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: ec5ae610 ("net: dsa: sja1105: save/restore VLANs using a delta commit method") Signed-off-by:Vladimir Oltean <vladimir.oltean@nxp.com> Signed-off-by: