netdev - Re: [PATCH v2 net-next 3/6] net: dsa: stop updating master MTU from master.c

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Date:   Thu, 31 Mar 2022 02:53:46 -0300
From:   Luiz Angelo Daros de Luca <luizluca@...il.com>
To:     Vladimir Oltean <vladimir.oltean@....com>,
        Alvin Šipraga <ALSI@...g-olufsen.dk>
Cc:     "open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH v2 net-next 3/6] net: dsa: stop updating master MTU from master.c

Hi Vladimir,

I think I found an issue with this patch.

> At present there are two paths for changing the MTU of the DSA master.
>
> The first is:
>
> dsa_tree_setup
> -> dsa_tree_setup_ports
>    -> dsa_port_setup
>       -> dsa_slave_create
>          -> dsa_slave_change_mtu
>             -> dev_set_mtu(master)

The first code from dsa_slave_change_mtu() is:

        if (!ds->ops->port_change_mtu)
               return -EOPNOTSUPP;

So, when the switch does not implement ds->ops->port_change_mtu, the
master MTU will never be updated. This is the case for
drivers/net/dsa/realtek/rtl8365mb.c. Before this patch,
ops->port_change_mtu was optional. We either need to turn it into a
mandatory function (even if it is a no-op that fails when mtu is
different) or change the dsa_slave_change_mtu to only return
-EOPNOTSUPP when the new slave MTU differs from current slave MTU.

Regards,

Luiz