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Date: Tue, 15 Nov 2011 09:45:16 -0800
From: Stephen Hemminger <shemminger@vyatta.com>
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Subject: Re: [Bridge] sky2 hw csum failure
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To: "Yan, Zheng" <zheng.z.yan@intel.com>
Cc: Martin Volf <martin.volf.42@gmail.com>, "bridge@lists.linux-foundation.org" <bridge@lists.linux-foundation.org>, "wcang@sfc.wide.ad.jp" <wcang@sfc.wide.ad.jp>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, "shemminger@linux-foundation.org" <shemminger@linux-foundation.org>, "davem@davemloft.net" <davem@davemloft.net>

On Tue, 15 Nov 2011 18:28:55 +0800
"Yan, Zheng" <zheng.z.yan@intel.com> wrote:

> I re-tested the checksum code, both CHECKSUM_NONE and CHECKSUM_COMPLETE
> cases are OK. Maybe the bug is related to sky2.
> 
> Regards
> Yan, Zheng

There are three types of receive checksumming:
  1. Hardware does not do checksumming (CHECKSUM_NONE)
  2. Hardware validates checksum (CHECKSUM_UNNECESSARY)
  3. Hardware computes sum of bytes in skb (CHECKSUM_COMPLETE)

Most hardware does #2, but sky2 uses #3.
For the second case, the hardware does not look at headers but only
reports the one's complement value in ip_summed. It is up to the
protocol layers to adjust accordingly.  This means if data is removed
or added the checksum needs to be adjusted.