Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Mon, 2010-10-18 at 16:27 +0100, Catalin Marinas wrote: >> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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. > > 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). The M3 may have a local exclusive monitor (like the main CPU) but it isn't cleared by memory accesses from the main CPU. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
