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SubjectRe: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel()
On Tue, 2012-03-06 at 14:03 +0100, Guennadi Liakhovetski wrote:
<adding few folks to thread, who i may be intrested in this discussion>
> But the DMAC is certainly a better match for making channel-selection
> decisions.
I am not sure about that as well...
> > Bigger question is who knows about this mapping and how do we
> > incorporate this mapping into channel allocation
> The platform does. And this knowledge has to be passed to the relevant
> driver. But I think it's the DMAC driver, that is relevant, not the client
> driver. The platform would supply information like
> DMAC #1
> channel #1
> (can be used for) device #1
> device #2
> ...
> channel #2
> ...
> ...
right :-)
and we need to ensure that somehow this information is presented to
dmaengine and dmaengine uses this information to filter the channel
requests. In past we had good discussion [1], [2], [3] on this topic but
unfortunately nothing came out of it.

I like the approach outlined by Linus W [1], where we can get the
information from platform (DT, FW,....) and its presented to dmaengine.

I think we need to solve this _now_. There are two aspects
a) to ensure dmaengine understand channel-client mapping. For this we
can start with idea in [1] and see if this suits everyones needs
b) how to ensure the platform gives this information in variety of arch
we have (arm, x86, sh-....)

> And I don't think, it would be reasonable to let every slave driver use
> this information. These lists can also be optimised for specific
> platforms. E.g., on some sh-mobile SoCs you have two DMAC types. One of
> them can serve devices from list A on any channel, the other one - from
> list B. So, all you have to do, is to reference either A or B from your
> DMAC platform data. Whereas doing a reverse mapping: for each (potential)
> DMA user reference a list of channels, that it can use - would be really
> clumsy.


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