On Wednesday 26 August 2009 00:02:48 David Xiao wrote: > On Tue, 2009-08-25 at 05:53 -0700, Steven Walter wrote: > > On Thu, Aug 6, 2009 at 6:25 PM, Russell King - ARM > > Linux<linux@xxxxxxxxxxxxxxxx> 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? > > page_address() is basically returning page->virtual, which records the > virtual/physical mapping for both user/kernel space; and what only > matters there is highmem or not. I'm not sure to get it. Are you implying that a physical page will then be mapped to the same address in all contexts (kernelspace and userspace processes) ? Is that even possible ? And if not, how could page->virtual store both the initial kernel map and all the userspace mappings ? -- Laurent Pinchart -- 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
