[PATCH] Documentation: dmaengine: Add a documentation for the dma controller API

[Russell King - ARM Linux]

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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