To: Jes Sorensen <jes@sgi.com>
Cc: "Bryan O'Sullivan" <bos@pathscale.com>, Andrew Morton <akpm@osdl.org>,
       Andi Kleen <ak@suse.de>, linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining
References: <1140841250.2587.33.camel@localhost.localdomain>
	<yq08xrvhkee.fsf@jaguar.mkp.net>
From: Roland Dreier <rdreier@cisco.com>
Date: Tue, 28 Feb 2006 07:42:52 -0800
In-Reply-To: <yq08xrvhkee.fsf@jaguar.mkp.net> (Jes Sorensen's message of "28
 Feb 2006 05:01:29 -0500")
Message-ID: <adar75nlcar.fsf@cisco.com>
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    Jes> Could you explain why the current mmiowb() API won't suffice
    Jes> for this?  It seems that this is basically trying to achieve
    Jes> the same thing.

I don't believe mmiowb() is at all the same thing.  mmiowb() is all
about ordering writes between _different_ CPUs without incurring the
cost of flushing posted writes by issuing a read on the bus.  wc_wmb()
would just act like a true wmb(), even when using write-combining
regions on x86 -- in other words, there would be no cross-CPU synchronization.

 - R.
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