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

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Date:	Thu, 8 Mar 2012 14:18:48 +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 17:04 +0530, Vinod Koul wrote:
> > 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?
> Assuming I didn't miss...
> 
> The case B can be handled without sweat by platforms channel mapping
> information.
> 
> Case A where we don't find that devices exist in map, thus being treated
> as generic DMA channels and can be handled easily in sequence. So when
> someone in Q request a channel it would get first channel in Ps
> 
> This way we handle both of them in a transparent manner to both clients
> and controllers. 
> 
> Perhaps we can also add capability to know that if channel is to be
> searched in map or not - would be anyway required for non slave cases.

Right, but I don't understand then what this gives us. You propose some 
channel maps, that will not be used for your "case A." Which means, for 
"case A" nothing changes. So, the reason for this whole thread hasn't been 
addressed: how to pass channel configuration to the DMA controller driver.

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