linux-kernel - Re: [PATCH v4 1/3] dsa: marvell: Provide per device information about max frame size

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Date:   Fri, 13 Jan 2023 15:59:30 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Andrew Lunn <andrew@...n.ch>, Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] dsa: marvell: Provide per device information
 about max frame size

On Fri, Jan 13, 2023 at 02:20:17PM +0100, Lukasz Majewski wrote:
> The fixed function maybe should look like below:
> 
> static int mv88e6xxx_get_max_mtu(struct dsa_switch *ds, int port)
> {
> 	....
> 	
> 	int max_mtu;
> 
> 	max_mtu = chip->info->max_frame_size - VLAN_ETH_HLEN -
> 		  ETH_FCS_LE;
> 
> 	if (dsa_is_dsa_port(ds, port) || dsa_is_cpu_port(ds, port))
> 		  max_mtu -= EDSA_HLEN;
> 
> 	return max_mtu;
> }
> 
> Comments more than welcome.

I suspect that looking at the DSA code which calls these methods will
answer a lot of your questions. ds->ops->port_max_mtu() is only called
for user ports. As for ds->ops->port_change_mtu(), this will always be
called with the requested L2 payload length (default 1500) on user ports,
and with the maximum among user ports for DSA and CPU ports.