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. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
