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    SubjectRe: dma_sync_sg_for_cpu applied to a single scatterlist element
    On Mon, 15 Mar 2010, James Bottomley wrote:

    > On Mon, 2010-03-15 at 17:30 -0400, Alan Stern wrote:
    > > This is addressed to James Bottomley as he is the author of
    > > Documentation/DMA-API.txt, but anyone else who can contribute is
    > > invited to do so.
    > >
    > > Suppose a scatter-gather transfer with multiple scatterlist elements
    > > has been mapped via dma_map_sg(). Is it then valid to call
    > > dma_sync_sg_for_cpu() with the "sg" argument pointing to one of the
    > > mapped scatterlist elements (not necessarily the first one) and the
    > > "nelems" argument set to 1?
    >
    > It's not the design of the API, but I'm guessing, given the way the API
    > works on most arch's that it will work. However, if you just want a
    > single element sync'd, won't dma_sync_single_for_cpu do that
    > transparently (as in just feed in the address and length from the sg
    > list), without mucking with the sg API?

    On Tue, 16 Mar 2010, FUJITA Tomonori wrote:

    > As James said, probably it works. As long as passed scatterlist
    > elements points to mapped regions, it works.
    >
    > However, I think that the latest DMA-API.txt makes it clear that only
    > dma_sync_single_for_{cpu|device} supports a partial sync. So it's not
    > recommended, I guess.

    What the documentation actually says about the dma_sync_* functions is:

    All the parameters must be the same as those passed into the
    single mapping API.

    So it isn't clear that dma_sync_sg_for_cpu(dev, sg, 1, dir) can be used
    on a mapping created by dma_map_sg(dev, sg, n, dir), and it isn't
    clear that dma_sync_single_for_cpu() can be used on a mapping created
    by dma_map_sg().

    But if you guys say it will work, I'll go ahead and use
    dma_sync_single_for_cpu().

    Thanks,

    Alan Stern



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