Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Aug 06, 2009 at 10:59:26PM -0700, David Xiao wrote:
> The V7 speculative prefetching will then probably apply to DMA coherency
> issue in general, both kernel and user space DMAs. Could this be
> addressed by inside the dma_unmap_sg/single() calling dma_cache_maint()
> when the direction is DMA_FROM_DEVICE/DMA_BIDIRECTIONAL, to basically
> invalidate the related cache lines in case any filled by prefetching?
> Assuming dma_unmap_sg/single() is called after each DMA operation is
> completed. 

It's something that I was going to look at, and it's probably going to
have to be something I do blind - I currently have no MPCore platform,
and even if my Realview EB worked, it doesn't use DMA at all.

However, it's not trivial - the unmap functions don't have all the
necessary information.  dma_unmap_single() has the DMA address, which
we can convert to the original virtual address via dma_to_virt().
However, dma_unmap_page() can't translate back to a virtual page
since we're missing some information there.

It bugs me that the DMA API is restrictive in the information which
architectures can retain across a mapping which makes this non-trivial.
Had I known of these issues when the DMA API was originally being
discussed, I'd have suggested that we have an arch-specific dma_map
struct which could contain whatever information was required, rather
than requiring the driver to maintain the handle/size/direction/etc
between each of the calls.  That would mean we could retain the virtual
address/struct page rather than having to work it back in some way.