linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Message-Id: <1392906098@web.de>
Date:	Mon, 19 Oct 2009 16:59:57 +0200
From:	Thomas Schlichter <thomas.schlichter@....de>
To:	Jan Beulich <JBeulich@...ell.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	arjan@...ux.intel.com, dri-devel@...ts.sourceforge.net,
	hancockrwd@...il.com, hmh@....eng.br, hpa@...or.com,
	jbarnes@...tuousgeek.org, jeremy.fitzhardinge@...rix.com,
	linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	thellstrom@...are.com, tj@...nel.org,
	venkatesh.pallipadi@...el.com, x86@...nel.org, yinghai@...nel.org
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not available

Suresh Siddha wrote:
> If we are going to make ioremap() and set_memory_wc() add mtrr's in
> non-pat case, then we need to delete the added mtrr(s) in the
> corresponding iounmap() and set_memory_wb() aswell.
> 
> hmm, this is becoming too complex. The way i915 and other graphics
> drivers are using set_memory_wc(), it is def a bad idea to start adding
> mtrr's behind the back for non-pat case.

Yes, maybe it's better to drop it for ioremap() and set_memory_wc(). But I'd like
to keep it for mmapping the PCI region. It should help all the people with
PAT-incapable CPUs and graphics chips without DRM support (for them there
simply is no driver that should set up the MTRR entries...).

> Can't we just force PAT option always and we probably don't care about
> ioremap_wc() on processors were PAT doesn't get enabled because of known
> errata.

I don't think this is a good idea, Robert Hancock wrote there may be millions of
such Laptops (Core Solo/Duo erratum AE7, Pentium M erratum Y31) :
  http://marc.info/?l=linux-kernel&m=125537136105246&w=2

> or Perhaps just try to add mtrr only for the pci mmap case like the 4th
> patch in this series..

I'd prefer this! ;-)

Kind regards,
  Thomas
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