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    SubjectRe: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA
    Hugh Dickins wrote:
    > On Thu, 4 Dec 2008, Peter Zijlstra wrote:
    >> On Wed, 2008-12-03 at 14:39 -0700, edward_estabrook@agilent.com wrote:
    >>> The gist of this implementation is to overload uio's mmap
    >>> functionality to allocate and map a new DMA region on demand. The
    >>> bus-specific DMA address as returned by dma_alloc_coherent is made
    >>> available to userspace in the 1st long word of the newly created
    >>> region (as well as through the conventional 'addr' file in sysfs).
    >>>
    >>> To allocate a DMA region you use the following:
    >>> /* Pass this magic number to mmap as offset to dynamically allocate a
    >>> chunk of memory */ #define DMA_MEM_ALLOCATE_MMAP_OFFSET 0xFFFFF000UL
    >>> ...
    >>> Comments appreciated!
    >> Yuck!
    >>
    >> Why not create another special device that will give you DMA memory when
    >> you mmap it? That would also allow you to obtain the physical address
    >> without this utter horrid hack of writing it in the mmap'ed memory.
    >>
    >> /dev/uioN-dma would seem like a fine name for that.
    >
    > I couldn't agree more. It sounds fine as a local hack for Edward to
    > try out some functionality he needed in a hurry; but as something
    > that should enter the mainline kernel in that form - no.

    Agree with Peter and Hugh here. Also I have a use case where I need to share
    DMA buffers between two or more devices. So I think we need a generic DMA
    device that does operations like alloc, mmap, etc. Mmapped regions can then be
    used with UIO devices.
    I'll put together a prototype of that some time early next year.

    Max






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