netdev - S/G operation, skb checksums, data copying

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Message-Id: <1209925725.3655.15.camel@johannes.berg>
Date:	Sun, 04 May 2008 20:28:45 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	netdev <netdev@...r.kernel.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	linux-wireless <linux-wireless@...r.kernel.org>
Subject: S/G operation, skb checksums, data copying

Hi,

Another thought I had while looking into this: many wireless devices are
actually able to do s/g operation but are not able to checksum IP (they
already have to checksum 802.11.)

Right now, we don't announce S/G support to the netstack, but as far as
I know it wouldn't help us anyway since we cannot checksum packets, cf.
register_netdevice:

        /* Fix illegal SG+CSUM combinations. */
        if ((dev->features & NETIF_F_SG) &&
            !(dev->features & NETIF_F_ALL_CSUM)) {
                printk(KERN_NOTICE "%s: Dropping NETIF_F_SG since no checksum feature.\n",
                       dev->name);
                dev->features &= ~NETIF_F_SG;

However, mac80211 needs to change the skb header before the skb is
handed to the hardware. Often enough, the skb it will get is cloned, so
it has to copy the header. Hence, it would benefit from having the
actual skb data not be in the header, but that is unavoidable currently
since we have no checksum features.

Should we do IP checksumming in software within mac80211, announce s/g
support and hardware checksumming to the rest of the networking layer
and hope that we only have to copy the header instead of the data for
many packets then, if the underlying hardware is capable enough?

johannes

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