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    SubjectRe: [Linaro-mm-sig] [RFC v3 0/2] Introduce DMA buffer sharing mechanism
    On Tue, Dec 20, 2011 at 2:20 PM, Dave Airlie <> wrote:
    >>> This is RFC v3 for DMA buffer sharing mechanism - changes from v2 are in the
    >>> changelog below.
    >>> Various subsystems - V4L2, GPU-accessors, DRI to name a few - have felt the
    >>> need to have a common mechanism to share memory buffers across different
    >>> devices - ARM, video hardware, GPU.
    >>> This need comes forth from a variety of use cases including cameras, image
    >>> processing, video recorders, sound processing, DMA engines, GPU and display
    >>> buffers, and others.
    >>> This RFC is an attempt to define such a buffer sharing mechanism- it is the
    >>> result of discussions from a couple of memory-management mini-summits held by
    >>> Linaro to understand and address common needs around memory management. [1]
    >>> A new dma_buf buffer object is added, with operations and API to allow easy
    >>> sharing of this buffer object across devices.
    >>> The framework allows:
    >>> - a new buffer-object to be created with fixed size.
    >>> - different devices to 'attach' themselves to this buffer, to facilitate
    >>>   backing storage negotiation, using dma_buf_attach() API.
    >>> - association of a file pointer with each user-buffer and associated
    >>>    allocator-defined operations on that buffer. This operation is called the
    >>>    'export' operation.
    >>> - this exported buffer-object to be shared with the other entity by asking for
    >>>    its 'file-descriptor (fd)', and sharing the fd across.
    >>> - a received fd to get the buffer object back, where it can be accessed using
    >>>    the associated exporter-defined operations.
    >>> - the exporter and user to share the scatterlist using map_dma_buf and
    >>>    unmap_dma_buf operations.
    >>> Documentation present in the patch-set gives more details.
    >>> This is based on design suggestions from many people at the mini-summits,
    >>> most notably from Arnd Bergmann <>, Rob Clark <> and
    >>> Daniel Vetter <>.
    >>> The implementation is inspired from proof-of-concept patch-set from
    >>> Tomasz Stanislawski <>, who demonstrated buffer sharing
    >>> between two v4l2 devices. [2]
    >>> References:
    >>> [1]:
    >>> [2]:
    >>> Patchset based on top of 3.2-rc3, the current version can be found at
    >>> Branch: dma-buf-upstr-v2
    >>> Earlier versions:
    >>> v2 at:
    >>> v1 at:
    >>> Best regards,
    >>> ~Sumit Semwal
    >> I think this is a really good v1 version of dma_buf. It contains all the
    >> required bits (with well-specified semantics in the doc patch) to
    >> implement some basic use-cases and start fleshing out the integration with
    >> various subsystem (like drm and v4l). All the things still under
    >> discussion like
    >> - userspace mmap support
    >> - more advanced (and more strictly specified) coherency models
    >> - and shared infrastructure for implementing exporters
    >> are imo much clearer once we have a few example drivers at hand and a
    >> better understanding of some of the insane corner cases we need to be able
    >> to handle.
    >> And I think any risk that the resulting clarifications will break a basic
    >> use-case is really minimal, so I think it'd be great if this could go into
    >> 3.3 (maybe as some kind of staging/experimental infrastructure).
    >> Hence for both patches:
    >> Reviewed-by: Daniel Vetter <>
    > Yeah I'm with Daniel, I like this one, I can definitely build the drm
    > buffer sharing layer on top of this.
    > How do we see this getting merged? I'm quite happy to push it to Linus
    > if we don't have an identified path, though it could go via a Linaro
    > tree as well.
    > so feel free to add:
    > Reviewed-by: Dave Airlie <>

    fwiw, patches to share buffers between drm and v4l2 are here:

    (need a bit of cleanup before the vb2 patches are submitted.. but that
    is unrelated to the dmabuf patches)


    Reviewed-and-Tested-by: Rob Clark <>

    > Dave.
    > --
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