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    SubjectRe: [PATCH] dma-buf: mmap support
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    On Wednesday 18 April 2012, Daniel Vetter wrote:
    > + Because existing importing subsystems might presume coherent mappings for
    > + userspace, the exporter needs to set up a coherent mapping. If that's not
    > + possible, it needs to fake coherency by manually shooting down ptes when
    > + leaving the cpu domain and flushing caches at fault time. Note that all the
    > + dma_buf files share the same anon inode, hence the exporter needs to replace
    > + the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
    > + requred. This is because the kernel uses the underlying inode's address_space
    > + for vma tracking (and hence pte tracking at shootdown time with
    > + unmap_mapping_range).
    > +
    > + If the above shootdown dance turns out to be too expensive in certain
    > + scenarios, we can extend dma-buf with a more explicit cache tracking scheme
    > + for userspace mappings. But the current assumption is that using mmap is
    > + always a slower path, so some inefficiencies should be acceptable.
    > +
    > + Exporters that shoot down mappings (for any reasons) shall not do any
    > + synchronization at fault time with outstanding device operations.
    > + Synchronization is an orthogonal issue to sharing the backing storage of a
    > + buffer and hence should not be handled by dma-buf itself. This is explictly
    > + mentioned here because many people seem to want something like this, but if
    > + different exporters handle this differently, buffer sharing can fail in
    > + interesting ways depending upong the exporter (if userspace starts depending
    > + upon this implicit synchronization).

    How do you ensure that no device can do DMA on the buffer while it's mapped
    into user space in a noncoherent manner?

    Arnd


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