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 09:32:18 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Ben Hutchings" <bhutchings@...arflare.com>,
	"David Miller" <davem@...emloft.net>,
	"Heiko Carstens" <heiko.carstens@...ibm.com>,
	"Geert Uytterhoeven" <geert@...ux-m68k.org>
Cc:	<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

> 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.

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.

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

	David