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SubjectRe: ioremap_nocache problem?
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Timur Tabi <ttabi@interactivesi.com> writes:
> ** Reply to message from Roman Zippel <zippel@fh-brandenburg.de> on
> Tue, 23 Jan 2001 19:12:36 +0100 (MET)
> > ioremap creates a new mapping that shouldn't interfere with MTRR,
> >whereas you can map a MTRR mapped area into userspace. But I'm not
> >sure if it's correct that no flag is set for boot_cpu_data.x86 <=
> >3...
>
> I was under the impression that the "don't cache" bit that
> ioremap_nocache sets overrides any MTRR.

Nope. There's a table explaining how page flags and MTRRs interact in
the Intel x86 manual, volume 3 (it's in section 9.5.1 "Precedence of
Cache Controls" in the fairly recent edition I have here).

For example, with PCD set, PWT clear, and the MTRRs saying WC, the
effective memory type is WC. In addition, there's a note saying this
may change in future models. So you have to set PCD | PWT if you want
to get uncached in all cases.


David Wragg
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