net: dsa: felix: dynamically determine tag_8021q CPU port for traps

net: dsa: felix: dynamically determine tag_8021q CPU port for traps

Ocelot switches support a single active CPU port at a time (at least as
a trapping destination, i.e. for control traffic). This is true
regardless of whether we are using the native copy-to-CPU-port-module
functionality, or a redirect action towards the software-defined
tag_8021q CPU port.

Currently we assume that the trapping destination in tag_8021q mode is
the first CPU port, yet in the future we may want to migrate the user
ports to the second CPU port.

For that to work, we need to make sure that the tag_8021q trapping
destination is a CPU port that is active, i.e. is used by at least some
user port on which the trap was added. Otherwise, we may end up
redirecting the traffic to a CPU port which isn't even up.

Note that due to the current design where we simply choose the CPU port
of the first port from the trap's ingress port mask, it may be that a
CPU port absorbes control traffic from user ports which aren't affine to
it as per user space's request. This isn't ideal, but is the lesser of
two evils. Following the user-configured affinity for traps would mean
that we can no longer reuse a single TCAM entry for multiple traps,
which is what we actually do for e.g. PTP. Either we duplicate and
deduplicate TCAM entries on the fly when user-to-CPU-port mappings
change (which is unnecessarily complicated), or we redirect trapped
traffic to all tag_8021q CPU ports if multiple such ports are in use.
The latter would have actually been nice, if it actually worked, but it
doesn't, since a OCELOT_MASK_MODE_REDIRECT action towards multiple ports
would not take PGID_SRC into consideration, and it would just duplicate
the packet towards each (CPU) port, leading to duplicates in software.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>