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    SubjectRe: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*
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    Hi Christoph,

    On Thu, Nov 29, 2018 at 05:57:15PM +0100, Christoph Hellwig wrote:
    >
    > As for the buffer sharing: at least for the DMA API side I want to
    > move the current buffer sharing users away from dma_alloc_coherent
    > (and coherent dma_alloc_attrs users) and the remapping done in there
    > required for non-coherent architectures. Instead I'd like to allocate
    > plain old pages, and then just dma map them for each device separately,
    > with DMA_ATTR_SKIP_CPU_SYNC passed for all but the first user to map
    > or last user to unmap. On the iommu side it could probably work
    > similar.
    >
    > I have done some preliminary work on this, and want to get it into this
    > merge window, but there is a few other bits I need to sort out first.
    >

    This sounds very useful for ion, to avoid CPU cache maintenance as
    long as the buffer stays in device-land.

    One question though: How would you determine "the last user to unmap"
    to know when to do the final "make visible to CPU" step?

    Thanks,
    -Brian

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