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 13:31:05 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Luiz Angelo Daros de Luca <luizluca@...il.com>
CC:     Alvin ┼áipraga <ALSI@...g-olufsen.dk>,
        "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

On Thu, Mar 31, 2022 at 02:53:46AM -0300, Luiz Angelo Daros de Luca wrote:
> 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

Thanks Luiz, you are right, I've sent a revert patch here:
https://patchwork.kernel.org/project/netdevbpf/patch/20220331132854.1395040-1-vladimir.oltean@nxp.com/
My only problem with updating the MTU from dsa_master_setup() was that
it was taking rtnl_lock(), but it looks like I went too far by deleting
it entirely. I restored the old code structure, which should need no
further changes.