linux-kernel - Re: pgprot_writecombine() and PATs on x86

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Message-Id: <200704252019.28017.ak@suse.de>
Date:	Wed, 25 Apr 2007 20:19:27 +0200
From:	Andi Kleen <ak@...e.de>
To:	Roland Dreier <rdreier@...co.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, mst@...lanox.co.il,
	jackm@...lanox.co.il
Subject: Re: pgprot_writecombine() and PATs on x86

On Wednesday 25 April 2007 20:02:26 Roland Dreier wrote:
> Hi Eric,
> 
> Where do your patches to add an implementation of
> pgprot_writecombine() using PATs on x86 stand?  

It's on my todo list.

> The mlx4 driver I'm 
> planning on merging for 2.6.22 would really like writecombining, and
> I'm interested in doing the work to finally get the PAT stuff merged
> (probably for 2.6.23 I guess).
> 
> Just to give a little background on my motivation: the mlx4 hardware
> allows a page in its PCI space to be mapped, where the driver can write
> descriptors and payloads directly, instead of ringing a doorbell and
> having the HW fetch the descriptor from system memory, for better latency.

When it's PCI space you can likely just use MTRRs. PAT is mostly useful
for applications that do IO with random memory pages

-Andi
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