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

[Vinod Koul]

On Thu, 2012-03-08 at 12:22 +0100, Guennadi Liakhovetski wrote:
> On Thu, 8 Mar 2012, Vinod Koul wrote:
> 
> > On Thu, 2012-03-08 at 11:16 +0100, Guennadi Liakhovetski wrote:
> > > I still have the impression, that my specific use-case (sh-mobile), where 
> > > channels can be freely configured for use by _ANY_ client on one of 
> > > _SEVERAL_ DMAC instances, is not fully understood or taken into account. 
> > > For this driver any kind of fixed mapping means, that we'd have to use 
> > > both virtual channels and controllers, adding _a lot_ of complexity to the 
> > > DMAC driver and making the dmaengine core just an "obfuscation layer." 
> > > Yes, I remember Russell proposing core helpers for this. They would help, 
> > > but (1) when would they be available, (2) how well would they be suitable 
> > > for us, (3) they'd take the coding / maintainance burden away, but 
> > > wouldn't reduce complexity and run-time overhead. 
> > Lets try to address you case as well.
> > On a typical platform
> 
> Let's take the mackerel board with the sh7372 SoC. it's not the state of 
> the art, but that's what I'm currently working with and it should give us 
> a good enough idea
> 
> > 1) how many dma controllers you have?
> 
> currently supported 5 of 3 types (3 of type A, 1 of each of the types B 
> and C), all handled by the same driver
> 
> > 2) how many clients you have
> 
> huh... many. Maybe like 20 or more, and more, that are not yet supported, 
> using type A, and 1 for each of types B and C
> 
> > 3) which client can use what controller channel? How is mapping decided,
> > do you have a mux, is it hard wired by soc designers,....?
> 
> In general - with all the current sh-mobile hardware, that I'm aware of - 
> there can be several controller instances on an SoC of each controller 
> type. Inside each type all instances and all channels are freely 
> configurable. So, of 20 Type A clients they can use any channels on any 
> one of the 3 type A controllers. Types B and C are "degenerate" cases, 
> there clients are practically hard-wired to a specific DMA controller.
> 
> So, we don't have to decide on mappings for type A. We just pick up any 
> free channels on any controller and configure them accordingly. Whether 
> there's a mux somewhere - you can say so, but it's all inside the SoC, and 
> it's configured automatically ones you configure a physical channel to 
> serve a specific client.
> 
> > Can you pls give a description so that we ensure all models fit in the
> > final solution?
> 
> That's what I've been trying to do since several days now... I've been 
> saying "multiple controllers with multiple channels all freely 
> configurable for any device from a list" again and again... Seems I'm 
> speaking some strange language, that noone understands.
Okay. One more question before I can tell you how it can work for you
without you sweating it out :-)

So you have:
case A: Here you have N dmacs and M controllers, any controller can use
any channel, No constraints on channel assignments, right?
case B: Some hardwired controllers P which can only be used by a set
clients Q?

Anything else I missed in your description?



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