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    SubjectRe: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")
    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@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).

    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.)
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