[PATCH 0/3] Add OMAP hardware spinlock misc driver

[Catalin Marinas]

Peter Zijlstra <peterz at infradead.org> wrote:
> On Mon, 2010-10-18 at 14:35 +0100, Russell King - ARM Linux wrote:
>> In any case, Linux's spinlock API (or more accurately, the ARM exclusive
>> access instructions) relies upon hardware coherency support (a piece of
>> hardware called an exclusive monitor) which isn't present on the M3 nor
>> DSP processors.  So there's no way to ensure that updates from the M3
>> and DSP are atomic wrt the A9 updates.
>
> Right, so the problem is that there simply is no way to do atomic memory
> access from these auxiliary processing units wrt the main CPU? Seeing as
> they operate on the same memory space, wouldn't it make sense to have
> them cache-coherent and thus provide atomicy guarantees through that?

With cache coherency you may get atomicity of writes or reads but
usually not atomic modifications.

-- 
Catalin