netdev - Re: [PATCH 0/7] via-velocity performance fixes

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Date:	Mon, 23 Nov 2009 14:39:22 +0100
From:	Simon Kagstrom <simon.kagstrom@...insight.net>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, davej@...hat.com,
	romieu@...zoreil.com
Subject: Re: [PATCH 0/7] via-velocity performance fixes

Hi Stephen and David!

Thanks for the comments, I'll address these in the next version.

On Fri, 20 Nov 2009 09:03:48 -0800
Stephen Hemminger <shemminger@...tta.com> wrote:
 
> > 1. Correct setting of skipped checksums (unsure about this). The
> >    mainline driver sets CHECKSUM_UNNECESSARY if this is an IP packet
> >    except if the TCP checksum is NOT ok.
> > 
> >    The VIA driver sets CHECKSUM_UNNECESSARY if this is an UDP/TCP
> >    packet except if the TCP checksum is not OK. The patch selects the
> >    VIA behavior.
>
> The mainline driver is already doing the correct thing. 
> The VIA driver would send packet
> with known bad checksum up the stack with CHECKSUM_UNNECESSARY.
> What is supposed to happen is:
> 
> Checksum good:  set CHECKSUM_UNNECESSARY
>           bad:  leave CHECKSUM_NONE

Yes, I see now that mainline is correct and I'll remove the patch from
the upcoming version 2 of the series. However, I don't think the driver
would send bad packets upwards - it would rather be more conservative.
What the current driver does is

	ip_summed = CHECKSUM_NONE;
	if (hardware sees that this is an IP packet):
		if (hw IP checksum is OK):
			if (hw sees that this is a TCP/UDP packet):
				if (hw TCP/UDP checksum is NOT OK):
					return  # Returning CHECKSUM_NONE
			ip_summed = CHECKSUM_UNNECESSARY

while the VIA driver does

	ip_summed = CHECKSUM_NONE;
	if (hardware sees that this is an IP packet):
		if (hw IP checksum is OK):
			if (hw sees that this is a TCP/UDP packet):
				if (hw TCP/UDP checksum is NOT OK):
					return  # Returning CHECKSUM_NONE
				ip_summed = CHECKSUM_UNNECESSARY

i.e., it will return CHECKSUM_NONE also for non-TCP/UDP IP packets.

// Simon
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