When the switchdev_handle_fdb_event_to_device() event replication helper
was created, my original thought was that FDB events on LAG interfaces
should most likely be special-cased, not just replicated towards all
switchdev ports beneath that LAG. So this replication helper currently
does not recurse through switchdev lower interfaces of LAG bridge ports,
but rather calls the lag_mod_cb() if that was provided.

No switchdev driver uses this helper for FDB events on LAG interfaces
yet, so that was an assumption which was yet to be tested. It is
certainly usable for that purpose, as my RFC series shows:

https://patchwork.kernel.org/project/netdevbpf/cover/20220210125201.2859463-1-vladimir.oltean@nxp.com/

however this approach is slightly convoluted because:

- the switchdev driver gets a "dev" that isn't its own net device, but
  rather the LAG net device. It must call switchdev_lower_dev_find(dev)
  in order to get a handle of any of its own net devices (the ones that
  pass check_cb).

- in order for FDB entries on LAG ports to be correctly refcounted per
  the number of switchdev ports beneath that LAG, we haven't escaped the
  need to iterate through the LAG's lower interfaces. Except that is now
  the responsibility of the switchdev driver, because the replication
  helper just stopped half-way.

So, even though yes, FDB events on LAG bridge ports must be
special-cased, in the end it's simpler to let switchdev_handle_fdb_*
just iterate through the LAG port's switchdev lowers, and let the
switchdev driver figure out that those physical ports are under a LAG.

The switchdev_handle_fdb_event_to_device() helper takes a
"foreign_dev_check" callback so it can figure out whether @dev can
autonomously forward to @foreign_dev. DSA fills this method properly:
if the LAG is offloaded by another port in the same tree as @dev, then
it isn't foreign. If it is a software LAG, it is foreign - forwarding
happens in software.

Whether an interface is foreign or not decides whether the replication
helper will go through the LAG's switchdev lowers or not. Since the
lan966x doesn't properly fill this out, FDB events on software LAG
uppers will get called. By changing lan966x_foreign_dev_check(), we can
suppress them.

Whereas DSA will now start receiving FDB events for its offloaded LAG
uppers, so we need to return -EOPNOTSUPP, since we currently don't do
the right thing for them.

Cc: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>