Messages in this thread | | | Date | Tue, 25 Aug 2009 08:53:29 -0400 | 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 | Steven Walter <> |
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On Thu, Aug 6, 2009 at 6:25 PM, Russell King - ARM Linux<linux@arm.linux.org.uk> wrote: [...] > As far as userspace DMA coherency, the only way you could do it with > current kernel APIs is by using get_user_pages(), creating a scatterlist > from those, and then passing it to dma_map_sg(). While the device has > ownership of the SG, userspace must _not_ touch the buffer until after > DMA has completed. [...]
Would that work on a processor with VIVT caches? It seems not. In particular, dma_map_page uses page_address to get a virtual address to pass to map_single(). map_single() in turn uses this address to perform cache maintenance. Since page_address() returns the kernel virtual address, I don't see how any cache-lines for the userspace virtual address would get invalidated (for the DMA_FROM_DEVICE case).
If that's true, then what is the correct way to allow DMA to/from a userspace buffer with a VIVT cache? If not true, what am I missing?
Thanks -- -Steven Walter <stevenrwalter@gmail.com> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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