netdev - RE: [PATCH RFC 1/3] net: Add dev_set_env_hdr

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Message-ID: <BL2PR07MB2306030D4912AD8F574BAF9D8DCC0@BL2PR07MB2306.namprd07.prod.outlook.com>
Date:   Tue, 27 Sep 2016 14:23:43 +0000
From:   "Mintz, Yuval" <Yuval.Mintz@...ium.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Patrick McHardy <kaber@...sh.net>,
        "Stephen Hemminger" <stephen@...workplumber.org>,
        Vlad Yasevich <vyasevich@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: RE: [PATCH RFC 1/3] net: Add dev_set_env_hdr_len to accept envelope
 frames

> +/* return envelope header length */
> +static inline int netif_get_env_hdr_len(struct net_device *dev) {
> +	if (dev->netdev_ops->ndo_set_env_hdr_len)
> +		return dev->env_hdr_len;
> +
> +	if (netif_reduces_vlan_mtu(dev))
> +		return 0;
> +
> +	return 4; /* VLAN_HLEN */
> +}

Why claim that `4' is the default?
What's the benefit of propagating this value to userspace if
driver doesn't actually support it?

Assuming this *is* a good default [meaning your analysis indicated
basically every driver has at least so many octets reserved]
perhaps we should consider initializing the env_hdr_len in some
common function [e.g., in setup_ether() next to the mtu] to this value?