linux-kernel - Re: [RFC] MMIO accessors & barriers documentation

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Message-ID: <m1pse17hzi.fsf@ebiederm.dsl.xmission.com>
Date:	Mon, 11 Sep 2006 23:48:33 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jeff Garzik <jgarzik@...ox.com>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [RFC] MMIO accessors & barriers documentation

Alan Cox <alan@...rguk.ukuu.org.uk> writes:

>> > "Except where the underlying device is marked as cachable or
>> > prefetchable"
>> 
>> You aren't supposed to use MMIO accessors on cacheable memory, are you ?
>
> Why not. Providing it is in MMIO space, consider ROMs for example or
> write path consider frame buffers.

Frame buffers are rarely cachable as such, on x86 they are usually
write-combining.   Which means that the writes can be merged and
possibly reordered while they are being written but they can't be
cached.  Most arches I believe have something that roughly corresponds
to write combining.

Ensuring we can still use this optimization to mmio space is
moderately important.

Eric



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