netdev - Re: [PATCH 01/20] ethernet: ucc_geth: set dev->max

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Date:   Thu, 10 Dec 2020 02:25:02 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     Li Yang <leoyang.li@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Murali Krishna Policharla <murali.policharla@...adcom.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Zhao Qiang <qiang.zhao@....com>, netdev@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/20] ethernet: ucc_geth: set dev->max_mtu to 1518

On Sat, Dec 05, 2020 at 08:17:24PM +0100, Rasmus Villemoes wrote:
> All the buffers and registers are already set up appropriately for an
> MTU slightly above 1500, so we just need to expose this to the
> networking stack. AFAICT, there's no need to implement .ndo_change_mtu
> when the receive buffers are always set up to support the max_mtu.
> 
> This fixes several warnings during boot on our mpc8309-board with an
> embedded mv88e6250 switch:
> 
> mv88e6085 mdio@...02120:10: nonfatal error -34 setting MTU 1500 on port 0
> ...
> mv88e6085 mdio@...02120:10: nonfatal error -34 setting MTU 1500 on port 4
> ucc_geth e0102000.ethernet eth1: error -22 setting MTU to 1504 to include DSA overhead
> 
> The last line explains what the DSA stack tries to do: achieving an MTU
> of 1500 on-the-wire requires that the master netdevice connected to
> the CPU port supports an MTU of 1500+the tagging overhead.
> 
> Fixes: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
> Cc: Vladimir Oltean <vladimir.oltean@....com>
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew