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

[Lars-Peter Clausen]

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.

- Lars

[1]
https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2016-July/003029.html

[2]
https://github.com/analogdevicesinc/linux/blob/xcomm_zynq/drivers/iio/buffer/hw_consumer.c