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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-10-18 at 16:51 +0100, Catalin Marinas wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, 2010-10-18 at 16:27 +0100, Catalin Marinas wrote:
> >> Peter Zijlstra <peterz@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.

Right, so you forgot the qualifying part of your stmt: on ARM.

> > Sure, but you can 'easily' extend your coherency protocols with support
> > for things like ll/sc (or larger transactions).
> >
> > Have ll bring the cacheline into exclusive state and tag it, then
> > anything that demotes the cacheline will clear the tag and make sc fail.
> 
> For the ll/sc operations on ARM (exclusive load/store) there is a
> per-CPU local exclusive monitor and a (virtual) global one. The global
> one may either be a separate piece of hardware or emulated via cache
> lines as you said. 

> But if you need synchronisation with a CPU (or DSP)
> like Cortex-M3 which doesn't have any built-in caches, you can only get
> atomic operations on the main processor (A9) but not on the M3 (as you
> can't have a cache line in exclusive state on the M3).

Right, and I take it that modifying the M3 to participate in the full
coherency/exclusive monitor thing would have been more work.

> The M3 may have a local exclusive monitor (like the main CPU) but it
> isn't cleared by memory accesses from the main CPU.

Sounds like asking for trouble if you ask me ;-)