Re: [PATCH/RFC] dmaengine: add a slave parameter to__dma_request_channel()

[Vinod Koul]

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@xxxxxx> 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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