netdev - Re: warnings from MTU setting on switch ports

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Date:   Tue, 1 Dec 2020 00:11:14 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Network Development <netdev@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: warnings from MTU setting on switch ports

> > A thought: Shouldn't the initialization of slave_dev->max_mtu in
> > dsa_slave_create() be capped by master->max_mtu minus tag overhead?
> 
> Talk to Andrew:
> https://www.spinics.net/lists/netdev/msg645810.html

Yes, this is historic. DSA started life with Marvell switches
connected to Marvell MACs. And Marvell MACs always allowed frames
bigger than the MTU to be sent/received. A few more MACs were paired
with Marvell switches, and they also happened to allow bigger frames
than the MTU to be used. So it all worked fine until a MAC/Switch pair
came along where the MAC did not pass frames bigger than the MTU. We
could not break backwards compatibility, so decided to ask the MAC to
up its MTU, but not error out if it failed. It was like that for a
long time, until Vladimir added jumbo support. Then you do need to
know if the MAC supports bigger MTU, so you can disallow jumbo. So
this warning got added.

     Andrew