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    SubjectRe: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel()
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    On Fri, 2012-03-16 at 11:16 +0100, Linus Walleij wrote:
    > On Fri, Mar 16, 2012 at 10:36 AM, Guennadi Liakhovetski
    > <g.liakhovetski@gmx.de> wrote:
    >
    > > 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.
    >
    > I fail to see why this would not be solved by a one-to-many mapping?
    >
    > Flag for each device which channels it may use in a mapping
    > table in platform data or device tree, I don't see the problem.
    >
    > You don't even have to specify that on a per-channel basis if
    > you can come up with something more clever in the mapping
    > table, such as "this device can use any channel on this DMAC,
    > and channels 1-7 on that DMAC" - no problem?
    Thats why added channel number to your proposal :)
    >
    > > 3. this will mean a substantial driver and platform code modification.
    > > Nothing super-complex, but still some.
    >
    > Big deal. Refactoring is fun... ;-)
    >
    > > 4. we'll need a 3-stage channel allocation / configuration: request,
    > > filter, config.
    >
    > In my world: channel request with *NO* filter function.
    >
    > Filter functions are part of the problem. So we refactor these
    > away as part of this change. That's the whole point...
    >
    > The core gathers information from the platform and the
    > DMAC driver(s) to build up the constraints necessary to
    > hand out workling channels to each device that request
    > one.
    >
    > And Russell IIRC already suggested a request-and-config
    > channel inline for the simple cases, and if you still need to
    > explicitly runtime-reconfigure then that's for a good
    > reason.
    >
    > > Whereas with my configuration-parameter proposal it's just
    > > one stage: allocate-and-configure.
    >
    > For one specific hardware, yes. For DMAengine at large
    > and the majority of the drivers, no.
    right, and that is my main concern.
    >
    > Yours,
    > Linus Walleij
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    --
    ~Vinod



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