[PATCH 2/7] MFD: add STM32 DFSDM support

[Arnaud Pouliquen]

Hello,

Thanks everyone for you feedback!
My comments below.

On 01/29/2017 03:34 PM, Lars-Peter Clausen wrote:
> On 01/29/2017 03:19 PM, Lars-Peter Clausen wrote:
>> On 01/29/2017 01:28 PM, Jonathan Cameron wrote:
>> [...]
>>>>> Jonathan, Mark, Please could you share your opinion on this topic?
>>> Hmm - based on a fairly quick read through of the code (which is never
>>> ideal!). I can see that the ideal would indeed be as Lee says, to
>>> expand the IIO interfaces sufficiently to support what you need.
>>>
>>>
>>> So, reading the code (fairly quickly I'm afraid as had a lot of reviews
>>> to catch up on this weekend).
>>> What we need:
>>> 1) DMA support in the ADC driver.  This would be a good anyway!
>>> 2) DMA consumer support - I defer to Lars for comments on this.
>>> 3) Means of describing and controlling the sinc filters applied. 
>>> 4) Appropriate channel support.  I'm not convinced that it doesn't make
>>> sense to have IIO channels for the microphones - at least in a streaming
>>> mode.  It's data - I don't really care what ;)
>>> Coarsely it's a filtered pulse per period counter which is
>>> a perfectly valid type to have a channel for.
>>>
>>> The big question to my mind is the DMA consumer support. How would
>>> it work. It it wouldn't this is somewhat of a non starter.
>>>
>>> To bring up another slightly ugly MFD case where it is borderline
>>> on whether an MFD makes sense (just as a reference point of something
>>> we have discussed a few times before)
>>>
>>> ADCs with features directed at touchscreen support.
>>> These are odd as the ADC bit is generic, but the specific output
>>> and read sequences used for touchscreen reading don't correspond to
>>> anything that makes any real sense for other applications.
>>>
>>> We have started to get hybrid drives that have an MFD underneath but
>>> do the ADC reads through IIO consumer interfaces, and the timing
>>> control from a touchscreen driver.  We haven't really gotten this
>>> one right yet either.
>>>
>>> Here however, to my mind things are different - as I read it
>>> (and feel free to point out what I'm missing), the sound usecase
>>> is just a question of setting up sampling frequencies and filters
>>> appropriate to the microphones and what ASoC expects?
>>>
>>> That's not to say the IIO dma stuff is flexible enough (yet) to
>>> handle the data flows, but perhaps we can work towards that.
>>
>> Yeah, so this is a bit different, but not unexpected. And I'm sure we'll see
>> more similar hardware in the future. I've talked about this before[1], the
>> cost structure of creating and manufacturing new hardware drives the design
>> in a certain direction so that we end up with general purpose hardware that
>> suddenly has applications in multiple frameworks that were previously fully
>> orthogonal.
>>
>> This device is certainly not a multi-function-device. It only has one
>> function, it's a sigma-delta demodulator. It is rather a
>> multi-purpose-device. It can be used for sigma-delta demodulation in audio
>> applications as well as more specialized data capture applications.
>>
>> It's comparable to something like a GPIO that can be used to control a reset
>> pin or turn on and off a LED. The GPIO chip is not considered
>> multi-function-device though, even though it can be used for many different
>> applications.
>>
>> As for DMA we already have a lot of DMA infrastructure on the audio side and
>> we probably want to reuse that rather than inserting IIO as a middle layer
>> since audio buffer capture as different requirements from IIO buffer and
>> we'd have to go the route of the least common denominator and loose
>> expressibility in the process.
>>
>> I've created a IIO buffer[2] that does not capture data to memory but is
>> only used to enable/disable the data capture process. We use this in setups
>> where the data is passed from the converter to a application specific
>> processing chain without ever going through system memory. This buffer could
>> probably also be used here on the audio side to control the converter state.
> 
> I forgot to mention. I think the first thing we should do is work on
> terminology. This is not an ADC, this is a configurable low-pass-filter.
> 
> It works in conjunction with a analog frontend (ADC) that produces a 1-bit
> pulse-density-modulated stream, takes that stream and converts it into N-bit
> PCM samples. The PCM samples are generated at a fraction of the PDM stream
> samplerate that corresponds to the decimation factor.
> 
> This is not an unusual device. Many audio CODEC and audio controllers
> contain such a core as well as most SigmaDelta converters supported by IIO.
> What is special about this part is that it is a dedicated core that is not
> embedded in some other hardware component. This creates greater flexibility,
> but of course also greater complexity that is required to manage all that
> flexibility.
> 
> We shouldn't codify anything about the kernel internal frameworks through
> which the device might be exposed into the devicetree. We should accurately
> describe the hardware (including the analog frontend) and then create a
> appropriate software structures to handle them.	
> 

So if everyone is aligned, i will abandon the MFD driver and try to bind
an ASoC driver on IIO interface. The "challenge" is to
define appropriate relation ship between ASoC and IIO...

In a first step, a lot of question to answers and points to clarify... i
will reply to associated mails.

Then I see two main topics to clarify:
	- DFSDM integration in IIO in a generic way. Lars, if i well interpret,
your proposal should be to introduce front-end and filter notions in
IIO, to support this kind of hardware?
	- DMA engine for audio purpose.
I propose to come back with RFC for both subjects.

If you want to have more detail on DFSDM: DFSDM datasheet is included in
STM32F413 datasheet available here:
http://www.st.com/content/ccc/resource/technical/document/reference_manual/group0/81/ea/88/1f/97/9e/4a/d0/DM00305666/files/DM00305666.pdf/jcr:content/translations/en.DM00305666.pdf

DFSDM Block diagram p 384

Regards
Arnaud