Currently the ocelot driver does support the 'vlan modify' action, but
in the ingress chain, and it is offloaded to VCAP IS1. This action
changes the classified VLAN before the packet enters the bridging
service, and the bridging works with the classified VLAN modified by
VCAP IS1.

That is good for some use cases, but there are others where the VLAN
must be modified at the stage of the egress port, after the packet has
exited the bridging service. One example is simulating IEEE 802.1CB
active stream identification filters ("active" means that not only the
rule matches on a packet flow, but it is also able to change some
headers). For example, a stream is replicated on two egress ports, but
they must have different VLAN IDs on egress ports A and B.

This seems like a task for the VCAP ES0, but that currently only
supports pushing the ES0 tag A, which is specified in the rule. Pushing
another VLAN header is not what we want, but rather overwriting the
existing one.

It looks like when we push the ES0 tag A, it is actually possible to not
only take the ES0 tag A's value from the rule itself (VID_A_VAL), but
derive it from the following formula:

ES0_TAG_A = Classified VID + VID_A_VAL

Otherwise said, ES0_TAG_A can be used to increment with a given value
the VLAN ID that the packet was already classified to, and the packet
will have this value as an outer VLAN tag. This new VLAN ID value then
gets stripped on egress (or not) according to the value of the native
VLAN from the bridging service.

While the hardware will happily increment the classified VLAN ID for all
packets that match the ES0 rule, in practice this would be rather
insane, so we only allow this kind of ES0 action if the ES0 filter
contains a VLAN ID too, so as to restrict the matching on a known
classified VLAN. If we program VID_A_VAL with the delta between the
desired final VLAN (ES0_TAG_A) and the classified VLAN, we obtain the
desired behavior.

It doesn't look like it is possible with the tc-vlan action to modify
the VLAN ID but not the PCP. In hardware it is possible to leave the PCP
to the classified value, but we unconditionally program it to overwrite
it with the PCP value from the rule.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>