To: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: "Bryan O'Sullivan" <bos@pathscale.com>, Andi Kleen <ak@suse.de>,
       Andrew Morton <akpm@osdl.org>,
       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>
	<200602251428.01767.ak@suse.de>
	<1140894083.9852.30.camel@localhost.localdomain>
	<200602280944.32210.jbarnes@virtuousgeek.org>
From: Roland Dreier <rdreier@cisco.com>
Date: Tue, 28 Feb 2006 09:50:48 -0800
In-Reply-To: <200602280944.32210.jbarnes@virtuousgeek.org> (Jesse Barnes's
 message of "Tue, 28 Feb 2006 09:44:31 -0800")
Message-ID: <ada7j7fl6dj.fsf@cisco.com>
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    Jesse> I don't think it addresses the flushing issue you seem to
    Jesse> be concerned about though.  I don't know the exact
    Jesse> semantics of sfence, but I think bcrl is likely right that
    Jesse> it won't absolutely guarantee that your writes have hit the
    Jesse> device before proceeding (though it may do that on some CPU
    Jesse> implementations).

Yeah, I think that Bryan just wrote something different than what he
meant: there is no desire for wc_wmb() to make sure that writes via a
write-combining mapping have gone all the way to the device, any more
than a normal wmb() makes sure normal writes have gone all the way to
the device.  All that wc_wmb() needs to do is make sure that writes
via a write-combining mapping don't get passed by later writes.

This does speak to the need for precise documentation though :)

 - R.
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