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

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Date:	Fri, 16 Mar 2012 10:36:40 +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 Mon, 12 Mar 2012, Vinod Koul wrote:

> On Fri, 2012-03-09 at 13:20 +0100, Guennadi Liakhovetski wrote:
> > It can be made to work as long as there's only one DMAC group with 
> > configurable channels and all other DMACs are dedicated to specific 
> > peripherals, yes. I don't know whether there are already now or are 
> > approaching any platforms with multiple reconfigurable groups. 
> And that is what I am talking about.
> 
> Having specific channel mapping given by platform for all channels which
> are to be used dedicated. And a pool of channels which can be used by
> anyone (if they can be) on a platform.
> 
> Does this proposal sound good for others as well. I think we can target
> this for next merge cycle, we are too late for the current one.

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.

3. this will mean a substantial driver and platform code modification. 
Nothing super-complex, but still some.

4. we'll need a 3-stage channel allocation / configuration: request, 
filter, config. Whereas with my configuration-parameter proposal it's just 
one stage: allocate-and-configure.

So, yes, this would be doable, but it doesn't look like a very good 
solution to me.

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