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

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Date:	Mon, 19 Oct 2009 10:16:06 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Thomas Schlichter" <thomas.schlichter@....de>
Cc:	"Jeremy Fitzhardinge" <jeremy.fitzhardinge@...rix.com>,
	"Robert Hancock" <hancockrwd@...il.com>,
	"Henrique de Moraes Holschuh" <hmh@....eng.br>,
	"Suresh Siddha" <suresh.b.siddha@...el.com>,
	"Venkatesh Pallipadi" <venkatesh.pallipadi@...el.com>,
	"Tejun Heo" <tj@...nel.org>, <x86@...nel.org>,
	"Yinghai Lu" <yinghai@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Arjan van de Ven" <arjan@...ux.intel.com>,
	<dri-devel@...ts.sourceforge.net>,
	"Ingo Molnar" <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<jbarnes@...tuousgeek.org>,
	"Thomas Hellstrom" <thellstrom@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not
	 available

>>> Thomas Schlichter <thomas.schlichter@....de> 17.10.09 21:48 >>>
>Patches 0001-0003 introduce functionality/changes to the MTRR and sysfs 
>subsystems. Patch 0004 is the main patch which sets up the MTRR entries when 
>pci memory regions are mmapped. The Patches 0005-0006 also change ioremap() 
>and set_mempry_wc() to set up MTRR entries. These two are completely optional, 
>especially and there is currently no way to automatically remove MTRR entries 
>set up with them.
>
>What do you think about these patches?

Functionality-wise this looks fine to me; whether the core sysfs changes
are acceptable I can't judge, though.

However, I would recommend folding the last two arguments of
mtrr_add_unaligned(), i.e. use mtrr_usage != NULL as the increment
argument passed to mtrr_add().

And patches 5 and 6 would apparently not build right now, as they're
failing to pass the new 5th argument to mtrr_add_unaligned().

Also, why do you add x86-specific code to drivers/pci-sysfs.c when the
function called there (pci_mmap_page_range()) already is arch-specific?
Moving your addition there would also allow covering the related
(legacy?) procfs based functionality... pci_release() could also become
arch-specific, with a fall-back definition to NULL.

Additionally, I would suggest making those code portions depend on
CONFIG_X86_MTRR rather than CONFIG_X86, so that you don't
needlessly (try to) allocate the mtrr_usage vector.

Jan

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