netdev - Re: [PATCH net-next 06/12] net: mscc: ocelot: adjust MTU on the CPU port in NPI mode

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Date:   Tue, 12 Nov 2019 14:57:11 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Joergen Andreasen <joergen.andreasen@...rochip.com>,
        "Allan W. Nielsen" <allan.nielsen@...rochip.com>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        netdev <netdev@...r.kernel.org>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next 06/12] net: mscc: ocelot: adjust MTU on the CPU
 port in NPI mode

On Tue, Nov 12, 2019 at 03:52:51PM +0200, Vladimir Oltean wrote:
> On Tue, 12 Nov 2019 at 15:51, Andrew Lunn <andrew@...n.ch> wrote:
> >
> > On Tue, Nov 12, 2019 at 02:44:14PM +0200, Vladimir Oltean wrote:
> > > From: Vladimir Oltean <vladimir.oltean@....com>
> > >
> > > When using the NPI port, the DSA tag is passed through Ethernet, so the
> > > switch's MAC needs to accept it as it comes from the DSA master. Increase
> > > the MTU on the external CPU port to account for the length of the
> > > injection header.
> >
> > I think this is the only DSA driver which needs to do this. Generally,
> > the port knows it is adding/removing the extra header, and so
> > magically accepts bigger frames.
> >
> > Where i have seen issues is the other end, the host interface. It
> > sometimes drops 'full MTU' frames because the DSA header makes the
> > frame too big.
> >
> 
> No, that worked out of the box because DSA adjusts the MTU of the
> master interface by the amount of bytes specified in the overhead of
> the tagger.

Yes, i added that code, after having to modify too many MAC drivers!

     Andrew