Re: [PATCH 4/4] iio: adc: stm32-dfsdm: add scale and offset support

[Olivier MOYSAN]

Hi Jonathan,

> > > > If 'backend' option turns out to be the most appropriated to match DFSDM
> > > > constraints, I can prepare some patches to support it.
> > > > Would you have some guidelines or requirements for the implementation of
> > > > such feature, in this case ?
> > >
> > > Closest example is that rcar-gyroadc but in this case we'd want to define
> > > something standard to support the modulators so that if we have other filters
> > > in future we can reuse them.
> > >
> > > That means implementing them as child devices of the filter - probably put
> > > the on the IIO bus, but as different device type.  Take a look at how
> > > triggers are done in industrialio-trigger.c
> > > You need struct device_type sd_modulator
> > > and a suitable device struct (burred in an iio_sd_modulator struct probably).
> > >
> > > Also needed would be a bunch of standard callbacks to allow you to query things
> > > like scaling.   Keep that interface simple. Until we have a lot of modulator
> > > drivers it will be hard to know exactly what is needed.  Also whilst we don't
> > > have many it is easy to modify the interface.
> > >
> > > Then have your filter driver walk it's own dt children and instantiate
> > > appropriate new elements and register them on the iio_bus.  They will have
> > > the filter as their parent.
> > >
> > > There are various examples of this sort of thing in tree.
> > > If you want a good one, drivers/cxl does a lot of this sort magic to manage
> > > a fairly complex graph of devices including some nice registration stuff to
> > > cause the correct device drivers to load automatically.
> > >
> > > Hmm.  Thinking more on this, there is an ordering issue for driver load.
> > > Instead of making the modulator nodes children of the modulator, you may need
> > > to give them their own existence and use a phandle to reference them.
> > > That will let you defer probe in the filter driver until those
> > > modulator drivers are ready.
> > >
> > > This isn't going to be particularly simple, so you may want to have a look
> > > at how various other subsystems do similar things and mock up the dependencies
> > > to make sure you have something that doesn't end up with a loop of dependencies.
> > > In some ways the modulators are on a bus below the filter, but the filter driver
> > > needs them to be in place to do the rest.
> > > You may end up with some sort of delayed load.
> > > 1. Initial filter driver load + parsing of the modulator dt children (if done that way).
> > > 2. Filter driver goes to sleep until...
> > > 3. Modulator drivers call something on the filter driver to say they are ready.
> > > 4. Filter driver finishes loading and create the IIO device etc.
> > > You'll need some reference counting etc in there to make removal safe etc but it
> > > shouldn't be 'too bad'.
> > >
> > > Good luck!
> > >
> > > Jonathan
I'am on the way to prototype this proposal for DFSDM.
Looking at your advices, I see that the current topolgy based on 
hardware consumer, already meets most of the requirements.

- SD modulators are described in DT with their own nodes and are 
referred in DFSDM nodes through their phandle.
- Dependencies at probe are managed (defer probe through 
devm_iio_hw_consumer_alloc())
- SD modulator scaling is retrieved through iio_read_channel_scale() ABI.

So, it seems that the current implementation is not so far from this 
solution.
It remains the unwanted sysfs interface for SD modulator. Or more than 
that, if I missed something ?
Instead of introducing a new device type for SD modulator, could the 
mode field be used to identify devices not requesting an IIO sysfs ?
(A dedicated mode may be used to skip sysfs register in device registration)
Otherwise let's go for a new type.

Regards
Olivier