netdev - Re: TAHI CN-6-4-1 failed on Linux 2.6.32 kernel

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Date:	Fri, 20 Aug 2010 13:39:03 -0400
From:	Jesse Gross <jesse@...ira.com>
To:	Steve Chen <schen@...sta.com>
Cc:	David Miller <davem@...emloft.net>, brian.haley@...com,
	usagi-users-ctl@...linux-ipv6.org, netdev@...r.kernel.org
Subject: Re: TAHI CN-6-4-1 failed on Linux 2.6.32 kernel

On Fri, Aug 20, 2010 at 1:16 PM, Steve Chen <schen@...sta.com> wrote:
>
> On Thu, Aug 19, 2010 at 7:06 PM, David Miller <davem@...emloft.net> wrote:
> > From: Steve Chen <schen@...sta.com>
> > Date: Thu, 19 Aug 2010 13:35:14 -0500
> >
> >> I trace through the code.  It appears that the network driver (e1000e
> >> for my setup) always set ip_summed to CHECKSUM_UNNECESSARY.  I have
> >> been unsuccessful to get the driver to take the other branch where
> >> ip_summed is set to CHECKSUM_COMPLETE.  Even when I hard code
> >> ip_summed to CHECKSUM_COMPLETE, __skb_checksum_complete_head set
> >> ip_summed to CHECKSUM_UNNECESSARY after recomputing the checksum.
> >>
> >> So far the only way I'm able to get ICMP to recompute checksum is
> >> through the attached hack.  Even though I can get all the tests to
> >> pass, but it just seem wrong.
> >
> > If turning off hardware RX checksumming with ethtool has no effect,
> > and the problem is seen with multiple ethernet cards, the problem
> > is elsewhere.
> >
> > First of all, if you turn RX checksumming off, the checksum field
> > of the SKB should always be skb->ip_summed = 0.  If this is not
> > happening, find out why.
>
> Ahhh, thats my problem.  I incorrectly thought the ip_summed should be
> 2.  The ip_summed is set to 1 in
> __skb_checksum_complete_head.  Looking at the code, shouldn't
>
> if (likely(!sum))
>
> be
>
> if (likely(sum))
>
> Since sum == 0 would indicate an error?

sum == 0 indicates that the checksum is correct.

If you compute the checksum of a packet containing the correct
checksum the result is 0.  It's like a slightly more complicated
varient of a parity bit.
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