netdev - NETIF_F_SG not taking effect

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Date:	Thu, 2 May 2013 09:49:56 +0530
From:	Viswanath D <viswanathd@...vanlabs.com>
To:	netdev@...r.kernel.org
Subject: NETIF_F_SG not taking effect

Hi,

I need help in NETIF_F_UFO to work. After reading several materials, I
have come to understand that UFO is dependent on NETIF_F_SG, which in
turn is dependent on NETIF_F_HW_CSUM and when I turn on all these
flags in ndev->features during probe, I should be seeing fragments in
skb in xmit function. Also, if the packets are UDP/TCP, then the first
fragment would the header (34/66 bytes) and the rest would be the data
pointer as given by application (iperf in this case).

But, when I print "skb_shinfo(skb)->nr_frags" in my xmit function, I
always find it zero. Also all the packet content (header + data) is
completely contained within skb->data. I also removed UFO (keeping
just SG and HW_CSUM) and the behavior remains the same.
But interestingly the checksum is not computed by the stack. I
confirmed it by dumping the UDP/TCP headers in xmit and wireshark on
receive side also reflects this.
These observations are leading me to think NETIF_F_SG flag is being
ignored by the stack for some reason.

Are there any compilation flag which can be enabled to see what is
going wrong. Could I please get some pointers on how to go about
debugging this problem. The kernel version I am using is 2.6.37.

Regards
Viswa
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