linux-kernel - Re: [PATCH v7 01/10] ARM: davinci: move private EDMA API to arm/common

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Date:	Mon, 4 Feb 2013 15:45:02 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Felipe Balbi <balbi@...com>
Cc:	Sergei Shtylyov <sshtylyov@...sta.com>,
	Matt Porter <mporter@...com>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Chris Ball <cjb@...top.org>,
	"Cousson, Benoit" <b-cousson@...com>,
	Arnd Bergmann <arnd@...db.de>,
	Linux Documentation List <linux-doc@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Devicetree Discuss <devicetree-discuss@...ts.ozlabs.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Vinod Koul <vinod.koul@...el.com>,
	Rob Landley <rob@...dley.net>, Dan Williams <djbw@...com>,
	Linux SPI Devel List 
	<spi-devel-general@...ts.sourceforge.net>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 01/10] ARM: davinci: move private EDMA API to
	arm/common

On Mon, Feb 04, 2013 at 05:41:53PM +0200, Felipe Balbi wrote:
> Hi,
> 
> On Fri, Feb 01, 2013 at 09:30:03PM +0000, Russell King - ARM Linux wrote:
> > > > > I guess to make the MUSB side simpler we would need musb-dma-engine glue
> > > > > to map dmaengine to the private MUSB API. Then we would have some
> > > > > starting point to also move inventra (and anybody else) to dmaengine
> > > > > API.
> > > > 
> > > >    Why? Inventra is a dedicated device's private DMA controller, why make
> > > > universal DMA driver for it?
> > > 
> > > because it doesn't make sense to support multiple DMA APIs. We can check
> > > from MUSB's registers if it was configured with Inventra DMA support and
> > > based on that we can register MUSB's own DMA Engine to dmaengine API.
> > 
> > Hang on.  This is one of the DMA implementations which is closely
> > coupled with the USB and only the USB?  If it is...
> > 
> > I thought this had been discussed _extensively_ before.  I thought the
> > resolution on it was:
> > 1. It would not use the DMA engine API.
> > 2. It would not live in arch/arm.
> > 3. It would be placed nearby the USB driver it's associated with.
> > 
> > (1) because we don't use APIs just for the hell of it - think.  Do we
> > use the DMA engine API for PCI bus mastering ethernet controllers?  No.
> > Do we use it for PCI bus mastering SCSI controllers?  No.  Because the
> > DMA is integral to the rest of the device.
> 
> that's not really a fair comparison, however. MUSB is used with several
> DMA engines.

I only mentioned it because it _was_ brought up as an argument against
using the DMA engine API in the previous discussions.  I'm just reminding
people what was discussed.

> Considering all of the above, it's far better to use DMA engine and get
> rid of all the mess.

Which is what both you and I have been saying for the last 3 or so years
on this subject...
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