netdev - Re: sky2 hw csum failure

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Message-ID: <20111115094516.3eb54a1a@nehalam.linuxnetplumber.net>
Date:	Tue, 15 Nov 2011 09:45:16 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	Martin Volf <martin.volf.42@...il.com>,
	"shemminger@...ux-foundation.org" <shemminger@...ux-foundation.org>,
	"bridge@...ts.linux-foundation.org" 
	<bridge@...ts.linux-foundation.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"wcang@....wide.ad.jp" <wcang@....wide.ad.jp>
Subject: Re: sky2 hw csum failure

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

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