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

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Message-ID: <m1d5a17g5u.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 12 Sep 2006 00:27:57 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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

Benjamin Herrenschmidt <benh@...nel.crashing.org> writes:

> I've not gone too much in details about write combining (we need to do
> something about it but I don't want to mix problems) but I did define
> that the ordered accessors aren't guaranteed to provide write combining
> on storage mapped with WC enabled while the relaxed or non ordered ones
> are. That should be enough at this point.

Sounds good.

> Later, we should look into providing an ioremap_wc() and possibly page
> table flags for write combining userland mappings. Time to get rid of
> MTRRs for graphics :) And infiniband-style stuff seems to want that too.

ioremap_wc is actually the easy half.  I have an old patch that handles
that.  The trick is to make certain multiple people don't map the same
thing with different attributes.  Unfortunately I haven't had time to
work through that one yet.

Eric

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