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

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