netdev - Re: [PATCH v5 05/13] net: ethernet: aquantia: Support for NIC-specific code

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Date:   Sun, 15 Jan 2017 00:55:02 +0200
From:   Rami Rosen <roszenrami@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Alexander Loktionov <Alexander.Loktionov@...antia.com>,
        Netdev <netdev@...r.kernel.org>,
        David VomLehn <vomlehn@...as.net>,
        "David S . Miller" <davem@...emloft.net>,
        Simon Edelhaus <Simon.Edelhaus@...antia.com>,
        Dmitrii Tarakanov <Dmitrii.Tarakanov@...antia.com>,
        Pavel Belous <Pavel.Belous@...antia.com>,
        Dmitry Bezrukov <Dmitry.Bezrukov@...antia.com>
Subject: Re: [PATCH v5 05/13] net: ethernet: aquantia: Support for
 NIC-specific code

Hi, Florian,

> +}
> +
> +static int aq_ndev_change_mtu(struct net_device *ndev, int new_mtu)
> +{
> +     struct aq_nic_s *aq_nic = (struct aq_nic_s *)netdev_priv(ndev);
> +     int err = 0;
> +
> +     if (new_mtu == ndev->mtu) {
> +             err = 0;
> +             goto err_exit;
> +     }
> +     if (new_mtu < 68) {
> +             err = -EINVAL;
> +             goto err_exit;
> +     }

> What's so special about 68 here?

I think that the check that the passed MTU is at least 68 bytes is
justified and correct.
This convention is followed by a large number of Ethernet drivers; for example,
http://lxr.free-electrons.com/source/drivers/net/ethernet/intel/i40e/i40e_main.c#L2246
http://lxr.free-electrons.com/source/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c#L6049

The size of 68 bytes is originated from RFC 791:
https://tools.ietf.org/html/rfc791
...
Every internet module must be able to forward a datagram of 68 octets
without further fragmentation.
This is because an internet header may be up to 60 octets, and the
minimum fragment is 8 octets.
...

Regards,
Rami Rosen