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 14:51:07 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     jakub.kicinski@...ronome.com, davem@...emloft.net,
        alexandre.belloni@...tlin.com, f.fainelli@...il.com,
        vivien.didelot@...il.com, joergen.andreasen@...rochip.com,
        allan.nielsen@...rochip.com, horatiu.vultur@...rochip.com,
        claudiu.manoil@....com, 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 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.

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

    Andrew