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    SubjectRe: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel()
    On Mon, 12 Mar 2012, Vinod Koul wrote:

    > On Fri, 2012-03-09 at 13:20 +0100, Guennadi Liakhovetski wrote:
    > > It can be made to work as long as there's only one DMAC group with
    > > configurable channels and all other DMACs are dedicated to specific
    > > peripherals, yes. I don't know whether there are already now or are
    > > approaching any platforms with multiple reconfigurable groups.
    > And that is what I am talking about.
    > Having specific channel mapping given by platform for all channels which
    > are to be used dedicated. And a pool of channels which can be used by
    > anyone (if they can be) on a platform.
    > Does this proposal sound good for others as well. I think we can target
    > this for next merge cycle, we are too late for the current one.

    Ok, let me try to summarise, what this would mean for sh-mobile:

    1. this proposal introduces a new special case: with or without a mapping,
    that will have to be handled in affected client and DMA controller
    drivers. E.g., on sh-mobile some devices might on some systems use
    channels from "general purpose" DMA controllers (no mapping), on other
    systems it will be a dedicated controller (fixed mapping).

    2. this will break, if we get more than 1 "general purpose" type with
    different supported client sets. So, we develop a new API with a
    pre-programmed limitation.

    3. this will mean a substantial driver and platform code modification.
    Nothing super-complex, but still some.

    4. we'll need a 3-stage channel allocation / configuration: request,
    filter, config. Whereas with my configuration-parameter proposal it's just
    one stage: allocate-and-configure.

    So, yes, this would be doable, but it doesn't look like a very good
    solution to me.

    Guennadi Liakhovetski, Ph.D.
    Freelance Open-Source Software Developer

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