linux-kernel - Re: [PATCH/RFC 0/4] dmaengine: add DT support for DMA multiplexers

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Date:	Thu, 2 May 2013 21:58:37 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	linux-sh@...r.kernel.org, Magnus Damm <magnus.damm@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Tony Lindgren <tony@...mide.com>,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH/RFC 0/4] dmaengine: add DT support for DMA multiplexers

On Tue, Apr 30, 2013 at 02:17:13PM +0200, Guennadi Liakhovetski wrote:
> (added Russell to CC, sorry for not including initially)
> 
> Hi Vinod
> 
> On Tue, 30 Apr 2013, Vinod Koul wrote:
> 
> > On Tue, Apr 30, 2013 at 09:11:19AM +0200, Guennadi Liakhovetski wrote:
> > > Previously an issue has been discussed, arising on sh-/r-mobile ARM-based 
> > > systems. There we typically have multiple DMA controller instances with 
> > > exactly equal or very similar capabilities. Each of them can serve the same 
> > > slaves, using the same slave identifiers (request line IDs). With the 
> > > present DMA DT implementation _each_ such DMA slave would have to reference 
> > > _each_ of those DMA controllers in its DMA bindings, e.g.
> > But why... if that is the case then we havent define DT-bindings clearly enough
> 
> Sorry, what do you mean "why?" Why each slave has to reference each DMA 
> controller? We have discussed this A LOT before... My understanding is, 
> that we decided, that the sh-/r-mobile case of multiple equal DMA 
> controllers is an exception and that we don't want to punish everyone for 
> it. So, the design includes only explicit requesting of specific DMA 
> request lines on specific DMA controllers, no wild-cards. If a slave DMA 
> channel can be provided by several DMA controllers we decided to list them 
> all explicitly too. And for the sh-/r-mobile case a DMA-mux DT node has 
> been proposed. This is exactly what this patch series is implementing. Is 
> my understanding wrong?
> 
> > And we havent merged that yet, so why not fix that in first set itself
> 
> Sorry, don't understand. The series isn't merged yet, that's right. That's 
> why I explicitly mention this dependency here. But this isn't a fix. This 
> is a new feature. The first patch-series only touches a specific DMA 
> controller driver and relevant platforms. No core changes, so, it's not 
> that intrusive and can be applied quickly. Whereas this series affects the 
> core and might need a more careful consideration, discussion, etc.
What i mean from above is if we were already defining the sh-DT binding then why
wasnt this taken care in the orignal definition?

It would make sense to have proper binding which works well for both of these
case, why a two shot approach?

--
~Vinod
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