linux-kernel - Re: [PATCH 5/7 v2] dma: sh: use an integer slave ID to improve API compatibility

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Message-ID: <Pine.LNX.4.64.1207161040070.12302@axis700.grange>
Date:	Mon, 16 Jul 2012 10:47:48 +0200 (CEST)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Vinod Koul <vinod.koul@...ux.intel.com>
cc:	Magnus Damm <magnus.damm@...il.com>, linux-sh@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/7 v2] dma: sh: use an integer slave ID to improve API
 compatibility

On Mon, 16 Jul 2012, Vinod Koul wrote:

> On Mon, 2012-07-16 at 09:13 +0200, Guennadi Liakhovetski wrote:
> > > > They tell the driver how the channel has to be configured to
> > support this 
> > > > specific client. They are values of two specific registers. In
> > fact, CHCR 
> > > > means exactly that - CHannel Control Register.
> > > what exactly does the channel control register do in shdma? Should
> > shdma
> > > driver deduce this value rather than client giving it?
> > > Same question for mid_rid?
> > 
> > See, e.g.,
> > arch/arm/mach-shmobile/setup-sh7372.c::sh7372_dmae_slaves[]. 
> > Platforms are supplying these values with shdma driver platform data, 
> > together with slave IDs. Them when slaves request channels and supply 
> > their slave IDs, the driver searches the above array, looking for the 
> > matching slave ID, then it uses the rest of the information in those 
> > structs to configure the channel for this client. Again, this is
> > nothing 
> > new, this is how the driver has been functioning since a long time,
> > this 
> > driver is not modifying anything there. Any changes to this
> > procedure, 
> > like providing all thig information from clients themselves instead
> > of 
> > keeping it with DMACs, requires these patches to be committed first.
> That wasn't my question.
> 
> I want to know what does ccr and mid_rid mean to dmac here?

CHCR contains a few fields, some enable various interrupt sources, some 
specify repeat- and renew-modes, others yet specify transfer size, source 
and destination address-modes (incrementing, constant, decrementing), 
others yet select a DMA client category (slave / memcpy / ...), and a 
transfer flag. Some of these fields could be calculated, others are 
pre-defined for various slaves, the exact layout of those fields can also 
vary between SoCs.

MID_RID is actually a slave-selector, it contains a magic value, that 
cannot be calculated.

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