From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org ([203.10.76.45]:39949 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752140AbXHUJdy (ORCPT ); Tue, 21 Aug 2007 05:33:54 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <18122.45437.807239.395869@cargo.ozlabs.ibm.com> Date: Tue, 21 Aug 2007 19:33:49 +1000 From: Paul Mackerras Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures In-Reply-To: <20070821070555.GA32036@flint.arm.linux.org.uk> References: <46C516BA.60700@yahoo.com.au> <20070817235912.GA24314@linux.vnet.ibm.com> <20070818000913.GA25585@gondor.apana.org.au> <20070818010818.GQ8464@linux.vnet.ibm.com> <46C997B1.1010800@redhat.com> <417ebba299a7ad3c4b7a31c4f860a814@kernel.crashing.org> <20070820224859.GA16162@flint.arm.linux.org.uk> <2bdb04581125f22122f1d230e991ea92@kernel.crashing.org> <20070821070555.GA32036@flint.arm.linux.org.uk> Sender: linux-arch-owner@vger.kernel.org To: Russell King Cc: Segher Boessenkool , Christoph Lameter , heiko.carstens@de.ibm.com, horms@verge.net.au, linux-kernel@vger.kernel.org, "Paul E. McKenney" , ak@suse.de, netdev@vger.kernel.org, cfriesen@nortel.com, akpm@linux-foundation.org, rpjday@mindspring.com, Nick Piggin , linux-arch@vger.kernel.org, jesper.juhl@gmail.com, satyam@infradead.org, zlynx@acm.org, schwidefsky@de.ibm.com, Chris Snook , Herbert Xu , davem@davemloft.net, Linus Torvalds , wensong@linux-vs.org, wjiang@resilience.com List-ID: Russell King writes: > Let me say it more clearly: On ARM, it is impossible to perform atomic > operations on MMIO space. Actually, no one is suggesting that we try to do that at all. The discussion about RMW ops on MMIO space started with a comment attributed to the gcc developers that one reason why gcc on x86 doesn't use instructions that do RMW ops on volatile variables is that volatile is used to mark MMIO addresses, and there was some uncertainty about whether (non-atomic) RMW ops on x86 could be used on MMIO. This is in regard to the question about why gcc on x86 always moves a volatile variable into a register before doing anything to it. So the whole discussion is irrelevant to ARM, PowerPC and any other architecture except x86[-64]. Paul.