Re: warnings from MTU setting on switch ports

[Vladimir Oltean <vladimir.oltean@nxp.com>]

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 <olteanv@gmail.com>
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 <olteanv@gmail.com>
    Reviewed-by: Andrew Lunn <andrew@lunn.ch>
    Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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.