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

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Date:	Wed, 14 Oct 2009 09:13:40 +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> 13.10.09 23:29 >>>
>Jan Beulich wrote:
>> >>> Thomas Schlichter <thomas.schlichter@....de> 12.10.09 20:32 >>>
>> >+	if (!pat_enabled) {
>> >+		void __iomem *ret = ioremap_nocache(phys_addr, size);
>> >+		if (ret)
>> >+			mtrr_add(phys_addr, size, MTRR_TYPE_WRCOMB, false);
>> 
>> This won't work if phys_addr or size aren't page aligned, or if size isn't
>> a power of two.
>
>No, at least the comments in mtrr_add and mtrr_check state that it is just 
>required that phys_addr and size are multiple of PAGE_SIZE. And I'm not sure 
>if it is always safe to round these up/down to the next PAGE boundary. If it 
>is not, maybe it is better to fail...

That function isn't the limiting factor, generic_validate_add_page() is
what you need to look at (plus the spec on how the MTRR ranges are
calculated by the CPU from the base/mask register pairs).

Jan

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