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

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Message-ID: <20130204164712.GB4269@arwen.pp.htv.fi>
Date:	Mon, 4 Feb 2013 18:47:12 +0200
From:	Felipe Balbi <balbi@...com>
To:	Sergei Shtylyov <sshtylyov@...sta.com>
CC:	<balbi@...com>, Russell King - ARM Linux <linux@....linux.org.uk>,
	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

Hi,

On Mon, Feb 04, 2013 at 08:36:38PM +0300, Sergei Shtylyov wrote:
> > opted out of it. From the top of my head we have CPPI 3.x, CPPI 4.1,
> > Inventra DMA, OMAP sDMA and ux500 DMA engines supported by the driver.
> 
> > Granted, CPPI 4.1 makes some assumptions about the fact that it's
> > handling USB tranfers,
> 
>    What CPPI 4.1 code makes this assumptions? MUSB DMA driver? Then it's just

HW makes the asumptions

> > but nevertheless, the IP can be, and in fact is,
> > used with many different DMA engines and driver needs to cope with it.
> 
>    What IP, CPPI 4.1 or MUSB?

MUSB

> > Current DMA abstraction is quite poor, for example there's no way to
> > compile support for multiple DMA engines. Code also makes certain, IMO
> > unnecessary, assumptions about the underlying DMA engine (abstraction is
> > poor, as said above but it we could follow MUSB's programming guide when
> > it comes to programming DMA transfers).
> 
>    Don't know, I was quite content with the abstraction when writing CPPI 4.1
> driver for MUSB...

look closer. The whole:

if (is_cppi())
	foo();
else if (is_inventra())
	bar();
else if (is_omap_sdma())
	baz();

is bogus.

> > Considering all of the above, it's far better to use DMA engine and get
> > rid of all the mess.
> 
>    In my eyes, getting rid of the mess doesn't justify breaking the rules that
> Russell formulated above.

MUSB is no PCI, there is no single, standard interface to the DMA
engine (unlike Synopsys' dwc-usb3 and dwc-usb2, where we don't use DMA
engine), every DMA engine comes with its own set of registers, its own
programming model and so forth.

-- 
balbi

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