netdev - Re: [PATCH v3 1/3] dsa: marvell: Provide per device information about max frame size

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Date:   Tue, 3 Jan 2023 13:54:44 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        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>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] dsa: marvell: Provide per device information
 about max frame size

On Tue, Jan 03, 2023 at 10:02:51AM +0100, Lukasz Majewski wrote:
> Hi Andrew,
> 
> > > @@ -3548,7 +3548,9 @@ static int mv88e6xxx_get_max_mtu(struct
> > > dsa_switch *ds, int port) if (chip->info->ops->port_set_jumbo_size)
> > >  		return 10240 - VLAN_ETH_HLEN - EDSA_HLEN -
> > > ETH_FCS_LEN; else if (chip->info->ops->set_max_frame_size)
> > > -		return 1632 - VLAN_ETH_HLEN - EDSA_HLEN -
> > > ETH_FCS_LEN;
> > > +		return (max_t(int, chip->info->max_frame_size,
> > > 1632)
> > > +			- VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN);
> > > +
> > >  	return 1522 - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;  
> > 
> > I would also prefer if all this if/else logic is removed, and the code
> > simply returned chip->info->max_frame_size - VLAN_ETH_HLEN -
> > EDSA_HLEN - ETH_FCS_LEN;
> > 
> 
> So then the mv88e6xxx_get_max_mtu shall look like:
> 
> WARN_ON_ONCE(!chip->info->max_frame_size)
> 
> if (chip->info->ops->port_set_jumbo_size)
> ...
> else 
>     return chip->info->max_frame_size - VLAN_ETH_HLEN -
> 	EDSA_HLEN - ETH_FCS_LEN;

I think it should go even further:

{
	WARN_ON_ONCE(!chip->info->max_frame_size)

	return chip->info->max_frame_size - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;
}

If we are going to use info->max_frame_size, we should always use
info->max_frame_size.

	Andrew