Messages in this thread | | | Subject | Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?") | From | "David Xiao" <> | Date | Tue, 11 Aug 2009 11:23:10 -0700 |
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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
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