Hi Rasmus,

On Mon, Nov 30, 2020 at 03:30:50PM +0100, Rasmus Villemoes wrote:

Hi,

Updating our mpc8309 board to 5.9, we're starting to get

[    0.709832] mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU on port 0
[    0.720721] mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU on port 1
[    0.731002] mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU on port 2
[    0.741333] mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU on port 3
[    0.752220] mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU on port 4
[    0.764231] eth1: mtu greater than device maximum
[    0.769022] ucc_geth e0102000.ethernet eth1: error -22 setting MTU to include DSA overhead

So it does say "nonfatal", but do we have to live with those warnings on
every boot going forward, or is there something that we could do to
silence it?

It's a mv88e6250 switch with cpu port connected to a ucc_geth interface;
the ucc_geth driver indeed does not implement ndo_change_mtu and has
->max_mtu set to the default of 1500.

To suppress the warning:

commit 4349abdb409b04a5ed4ca4d2c1df7ef0cc16f6bd
Author: Vladimir Oltean [off-list ref]
Date:   Tue Sep 8 02:25:56 2020 +0300

    net: dsa: don't print non-fatal MTU error if not supported

    Commit 72579e14a1d3 ("net: dsa: don't fail to probe if we couldn't set
    the MTU") changed, for some reason, the "err && err != -EOPNOTSUPP"
    check into a simple "err". This causes the MTU warning to be printed
    even for drivers that don't have the MTU operations implemented.
    Fix that.

    Signed-off-by: Vladimir Oltean [off-list ref]
    Reviewed-by: Andrew Lunn [off-list ref]
    Reviewed-by: Florian Fainelli [off-list ref]
    Signed-off-by: Jakub Kicinski [off-list ref]

But you might also want to look into adding .ndo_change_mtu for
ucc_geth. If you are able to pass MTU-sized traffic through your
mv88e6085, then it is probably the case that the mpc8309 already
supports larger packets than 1500 bytes, and it is simply a matter of
letting the stack know about that. The warning is there to give people a
clue for the reason why MTU-sized traffic might not work over DSA.