net: dsa: mv88e6xxx: fix max_mtu of 1492 on 6165, 6191, 6220, 6250, 6290

net: dsa: mv88e6xxx: fix max_mtu of 1492 on 6165, 6191, 6220, 6250, 6290

There are 3 classes of switch families that the driver is aware of, as
far as mv88e6xxx_change_mtu() is concerned:

- MTU configuration is available per port. Here, the
  chip->info->ops->port_set_jumbo_size() method will be present.

- MTU configuration is global to the switch. Here, the
  chip->info->ops->set_max_frame_size() method will be present.

- We don't know how to change the MTU. Here, none of the above methods
  will be present.

Switch families MV88E6165, MV88E6191, MV88E6220, MV88E6250 and MV88E6290
fall in category 3.

The blamed commit has adjusted the MTU for all 3 categories by EDSA_HLEN
(8 bytes), resulting in a new maximum MTU of 1492 being reported by the
driver for these switches.

I don't have the hardware to test, but I do have a MV88E6390 switch on
which I can simulate this by commenting out its .port_set_jumbo_size
definition from mv88e6390_ops. The result is this set of messages at
probe time:

mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 1
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 2
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 3
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 4
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 5
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 6
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 7
mv88e6085 d0032004.mdio-mii:10: nonfatal error -34 setting MTU to 1500 on port 8

It is highly implausible that there exist Ethernet switches which don't
support the standard MTU of 1500 octets, and this is what the DSA
framework says as well - the error comes from dsa_slave_create() ->
dsa_slave_change_mtu(slave_dev, ETH_DATA_LEN).

But the error messages are alarming, and it would be good to suppress
them.

As a consequence of this unlikeliness, we reimplement mv88e6xxx_get_max_mtu()
and mv88e6xxx_change_mtu() on switches from the 3rd category as follows:
the maximum supported MTU is 1500, and any request to set the MTU to a
value larger than that fails in dev_validate_mtu().

Fixes: b9c587fed61c ("dsa: mv88e6xxx: Include tagger overhead when setting MTU for DSA and CPU ports")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>