Re: [PATCH v3 0/7] mux controller abstraction and iio/i2c muxes

[Peter Rosin]

On 2016-11-22 21:58, Lars-Peter Clausen wrote:
> 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.

This is a characterization without any specifics. But is the
characterization true? You have two complaints, complexity
and restrictions with bindings.

> 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.

So, there are three "new" concepts:

1. Sticking a mux in front of an AD-converter. That's not all that
novel, nor complex. Quite the opposite, I'd say. In fact, I find it
a bit amazing that there is no in-kernel support for it.

2. Reusing the same GPIO-pins to drive different muxes. There are
obviously chips that work this way (as Jonathan pointed out) and
these will at some point get used in Linux devices. I guess they
already are used, but that people handle them in userspace. Or
something? If this is complex, which I question, it will still need
support at some point. At least that's what I believe.

3. Using the same GPIO pins to mux things handled by different
subsystems. Right, this is a bit crazy, and I'd rather not have this
requirement, but this HW is what it is so I'll need to handle it in
some way. It is also what stops me from falling back to a userspace
solution, which is probably connected to why #1 and #2 is not supported
by the kernel; everybody probably does muxing in userspace. Which is
not necessarily a good idea, nor how it's supposed to be done...

So, the only thing that's out of the ordinary (as I see it), is #3.
The question that then needs an answer is how the in-kernel solution
for #1 and #2 would look if we do not consider #3.

And I claim that the desired solution to #1 and #2 is pretty close
to my proposal.

A. You do not want mux-controller drivers in every subsystem that
needs them.

B. You do not want every mux-consumer to know the specifics of how to
operate every kind of mux; there are muxes that are not controlled
with GPIO pins...

C. When you implement muxing in a subsystem, there will in some cases
be a need to handle parallel muxing, where there is a need to share
mux-controllers.

It just feels right to separate out the mux-controller and refer to
it from where a mux is needed. It solves #1 and #2. And, of course,
as a bonus #3 is also solved. But my bias is obvious.

And that leads us to the restrictions with the bindings. And the same
thing happens; the solution for #2 also solves #3.

So how do you refer to a mux-controller from where it's needed? My
first proposal used a dt phandle, for the second round I put them in
the parent node. It would be super if it was possible for the mux-
consumer to create the mux-controller device from the same dt
node that is already bound to the mux-consumer. The problem is that
the mux-consumer should not hard-code which mux-controller device it
should create. The best I can think of is some kind of 'mux-compatible'
attribute, that works like the standard 'compatible' attribute. That
would simplify the bindings for the normal case (#1) where the mux-
controller isn't shared (#2 and #3). Maybe it's possible to fix this
issue somehow? I simply don't know?

Cheers,
Peter