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

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Message-ID: <20070426054333.GN5217@mellanox.co.il>
Date:	Thu, 26 Apr 2007 08:43:33 +0300
From:	"Michael S. Tsirkin" <mst@....mellanox.co.il>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Roland Dreier <rdreier@...co.com>, Andi Kleen <ak@...e.de>,
	linux-kernel@...r.kernel.org,
	Jack Morgenstein <jackm@...lanox.co.il>
Subject: Re: pgprot_writecombine() and PATs on x86

> So in general the pci prefetchable attribute means write-combining as
> well as prefetching is safe.  A sane BIOS will allocate prefetchable
> BARS contiguously in the address space.  So on a good day you
> can just use one MTRR to map all of the prefetchable BARs as write-combining.

Good point, and sounds easy enough.
So why does not linux do it automatically then where possible?

There are sure to be some broken devices, but if some device
can't live with WC, we can always disable WC system-wide.

-- 
MST
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