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

[Segher Boessenkool <segher@kernel.crashing.org>]

> > > > 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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