on the CPU
port if we only listen to tag_8021q, and unless we do something about it.

The decision taken at the time was to keep VLAN 1 in the list of
priv->dsa_8021q_vlans, and make it a pvid of the CPU port. This ensures
that STP and PTP frames can always be sent to the outside world.

However there is a problem. If we do the following while we are in
the best_effort_vlan_filtering=true mode:

ip link add br0 type bridge vlan_filtering 1
ip link set swp2 master br0
bridge vlan del dev swp2 vid 1

Then untagged and pvid-tagged frames should be dropped. But we observe
that they aren't, and this is because of the precaution we took that VID
1 is always installed on all ports.

So clearly VLAN 1 is not good for this purpose. What about VLAN 0?
Well, VLAN 0 is managed by the 8021q module, and that module wants to
ensure that 802.1p tagged frames are always received by a port, and are
always transmitted as VLAN-tagged (with VLAN ID 0). Whereas we want our
STP and PTP frames to be untagged if the stack sent them as untagged -
we don't want the driver to just decide out of the blue that it adds
VID 0 to some packets.

So what to do?

Well, there is one other VLAN that is reserved, and that is 4095:
$ ip link add link swp2 name swp2.4095 type vlan id 4095
Error: 8021q: Invalid VLAN id.
$ bridge vlan add dev swp2 vid 4095
Error: bridge: Vlan id is invalid.

After we made this change, VLAN 1 is indeed forwarded and/or dropped
according to the bridge VLAN table, there are no further alterations
done by the sja1105 driver.

Fixes: ec5ae61076 ("net: dsa: sja1105: save/restore VLANs using a delta commit method")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/sja1105/sja1105_main.c

@@ -26,6 +26,7 @@
 #include "sja1105_tas.h"
 
 #define SJA1105_UNKNOWN_MULTICAST	0x010000000000ull
+#define SJA1105_DEFAULT_VLAN		(VLAN_N_VID - 1)
 
 static const struct dsa_switch_ops sja1105_switch_ops;
 
@@ -322,6 +323,13 @@ static int sja1105_init_l2_lookup_params(struct sja1105_private *priv)
 	return 0;
 }
 
+/* Set up a default VLAN for untagged traffic injected from the CPU
+ * using management routes (e.g. STP, PTP) as opposed to tag_8021q.
+ * All DT-defined ports are members of this VLAN, and there are no
+ * restrictions on forwarding (since the CPU selects the destination).
+ * Frames from this VLAN will always be transmitted as untagged, and
+ * neither the bridge nor the 8021q module cannot create this VLAN ID.
+ */
 static int sja1105_init_static_vlan(struct sja1105_private *priv)
 {
 	struct sja1105_table *table;
@@ -331,17 +339,13 @@ static int sja1105_init_static_vlan(struct sja1105_private *priv)
 		.vmemb_port = 0,
 		.vlan_bc = 0,
 		.tag_port = 0,
-		.vlanid = 1,
+		.vlanid = SJA1105_DEFAULT_VLAN,
 	};
 	struct dsa_switch *ds = priv->ds;
 	int port;
 
 	table = &priv->static_config.tables[BLK_IDX_VLAN_LOOKUP];
 
-	/* The static VLAN table will only contain the initial pvid of 1.
-	 * All other VLANs are to be configured through dynamic entries,
-	 * and kept in the static configuration table as backing memory.
-	 */
 	if (table->entry_count) {
 		kfree(table->entries);
 		table->entry_count = 0;
@@ -354,9 +358,6 @@ static int sja1105_init_static_vlan(struct sja1105_private *priv)
 
 	table->entry_count = 1;
 
-	/* VLAN 1: all DT-defined ports are members; no restrictions on
-	 * forwarding; always transmit as untagged.
-	 */
 	for (port = 0; port < ds->num_ports; port++) {
 		struct sja1105_bridge_vlan *v;
 
@@ -367,15 +368,12 @@ static int sja1105_init_static_vlan(struct sja1105_private *priv)
 		pvid.vlan_bc |= BIT(port);
 		pvid.tag_port &= ~BIT(port);
 
-		/* Let traffic that don't need dsa_8021q (e.g. STP, PTP) be
-		 * transmitted as untagged.
-		 */
 		v = kzalloc(sizeof(*v), GFP_KERNEL);
 		if (!v)
 			return -ENOMEM;
 
 		v->port = port;
-		v->vid = 1;
+		v->vid = SJA1105_DEFAULT_VLAN;
 		v->untagged = true;
 		if (dsa_is_cpu_port(ds, port))
 			v->pvid = true;