linux-kernel - Re: [PATCH v2 2/3] dt-bindings: leds: add dt schema for worldsemi,ws2812b-spi

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Date:   Sat, 3 Dec 2022 11:52:41 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Chuanhong Guo <gch981213@...il.com>
Cc:     linux-leds@...r.kernel.org, Pavel Machek <pavel@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Stanislav Jakubek <stano.jakubek@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Johan Hovold <johan+linaro@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Sven Schwermer <sven.schwermer@...ruptive-technologies.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] dt-bindings: leds: add dt schema for
 worldsemi,ws2812b-spi

On 02/12/2022 13:55, Chuanhong Guo wrote:
>>> +
>>> +      default-brightness:
>>> +        description:
>>> +          The default brightness that should be applied to the LED by the operating
>>> +          system on start-up. The brightness should not exceed the brightness the
>>> +          LED can provide.
>>> +        $ref: /schemas/types.yaml#/definitions/uint32
>>> +        minimum: 0
>>> +        maximum: 255
>>> +        default: 0
>>> +
>>> +      default-intensity:
>>> +        description: |
>>> +          An array of 3 integer specifying the default intensity of each color
>>> +          components in this LED. <255 255 255> if unspecified.
>>> +        $ref: /schemas/types.yaml#/definitions/uint32-array
>>
>> I am still not convinced these two properties are correct. Why this LED
>> is special and defines default brightness and intensity and other LEDs
>> do not? You explained you are doing it for user-space which is usually
>> not a valid reason for changes specific to one binding. Either all
>> bindings should support it or none.
> 
> There's already a default-state for simple LEDs without brightness
> control so I think it makes sense to add default-brightness for LEDs
> with brightness control and default-intensity for colored LEDs.
> The default-state seems to be implemented in various LED drivers,
> so I implemented these two properties in my LED driver.
> There's nothing device-specific about these two properties.

default-state has a bit different purpose - to prevent any
glitches/changes when probing driver. Other values should not be
controller by default, at least not for this specific LED. If you want
to change common LED bindings and LED maintainers agree, sure, that's
different case.

Your device does not require these default brightness/intensity
properties... or at least you did not provide rationale why your device
(not driver... you keep referring to driver for some reason) needs them.

>>
>>> +        maxItems: 3
>>> +        items:
>>> +          minimum: 0
>>> +          maximum: 255
>>> +
>>> +      reg:
>>> +        description: |
>>> +          Which LED this node represents. The reg of the first LED on the chain
>>> +          is 0.
>>> +        maxItems: 1
>>> +
>>> +    required:
>>> +      - reg
>>> +      - color
>>> +      - function
>>> +
>>> +required:
>>> +  - compatible
>>> +
>>> +additionalProperties: false
>>> +
>>> +examples:
>>> +  - |
>>> +    #include <dt-bindings/leds/common.h>
>>> +    spi {
>>> +        #address-cells = <1>;
>>> +        #size-cells = <0>;
>>> +
>>> +        leds@0 {
>>
>> git grep leds@ -- Documentation/devicetree/ | wc -l
>> 1
>> git grep led@ -- Documentation/devicetree/ | wc -l
>> 165
>>
>> so rather not the first one ("leds").
> 
> As you can see, this node describes a chain of LEDs, not
> a single LED, so the plural form is more appropriate than
> the singular form.
> 
>>
>> There is also:
>> git grep led-controller@ -- Documentation/devicetree/ | wc -l
>> 30
> 
> This also isn't appropriate. WS2812B is a single LED package
> of 3 diodes and a microcontroller. If we treat every package
> as a LED, the SPI MOSI is connected directly to the LED
> packages themselves with no controller in between.
> If we treat the microcontroller as a led-controller, every
> LED contains its own controller, instead of one controller
> controlling all LEDs, and the parent node still shouldn't
> be called a led-controller.
> 
> Here's a picture of the WS2812B LED package:
> https://cdn-shop.adafruit.com/970x728/1655-00.jpg
> and a chain of them:
> https://cdn-shop.adafruit.com/970x728/1463-00.jpg

Then your bindings and DTS do not represent the hardware.

Best regards,
Krzysztof