linux-kernel - Re: [RFC PATCH 1/6] dt-bindings: iio: light: Support ROHM BU27034

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Message-ID: <33abc8a3-39f7-af7c-9676-723228624b0f@linaro.org>
Date:   Thu, 23 Feb 2023 10:26:02 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>,
        Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shreeya Patel <shreeya.patel@...labora.com>,
        Zhigang Shi <Zhigang.Shi@...eon.com>,
        Paul Gazzillo <paul@...zz.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Liam Beguin <liambeguin@...il.com>,
        Peter Rosin <peda@...ntia.se>,
        Randy Dunlap <rdunlap@...radead.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/6] dt-bindings: iio: light: Support ROHM BU27034

On 23/02/2023 07:20, Vaittinen, Matti wrote:
> Hi dee Ho Krzysztof,
> 
> Thanks for the review! It's nice you had the time to take a look on RFC :)
> 
> On 2/22/23 20:57, Krzysztof Kozlowski wrote:
>> On 22/02/2023 17:14, Matti Vaittinen wrote:
>>> ROHM BU27034 is an ambient light sesnor with 3 channels and 3 photo diodes
>>> capable of detecting a very wide range of illuminance. Typical application
>>> is adjusting LCD and backlight power of TVs and mobile phones.
>>>
>>> Add initial dt-bindings.
>>
>> Driver can be "initial", but bindings better to be closer to complete,
>> even if not used by the driver currently.
> 
> Out of the curiosity - why is that? (Please, don't take me wrong, I am 
> not trying to argue against this - just learn the reason behind). I 
> can't immediately see the harm caused by adding new properties later 
> when we learn more of hardware. (and no, I don't expect this simple IC 
> to gain at least many properties).

Linux drivers change, but the hardware does not, thus DTS, which
describes the hardware, can be complete. It should be written based on
the hardware, not based on Linux drivers. If you add incomplete
bindings, this suggests you wrote them to match your driver, not to
match hardware. This in turn (adjusting bindings to driver) makes them
less portable, narrowed to one specific driver implementation and more
ABI-break-prone later.

Imagine you that clock inputs, which you skipped in the binding, were
actually needed but on your board they were enabled by bootloader. The
binding is then used on other systems or by out of tree users. On your
new system the clocks are not enabled by bootloader anymore, thus you
add them to the binding. They are actually required for device to work,
so you make them required. But all these other users cannot be fixed...

What's more, incomplete binding/DTS is then used together with other
pieces - DTS and driver, e.g. via some graphs or other
phandles/supplies/pinctrl. So some other DTS or driver code might rely
on your particular binding. Imagine you had only vdd-supply regulator,
but no reset pins, so the only way to power-cycle device was to turn
off/on regulator supply. Then you figure out that you have reset pins
and it would be useful to add and use it. But already drivers are
written to power cycle via regulator... or even someone wrote new driver
regulator-pwrseq to power cycle your device due to missing reset GPIOs...

Best regards,
Krzysztof