netdev - Re: [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP

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Date:	Tue, 14 May 2013 14:48:43 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	David Miller <davem@...emloft.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	<linux-net-drivers@...arflare.com>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	<linux-s390@...r.kernel.org>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to
 NET_IP_ALIGN

On Tue, 2013-05-14 at 09:32 +0100, David Laight wrote:
> > The two architectures that define CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> > (powerpc and x86) now both define NET_IP_ALIGN as 0, so there is no
> > need for this optimisation any more.
> 
> Hmmm.... even on x86 there will be a measurable cost
> in misaligned accesses - at least for some workloads.

When they cross cache-line boundaries, yes.

Maybe napi_get_frags() should be adding the 2 byte offset on all
architectures, as the skbs it allocates are never RX DMA buffers.

> If the DMA is able to write to a mis-aligned buffer and
> still perform aligned burst transfers mid-frame then
> 4n+2 aligning the rx buffer should be a win even on x86.

I don't think so.

> Note to hardware engineers: add an option to write two
> bytes of junk before the rx data :-)

There is some hardware with that option.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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