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

On Mon, Aug 20, 2007 at 05:05:18PM -0700, Paul E. McKenney wrote:

On Tue, Aug 21, 2007 at 01:02:01AM +0200, Segher Boessenkool wrote:

quoted

quoted

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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.

I thought that ARM's load exclusive and store exclusive instructions
were its equivalent of LL and SC, which RISC machines typically use to
build atomic sequences of instructions -- and which normally cannot be
applied to MMIO space.

Absolutely correct.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of: