linux-kernel - Re: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request

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Date:	Thu, 8 Mar 2012 12:22:00 +0100 (CET)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Vinod Koul <vinod.koul@...el.com>
cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org,
	'Jassi Brar' <jassisinghbrar@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel()

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.

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