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

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Date:	Wed, 21 Oct 2009 15:45:12 +0200
From:	Thomas Schlichter <thomas.schlichter@....de>
To:	Ingo Molnar <mingo@...e.hu>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	dri-devel@...ts.sourceforge.net,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"H. Peter Anvin" <hpa@...or.com>, jbarnes@...tuousgeek.org,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Tejun Heo <tj@...nel.org>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	x86@...nel.org, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not available

Suresh Siddha wrote:
> I think we can simplify this  by just using mtrr_add_page() and avoid
> all the complexity that comes with mtrr_add_unaligned().
> 
> pci_mmap_range() should call one mtrr_add_page() if the base and size
> are nicely aligned. Almost all the cases, this is the usage sequence
> here anyway.

Ingo Molnar wrote:
> Can we eliminate mtrr_add_unaligned() as Suresh suggested, and still 
> make it work on your testbox?

Yes, I had that in the first place, but Jan suggested to extend it to also
handle non-aligned, non-power-of-two cases:
  http://marc.info/?l=linux-kernel&m=125541951529918&w=2

So if it's OK for Jan, I'll reduce the functionality again and use mtrr_add()
instead. Btw. this simply means to drop mtrr_add_unaligned(), all the other
parts are still required for reference counting and a proper mtrr_del() on file
close.

Kind regards,
  Thomas
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