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Message-Id: <33412cad5894a9cbdc85482db5e9a0a0@kernel.crashing.org> Date: Tue, 21 Aug 2007 16:48:51 +0200 From: Segher Boessenkool <segher@...nel.crashing.org> To: Paul Mackerras <paulus@...ba.org> Cc: Russell King <rmk+lkml@....linux.org.uk>, Christoph Lameter <clameter@....com>, heiko.carstens@...ibm.com, horms@...ge.net.au, linux-kernel@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, ak@...e.de, netdev@...r.kernel.org, cfriesen@...tel.com, akpm@...ux-foundation.org, rpjday@...dspring.com, Nick Piggin <nickpiggin@...oo.com.au>, linux-arch@...r.kernel.org, jesper.juhl@...il.com, satyam@...radead.org, zlynx@....org, schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>, Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net, Linus Torvalds <torvalds@...ux-foundation.org>, wensong@...ux-vs.org, wjiang@...ilience.com Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures >> 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. This question is GCC PR33102, which was incorrectly closed as a duplicate of PR3506 -- and *that* PR was closed because its reporter seemed to claim the GCC generated code for an increment on a volatile (namely, three machine instructions: load, modify, store) was incorrect, and it has to be one machine instruction. > So the whole discussion is irrelevant to ARM, PowerPC and any other > architecture except x86[-64]. And even there, it's not something the kernel can take advantage of before GCC 4.4 is in widespread use, if then. Let's move on. Segher - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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