linux-kernel - Re: [PATCH v8 07/12] dt-bindings: i2c: i2c-mux-simple: document i2c-mux-simple bindings

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Date:   Mon, 6 Feb 2017 15:22:17 -0600
From:   Rob Herring <robh@...nel.org>
To:     Peter Rosin <peda@...ntia.se>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        Mark Rutland <mark.rutland@....com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v8 07/12] dt-bindings: i2c: i2c-mux-simple: document
 i2c-mux-simple bindings

On Fri, Feb 3, 2017 at 2:25 AM, Peter Rosin <peda@...ntia.se> wrote:
> On 2017-02-02 17:08, Rob Herring wrote:
>> On Tue, Jan 31, 2017 at 1:36 AM, Peter Rosin <peda@...ntia.se> wrote:
>>> If you see this new driver as something that is superseding the existing
>>> i2c-mux-gpio driver, I'm sad to inform you that the code is not simply
>>> not there. i2c-mux-gpio has acpi support and users may provide platform
>>> data from code. The existing i2c-mux-gpio driver also has the below
>>> mentioned locking heuristic. Adding all those things to the new driver
>>> would make it big and unwieldy and having even more unwanted tentacles
>>> into other subsystems. And why should it be only i2c-mux-gpio that is
>>> merged into this new i2c-mux driver? Why not implement a mux-pinctrl
>>> driver for the new mux subsustem and then merge i2c-mux-pinctrl as well?
>>> I say no, that way lies madness.
>>
>> Sounds like a good idea to me. I'm not saying you need to merge any of
>> them right now though (that's Wolfram's call).
>
> If we're pedantic I probably have some stake in it too, being the i2c-mux
> maintainer and all. But, agreed, I arrived quite late to the Linux kernel
> party and my opinion might perhaps not carry all that much weight...
>
>> None of this has anything to do with the binding though. Compatible
>> strings should be specific. That's not up for debate. Whether the
>
> Ok, I'm going to focus on the compatible string for a minute and leave
> the implementation details for some other discussion.
>
>> driver bound to a compatible string is common or specific to that
>> compatible string is completely up to the OS. That decision can change
>> over time, but the binding should not.
>
> So, there's the existing compatible "i2c-mux-gpio" ("i2c-gpio-mux" is
> wrong) that you seem to suggest is what I should stick to. I object to
> that.
>
> As you say, the bindings and compatible strings should describe hardware,
> and you also state they should be specific. I agree. But, why are you
> then apparently suggesting (by implication) that for this (hypothetical)
> hardware...
>
>     .----.
>     |SDA0|-----------.
>     |SCL0|---------. |
>     |    |         | |
>     |    |      .-------.
>     |    |      |adg792a|
>     |    |      |       |
>     |ADC0|------|D1  S1A|---- signal A
>     |    |      |    S1B|---- signal B
>     |    |      |    S1C|---- signal C
>     |    |      |    S1D|---- signal D
>     |    |      |       |
>     |SDA1|---+--|D2  S2A|---- i2s segment foo A
>     |SCL1|-. |  |    S2B|---- i2s segment foo B
>     '----' | |  |    S2C|---- i2s segment foo C
>            | |  |    S2D|---- i2s segment foo D
>            | |  |       |
>            | '--|D3  S3A|---- i2s segment bar A
>            |    |    S3B|---- i2s segment bar B
>            |    |    S3C|---- i2s segment bar C
>            |    |    S3D|---- i2s segment bar D
>            |    '-------'
>            |                  A B C D   A B C D  (feed SCL1 to each of
>            |                  | | | |   | | | |   the 8 muxed segments)
>            '------------------+-+-+-+---+-+-+-'
>
> ...the devicetree should be like below?
>
> &i2c0 {
>         mux: mux-controller@50 {
>                 compatible = "adi,adg792a";
>                 reg = <0x50>;
>                 #mux-control-cells = <1>;
>         };
> };
>
> adc-mux {
>         compatible = "io-channel-mux";
>         io-channels = <&adc 0>;
>         io-channel-names = "parent";
>
>         mux-controls = <&mux 0>;
>
>         ...
> };
>
> i2c-mux-foo {
>         compatible = "i2c-mux-gpio";
>         i2c-parent = <&i2c1>;
>
>         mux-controls = <&mux 1>;
>
>         ...
> };
>
> i2c-mux-bar {
>         compatible = "i2c-mux-gpio";
>         i2c-parent = <&i2c1>;
>
>         mux-controls = <&mux 2>;
>
>         ...
> };
>
> There must be some disconnect, because those "i2c-mux-gpio" compatible
> strings are just dead wrong. There simply are no gpio pins involved at
> all and that "gpio" suffix is just totally out of place.

Indeed. In the above case, that makes no sense.

> So, since you are not happy with "i2c-mux-simple", "i2c-mux-generic" or
> "i2c-mux" that I have suggested, can you please come up with something
> that is good enough for the above?

Let's go with just "i2c-mux". I've got nothing better and simple or
generic doesn't add anything.

Rob