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    SubjectRe: [PATCH v2 1/3] dmaengine: add new dma API for max_segment_number
    On Mon, Jun 06, 2011 at 05:06:03PM +0900, FUJITA Tomonori wrote:
    > max_segs isn't unrelated with the dma mapping API. I explained above,
    > IOMMUs doesn't increase the number of segments (could decrease the
    > number of segments by merging).
    > The limitation about the number of segment already lives elsewhere
    > (e.g. queue's limits.max_segments).

    I think you're missing the point entirely.

    Lets take the problem at hand: you have two devices. One of them is
    handled by the DMA engine code. One of them is a block device.

    The block layer needs to know the various parameters of what is
    allowable for DMA, including such things as the maximum size of a
    segment, and the _number_ of segments that can be placed into any
    one request.

    As the DMA provider is _entirely_ separate and unknown to the block
    device driver, the block device driver has no way to sanely provide
    these parameters to the block layer - they are not a property of the
    block device driver, but of the DMA provider.

    However, the DMA provider can't know that it'll be interacting with
    the block layer, so there's no way for the DMA provider to tell the
    block layer about its parameters.

    The only way this can happen is for there to be some way for the DMA
    provider to export this information to the block device driver, so
    the block device driver can then inform its upper layers what the DMA
    capabilities are.

    So to say that "it lives elsewhere" is completely missing the problem
    trying to be addressed by these patches.

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