On Thu, Aug 14, 2014 at 09:57:53AM +0100, Russell King - ARM Linux wrote:

On Thu, Aug 14, 2014 at 10:53:01AM +0200, Ludovic Desroches wrote:

quoted

On Wed, Jul 30, 2014 at 06:03:13PM +0200, Maxime Ripard wrote:

quoted

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.)

Yes this is my understanding too. Async API IIUC can reuse descriptors and
if engine doesnt support this is a way to tell them please do use that.

For slave dmanegine API we need to ignore it.

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.)

-- 
~Vinod