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

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Message-ID: <20221130001459.3wyxnwpjaxvla6oj@mercury.elektranox.org>
Date:   Wed, 30 Nov 2022 01:14:59 +0100
From:   Sebastian Reichel <sre@...nel.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Chuanhong Guo <gch981213@...il.com>, 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 2/3] dt-bindings: leds: add dt schema for
 worldsemi,ws2812b-spi

Hi,

On Tue, Nov 29, 2022 at 05:54:52PM +0100, Krzysztof Kozlowski wrote:
> On 29/11/2022 15:09, Chuanhong Guo wrote:
> > This patch adds dt binding schema for WorldSemi WS2812B driven using SPI
> > bus.

Nice, I have a hobby project for something similar (SK6812, which is
basically WS2812 with an extra white channel). I will switch to this
work and extend it once it lands :)

[...]

> > +      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
> > +        minItems: 3
> 
> Drop minItems.... but:
> 
> > +        maxItems: 3
> > +        items:
> > +          minimum: 0
> > +          maximum: 255
> 
> default: 255

I would argue, that the default should be 0 (off) instead of 255
(full power).

> What controls the intensity? Don't you have PWM there?

WS2812 is a RGB led, which contains a small Microcontroller. The µC
takes 24 byte intensity data from a serial input and then passes on
any following bits to the next LED. SPI clk and chip-select are
ignored (chip-select support can be trivially added though).

You can find them everywhere nowadays, since they are quite cheap
(a few cents per LED) and need only one MOSI pin to control hundreds
of LEDs.

[...]

-- Sebastian

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