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 10:02:51 +0100
From:   Lukasz Majewski <lukma@...x.de>
To:     Andrew Lunn <andrew@...n.ch>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     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

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;


Or shall I put WARN_ON_ONCE to the mv88e6xxx_probe() function?


The above approach is contrary to one proposed by Alexander, who wanted
to improve the defensive approach in this driver (to avoid situation
where the max_frame_size callback is not defined and max_frame_size
member of *_info struct is not added by developer).

Which approach is the recommended one for this driver?

> > +++ b/drivers/net/dsa/mv88e6xxx/chip.h
> > @@ -132,6 +132,7 @@ struct mv88e6xxx_info {
> >  	unsigned int num_gpio;
> >  	unsigned int max_vid;
> >  	unsigned int max_sid;
> > +	unsigned int max_frame_size;  
> 
> It might be worth adding a comment here what this value actually
> represents.

Ok. I will add proper comment.

> We don't want any mixups where the value already has the
> frame checksum removed for example.

Could you be more specific here about this use case?

The max_frame_size is the maximal size of the ethernet frame for which
the IC designer provided specified amount of RAM (it is a different
value for different SoCs in the Link Street family).

> 
>       Andrew


Best regards,

Lukasz Majewski

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