On Tue, Aug 25, 2009 at 08:53:29AM -0400, 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). You are correct. > 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? I don't think you read what I said (but I've also forgotten what I did say). To put it simply, the kernel does not support DMA direct from userspace pages. Solutions which have been proposed in the past only work with a sub-set of conditions (such as the one above only works with VIPT caches.) -- 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
