Speeding up dma_unmap

On Wed, Jan 27, 2016 at 06:09:45PM +0000, Russell King - ARM Linux wrote:

On Wed, Jan 27, 2016 at 04:06:30PM +0000, Catalin Marinas wrote:

quoted

On Wed, Jan 27, 2016 at 01:23:27PM +0100, Arnd Bergmann wrote:

quoted

up reading cache lines back in randomly on a speculative prefetch,
but as far as I can tell, the Cortex-A8 (or A5/A7) won't do that.

Are you sure about A5 and A7? I'm not even sure about the A8 but there
are good chances that A7 and A5 do speculative prefetches.

I thought when I was re-implementing the DMA API on ARM (which was
around early v7 times) that there were CPUs that did speculative
prefetching, which included the A8.  I seem to remember it was pretty
urgent to have the DMA API fixed for _any_ ARMv7 CPU because of the
speculative prefetching.

Indeed, it's a safe assumption to say that any ARMv7 CPU perform
speculative accesses. Even if some of them may only do I-cache
prefetching (just guessing), in the presence of a unified L2 this
distinction no longer matters.

-- 
Catalin