On 11/21/2016 02:17 PM, Peter Rosin wrote:
[...]

I have a piece of hardware that is using the same 3 GPIO pins
to control four 8-way muxes. Three of them control ADC lines
to an ADS1015 chip with an iio driver, and the last one
controls the SDA line of an i2c bus. We have some deployed
code to handle this, but you do not want to see it or ever
hear about it. I'm not sure why I even mention it. Anyway,
the situation has nagged me to no end for quite some time.

So, after first getting more intimate with the i2c muxing code
and later discovering the drivers/iio/inkern.c file and
writing a couple of drivers making use of it, I came up with
what I think is an acceptable solution; add a generic mux
controller driver (and subsystem) that is shared between all
instances, and combine that with an iio mux driver and a new
generic i2c mux driver. The new i2c mux I called "simple"
since it is only hooking the i2c muxing and the new mux
controller (much like the alsa simple card driver does for ASoC).

While abstracting this properly is all nice and good and the way it should
be done, but it also adds a lot of complexity and the devicetree adds a lot
of restrictions on what can actually be represented.

There is a certain point where the fabric on a PCB becomes so complex that
it deserves to be a device on its own (like the audio fabric drivers).
Especially when the hardware is built with a certain application in mind and
the driver is supposed to impose policy which reflects this application. The
latter can often not properly be described with the primitives the
devicetree can offer.

And I think your setup is very borderline what can be done in a declarative
way only and it adds a lot of complexity over a more imperative solution in
form of a driver. I think it is worth investigating about having a driver
that is specific to your fabric and handles the interdependencies of the
discrete components.