linux-kernel - Re: [PATCH v6 4/9] dt-bindings: iio: iio-mux: document iio-mux bindings

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Date:   Mon, 12 Dec 2016 13:18:36 +0100
From:   Peter Rosin <peda@...ntia.se>
To:     Jonathan Cameron <jic23@...nel.org>, Rob Herring <robh@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>,
        Mark Rutland <mark.rutland@....com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-iio@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 4/9] dt-bindings: iio: iio-mux: document iio-mux
 bindings

On 2016-12-10 19:21, Jonathan Cameron wrote:
> On 06/12/16 09:18, Peter Rosin wrote:
>> On 2016-12-06 00:26, Rob Herring wrote:
>>> On Wed, Nov 30, 2016 at 09:16:58AM +0100, Peter Rosin wrote:
>>>> Signed-off-by: Peter Rosin <peda@...ntia.se>
>>>> ---
>>>>  .../bindings/iio/multiplexer/iio-mux.txt           | 40 ++++++++++++++++++++++
>>>>  MAINTAINERS                                        |  6 ++++
>>>>  2 files changed, 46 insertions(+)
>>>>  create mode 100644 Documentation/devicetree/bindings/iio/multiplexer/iio-mux.txt
>>>
>>> I'm still not convinced about this binding, but don't really have more 
>>> comments ATM. Sending 6 versions in 2 weeks or so doesn't really help 
>>> either.
>>
>> Sorry about the noise, I'll try to be more careful going forward. On
>> the flip side, I haven't touched the code since v6.
>>
>> I don't see how bindings that are as flexible as the current (and
>> original) phandle link between the mux consumer and the mux controller
>> would look, and at the same time be simpler to understand. You need
>> to be able to refer to a mux controller from several mux consumers, and
>> you need to support several mux controllers in one node (the ADG792A
>> case). And, AFAICT, the complex case wasn't really the problem, it was
>> that it is overly complex to describe the simple case of one mux
>> consumer and one mux controller. But in your comment for v2 [1] you
>> said that I was working around limitations with shared GPIO pins. But
>> solving that in the GPIO subsystem would not solve all that the
>> phandle approach is solving, since you would not have support for
>> ADG792A (or other non-GPIO controlled muxes). So, I think listing
>> the gpio pins inside the mux consumer node is a non-starter, the mux
>> controller has to live in its own node with its own compatible.
>>
>> Would you be happier if I managed to marry the phandle approach with
>> the option of having the mux controller as a child node of the mux
>> consumer for the simple case?
>>
>> I added an example at the end of this message (the same as the first
>> example in v4 [2], at least in principle) for easy comparison between
>> the phandle and the controller-in-child-node approaches. I can't say
>> that I personally find the difference all that significant, and do not
>> think it is worth it. As I see it, the "simple option" would just muddy
>> the waters...
>>
>> [1] http://marc.info/?l=linux-kernel&m=147948334204795&w=2
>> [2] http://marc.info/?l=linux-kernel&m=148001364904240&w=2
>>
>>>> diff --git a/Documentation/devicetree/bindings/iio/multiplexer/iio-mux.txt b/Documentation/devicetree/bindings/iio/multiplexer/iio-mux.txt
>>>> new file mode 100644
>>>> index 000000000000..8080cf790d82
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/iio/multiplexer/iio-mux.txt
>>>> @@ -0,0 +1,40 @@
>>>> +IIO multiplexer bindings
>>>> +
>>>> +If a multiplexer is used to select which hardware signal is fed to
>>>> +e.g. an ADC channel, these bindings describe that situation.
>>>> +
>>>> +Required properties:
>>>> +- compatible : "iio-mux"
>>>
>>> This is a Linuxism. perhaps "adc-mux".
>>
>> No, that's not general enough, it could just as well be used to mux a
>> temperature sensor. Or whatever. Hmmm, given that "iio-mux" is bad, perhaps
>> "io-channel-mux" is better? That matches the io-channels property used to
>> refer to the parent channel.
> analog-mux maybe? Makes more sense out of context (though with io-channels defined on
> the next line you have plenty of context here ;)

Not that I care all that much about the name, but that doesn't really
fit if you take e.g. an IIO_INDEX channel. That sounds entirely non-analog
to me, but what do I know? Maybe that example doesn't make sense for some
reason, but I can't help but think that there will be some non-analog
channel in the future, if there isn't one already.

So, my preference is io-channel-mux, as that matches the previous dt
naming for what is muxed. But that's just my opinion, if I'm told that
it should be something else, then that's ok.

I'm more worried about other aspects, such as how to get reviewers and who
is going to take the core mux patches and what tree they are going to be
merged into etc. That is, if this series is going anywhere at all or if
someone is going to put up a road-block for some reason...

Cheers,
peda