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    SubjectRe: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel()
    On Fri, 16 Mar 2012, 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?

    Sure, everything is possible. So, would something like this make you
    happy:

    struct dma_channel_range {
    const char *dma_device;
    int channel_start;
    int channel_end;
    };

    struct dma_map {
    const char *name;
    const char *client;
    const struct dma_channel_range *chan_range;
    int chan_range_num;
    };

    You really want to do this?...

    And the least important question: who and when will implement the core
    support for this?

    > > 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.

    How??? Again:

    1. the client issues a dma_request_channel() with _just_ a capability mask
    and a filter and its argument as parameters - _nothing_ about channel
    restrictions.

    2. you propose to eliminate a filter - the core has no way to know, which
    channel to pick up...

    3. the wrapper, proposed by Russell, now calls dmaengine_slave_config(),
    which fails, because that's a wrong channel (hope I get this right this
    time - configuration has nothing to do with selection:-))

    4. that's it, if you start again - the dmaengine core will enumerate the
    same channels again and propose the same unsuitable channel to you -
    there's no way to continue to the next channel / device.

    What am I missing? How is the mapping going to be used, if you eliminate
    the 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.

    Sorry, why? I don't think I saw an answer to it apart from - maintenance
    burden... You can use that parameter to actually pass information to be
    used by the core to scan your mapping tables, I really don't see how you
    want to use those tables with the existing dmaengine channel-allocation
    API.

    Thanks
    Guennadi
    ---
    Guennadi Liakhovetski, Ph.D.
    Freelance Open-Source Software Developer
    http://www.open-technology.de/


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