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    SubjectRe: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access
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    On Fri, Mar 2, 2012 at 23:53, Rob Clark <rob.clark@linaro.org> wrote:
    > nitially the expectation was that userspace would not pass a buffer
    > to multiple subsystems for writing (or that if it did, it would get
    > the undefined results that one could expect)..  so dealing w/
    > synchronization was punted.

    Imo synchronization should not be part of the dma_buf core, i.e.
    userspace needs to ensure that access is synchronized.
    begin/end_cpu_access are the coherency brackets (like map/unmap for
    device dma). And if userspace asks for a gun and some bullets, the
    kernel should just deliver. Even in drm/i915 gem land we don't (and
    simply can't) make any promises about concurrent reads/writes/ioctls.

    > I expect, though, that one of the next steps is some sort of
    > sync-object mechanism to supplement dmabuf

    Imo the only reason to add sync objects as explicit things is to make
    device-to-device sync more efficient by using hw semaphores and
    signalling lines. Or maybe a quick irq handler in the kernel that
    kicks of the next device. I don't think we should design these to make
    userspace simpler.

    Cheers, Daniel
    --
    Daniel Vetter
    daniel.vetter@ffwll.ch - +41 (0) 79 365 57 48 - http://blog.ffwll.ch
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