Speeding up dma_unmap

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

On Wednesday 27 January 2016 00:32:56 Jason Holt wrote:

quoted

Failing that, I suppose a very dirty hack would be to
data_cache_clean_and_invalidate if the only thing I cared about was
getting data from my DMA peripheral as fast as possible.  (I'm on
AM335X and seeing no more than 200MB/s from device to CPU with
dma_unmap_single, whereas the PRUs can write to main memory at
600MB/s.)

On your Cortex-A8, we could come up with a way to not invalidate
the cache at all on unmap, as the comment in __dma_page_dev_to_cpu()
says:

        /* FIXME: non-speculating: not required */
        /* in any case, don't bother invalidating if DMA to device */
        if (dir != DMA_TO_DEVICE) {
                outer_inv_range(paddr, paddr + size);

                dma_cache_maint_page(page, off, size, dir, dmac_unmap_area);
        }

We already do a cache-invalidate operation on dma_map(), and the kernel
is not allowed to access the memory in the meantime. On CPU cores
that do speculative prefetching (Cortex-A9 and higher), we may end

                                   ^^^^^^^^^^^^^^^^^^^^
I would say "Cortex-A9 and newer".

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.

-- 
Catalin