linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-Id: <0e3d1956b0b85feb2bdbdba07fad6a08@kernel.crashing.org>
Date:	Tue, 21 Aug 2007 16:39:15 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	Christoph Lameter <clameter@....com>,
	Paul Mackerras <paulus@...ba.org>, 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

>>>> And no, RMW on MMIO isn't "problematic" at all, either.
>>>>
>>>> An RMW op is a read op, a modify op, and a write op, all rolled
>>>> into one opcode.  But three actual operations.
>>>
>>> Maybe for some CPUs, but not all.  ARM for instance can't use the
>>> load exclusive and store exclusive instructions to MMIO space.
>>
>> Sure, your CPU doesn't have RMW instructions -- how to emulate
>> those if you don't have them is a totally different thing.
>
> Let me say it more clearly: On ARM, it is impossible to perform atomic
> operations on MMIO space.

It's all completely beside the point, see the other subthread, but...

Yeah, you can't do LL/SC to MMIO space; ARM isn't alone in that.
You could still implement atomic operations on MMIO space by taking
a lock elsewhere, in normal cacheable memory space.  Why you would
do this is a separate question, you probably don't want it :-)


Segher

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