linux-kernel - Re: [PATCH] Documentation: dmaengine: Add a documentation for the dma controller API

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Message-ID: <20140814085753.GS30401@n2100.arm.linux.org.uk>
Date:	Thu, 14 Aug 2014 09:57:53 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Ludovic Desroches <ludovic.desroches@...el.com>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	dmaengine@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Antoine Ténart <antoine@...e-electrons.com>,
	Thomas Petazzoni <thomas@...e-electrons.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Boris Brezillon <boris@...e-electrons.com>,
	Matt Porter <matt.porter@...aro.org>,
	laurent.pinchart@...asonboard.com,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>
Subject: Re: [PATCH] Documentation: dmaengine: Add a documentation for the
	dma controller API

On Thu, Aug 14, 2014 at 10:53:01AM +0200, Ludovic Desroches wrote:
> On Wed, Jul 30, 2014 at 06:03:13PM +0200, Maxime Ripard wrote:
> > The dmaengine is neither trivial nor properly documented at the moment, which
> > means a lot of trial and error development, which is not that good for such a
> > central piece of the system.
> > 
> > Attempt at making such a documentation.
> 
> Good idea, many questions are asked when writing a new dmaengine driver.
> 
> For instance I didn't find how to use the DMA_CTRL_ACK flags.
> 
> - How this flag has to be managed? For instance, async_tx_ack is used in
> dmaengine driver but also in some devices.
> - Is it mandatory to deal with this flag? It seems some dmaengine
> drivers don't care about it.

This is one part of the DMA engine API that even I don't understand.

It's got something to do with the async engine API, and seems to be
something to do with whether a descriptor can have other transactions
added to it, and whether a descriptor can be re-used (async-tx engines
typically allocate a fixed set of descriptors and recycle them.)

I was never able to get to the bottom of what that flag meant or how
it should be used with the DMA slave API (and I suspect everyone else
just gave up trying to understand it as well.)

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