netdev - Re: [PATCH 0/3] x86_64, sfc: 128-bit memory-mapped I/O

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Message-ID: <1345599783.2659.82.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 22 Aug 2012 02:43:03 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, <netdev@...r.kernel.org>,
	<linux-net-drivers@...arflare.com>, <x86@...nel.org>
Subject: Re: [PATCH 0/3] x86_64, sfc: 128-bit memory-mapped I/O

On Tue, 2012-08-21 at 18:38 -0700, H. Peter Anvin wrote:
> On 08/21/2012 06:17 PM, Ben Hutchings wrote:
> > Current Solarflare network controllers have 128-bit memory-mapped
> > registers which are normally accessed through a series of I/O
> > operations.  However, it is also possible to access them with a single
> > MOVAPS instruction on x86_64, and this is measurably faster as it
> > requires only one PCIe transaction.
> 
> Also, have you considered doing this with write combining instead?

We tried it, and it goes horribly wrong.  On some systems, the writes
are not combined, but they are reordered in a way the hardware doesn't
support.  See the comment at the top of drivers/net/ethernet/sfc/io.h.

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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