linux-kernel - Re: [PATCH] serial: sh-sci: don't filter on DMA device, use only channel ID

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Date:	Mon, 5 Sep 2011 17:01:41 +0200 (CEST)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Vinod Koul <vinod.koul@...ux.intel.com>
cc:	Magnus Damm <magnus.damm@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>, linux-sh@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>,
	Magnus Damm <damm@...nsource.se>
Subject: Re: [PATCH] serial: sh-sci: don't filter on DMA device, use only
 channel ID

On Mon, 5 Sep 2011, Vinod Koul wrote:

> On Mon, 2011-09-05 at 15:48 +0200, Guennadi Liakhovetski wrote:
> > 
> > Let me try again. DMA channels on these DMA controllers are not dedicated. 
> > On one such SoC there can be several such DMA controllers of different 
> > kinds. One kind is "generic" - it can do memcpy(), besides channels can be 
> > freely configured for one of onboard peripherals: serial, mmc, etc. Some 
> > of them can also serve external DMA-capable devices. Another kind of DMA 
> > controllers, served by the same driver, can only be used with USB 
> > controllers. Now, if the MMC driver requests a DMA channel, let's say, the 
> > dmaengine core first finds the USB DMA controller. The MMC driver cannot 
> > know this. It assigns its MMC DMA configuration to the.private pointer and 
> > returns true. Next the DMA driver is entered, it checks the private 
> > pointer, sees an MMC channel request, looks at the DMA controller and 
> > sees, that it doesn't support MMC. So, .device_alloc_chan_resources() 
> > fails. When the same is attempted with a suitable DMA controller, the 
> > shdma driver recognises, that the controller can service MMC and uses the 
> > data, provided the MMC driver, to configure the DMA channel for MMC.
> Hmmm, Can't you know in filter function if the respective channel can do
> the dma for you or not? Maybe export a dma function or use platform data
> for this (wont you soc have these caps fixed), i prefer latter.
> That maybe a better approach. 

How? On a system you can have 3 suitable DMA controllers and 2 unsuitable. 
Do you want to pass a list of 3 suitable DMA controllers to each 
peripheral driver?...

> Once you have been allocated it should normally work, additional
> filtering confuses :(

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