Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755667Ab1ESFiz (ORCPT ); Thu, 19 May 2011 01:38:55 -0400 Received: from gate.crashing.org ([63.228.1.57]:33395 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753277Ab1ESFix (ORCPT ); Thu, 19 May 2011 01:38:53 -0400 Subject: Re: [PATCH 1/3] mpt2sas: remove the use of writeq, since writeq is not atomic From: Benjamin Herrenschmidt To: Roland Dreier Cc: Milton Miller , Hitoshi Mitake , Sam Ravnborg , Ingo Molnar , Ingo Molnar , "Desai, Kashyap" , "Prakash, Sathya" , James Bottomley , Matthew Wilcox , linux scsi dev , "paulus@samba.org" , linux powerpc dev , linux pci , linux kernel , linux-arch In-Reply-To: References: <20110504115324.GE17855@lsi.com> <1305616571.6008.23.camel@mulgrave.site> <20110518041551.GL15227@parisc-linux.org> <1305692584.2580.3.camel@mulgrave.site> <1305702010.2781.33.camel@pasglop> <4565AEA676113A449269C2F3A549520F80B66280@cosmail03.lsi.com> Content-Type: text/plain; charset="UTF-8" Date: Thu, 19 May 2011 15:34:02 +1000 Message-ID: <1305783242.7481.42.camel@pasglop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1371 Lines: 33 On Wed, 2011-05-18 at 21:16 -0700, Roland Dreier wrote: > On Wed, May 18, 2011 at 11:31 AM, Milton Miller wrote: > > So the real question should be why is x86-32 supplying a broken writeq > > instead of letting drivers work out what to do it when needed? > > Sounds a lot like what I was asking a couple of years ago :) > http://lkml.org/lkml/2009/4/19/164 > > But Ingo insisted that non-atomic writeq would be fine: > http://lkml.org/lkml/2009/4/19/167 Yuck... Ingo, I think that was very wrong. Those are for MMIO, which must almost ALWAYS know precisely what the resulting access size is going to be. It's not even about atomicity between multiple CPUs. I have seen plenty of HW for which a 64-bit access to a register is -not- equivalent to two 32-bit ones. In fact, in some case, you can get the side effects twice ... or none at all. The only case where you can be lax is when you explicitely know that there is no side effects -and- the HW cope with different access sizes. This is not the general case and drivers need at the very least a way to know what the behaviour will be. Cheers, Ben. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/