From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jes Sorensen Subject: Re: MMIO and gcc re-ordering issue Date: Fri, 30 May 2008 11:36:20 +0200 Message-ID: <483FCA94.1060506@sgi.com> References: <1211852026.3286.36.camel@pasglop> <20080526.184047.88207142.davem@davemloft.net> <1211854540.3286.42.camel@pasglop> <20080526.192812.184590464.davem@davemloft.net> <20080526204233.75b71bb8@infradead.org> <1211872130.3286.64.camel@pasglop> <1211922696.3286.82.camel@pasglop> <1212073289.3428.30.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from relay1.sgi.com ([192.48.171.29]:39361 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751160AbYE3Jg1 (ORCPT ); Fri, 30 May 2008 05:36:27 -0400 In-Reply-To: <1212073289.3428.30.camel@localhost.localdomain> Sender: linux-arch-owner@vger.kernel.org List-ID: To: James Bottomley Cc: Roland Dreier , benh@kernel.crashing.org, Arjan van de Ven , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, tpiepho@freescale.com, linuxppc-dev@ozlabs.org, scottwood@freescale.com, torvalds@linux-foundation.org, David Miller , alan@lxorguk.ukuu.org.uk, Jeremy Higdon James Bottomley wrote: >> The only way to guarantee ordering in the above setup, is to either >> make writel() fully ordered or adding the mmiowb()'s inbetween the two >> writel's. On Altix you have to go and read from the PCI brige to >> ensure all writes to it have been flushed, which is also what mmiowb() >> is doing. If writel() was to guarantee this ordering, it would make >> every writel() call extremely expensive :-( > > So if a read from the bridge achieves the same effect, can't we just put > one after the writes within the spinlock (an unrelaxed one). That way > this whole sequence will look like a well understood PCI posting flush > rather than have to muck around with little understood (at least by most > driver writers) io barriers? Hmmm, I think mmiowb() does some sort of status read from the bridge, I am not sure if it's enough to just do a regular readl(). I'm adding Jeremy to the list, he should know for sure. Cheers, Jes