On Tue, 2009-08-11 at 02:31 -0700, Catalin Marinas wrote: > On Thu, 2009-08-06 at 22:59 -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. > > Theoretically, with speculative prefetching on ARMv7 and the FROM_DEVICE > case we need to invalidate the corresponding D-cache lines both before > and after the DMA transfer, i.e. in both dma_map_sg and dma_unmap_sg, > otherwise there is a risk of stale data in the cache. > The dma_map_sg() code is already calling dma_cache_maint() to invalidate the cache lines in the DMA_FROM_DEVICE/DMA_BIDIRECTIONAL direction cases. And the suggestion was to do something similar in dma_unmap_sg() case to deal with the speculative prefetching on ARMv7, and Russel has other postings talking about the details of this in terms of feasibility/etc. Furthermore, duplicate MMU mappings in the kernel bring more twists to this problem as explained in this email chain as well, especially in the case of DMA-coherent memory (dma_alloc_coherent()). David -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
