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

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 30/11/2022 09:36, Chuanhong Guo wrote:
> Hi!
> 
> On Wed, Nov 30, 2022 at 12:54 AM Krzysztof Kozlowski
> <krzysztof.kozlowski@linaro.org> wrote:
>>> +description: |
>>> +  WorldSemi WS2812B is a individually addressable LED chip that can be chained
>>> +  together and controlled individually using a single wire.
>>> +  This driver simulates the protocol used by this LED chip with SPI bus.
>>
>> Drop references to Linux driver, unless important for the binding.
> 
> I think the SPI part is important. (I'll explain it below.) What about:
> 
> This binding describes a chain of WS2812B LEDs connected to the SPI MOSI pin.
> 
> instead?

OK

> 
>>> +  Typical setups includes connecting the data pin of the LED chain to MOSI as
>>> +  the only device or using CS and MOSI with a tri-state voltage-level shifter
>>> +  for the data pin.
>>> +  The SPI frequency needs to be 2.105MHz~2.85MHz for the timing to be correct
>>> +  and the controller needs to send all the bytes continuously.
>>> +
>>> +allOf:
>>> +  - $ref: /schemas/spi/spi-peripheral-props.yaml#
>>> +
>>> +properties:
>>> +  compatible:
>>> +    const: worldsemi,ws2812b-spi
>>
>> Drop "-spi". Compatibles are not supposed to include bus information.
>> The same for file name.
> 
> WS2812B isn't a SPI chip. It's controlled with only a single wire and
> can be driven
> using anything that can produce a long and a short pulse meeting its timing
> requirement.
> This driver uses a SPI bus to send the pulses, but it can also be
> controlled with
> I2S and the PIO pins on a Raspberry Pi Pico.
> This spi suffix is to distinguish it from other possible
> implementations if someone
> else submits a support with a different peripheral.

And that's exactly what I said - the compatibles should not include bus
information. The bus information comes from... the bus!

> 
>>
>>> +
>>> +  reg:
>>> +    description: The chip-select line on the SPI bus
>>
>> Drop description, it's obvious.
> 
> OK.
> 
>>
>>> +    maxItems: 1
>>> +
>>> +  spi-max-frequency:
>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>> +    description:
>>> +      Maximum SPI clocking speed of the device in Hz.
>>
>> No need for ref and description. It comes from spi-peripheral-props.
> 
> OK.
> 
>>
>>> +    minimum: 2105000
>>> +    maximum: 2850000
>>> +
>>> +  "#address-cells":
>>> +    const: 1
>>> +
>>> +  "#size-cells":
>>> +    const: 0
>>> +
>>> +patternProperties:
>>> +  "^multi-led(@[0-9a-f])?$":
>>
>> Why unit address is optional?
> 
> It isn't. I copy-pasted it from led-class-multicolor.yaml and
> didn't check the exact regex.
> I'll fix it in the next version.

Make it required and matching your case.

> 
>>
>>> +    type: object
>>> +    $ref: leds-class-multicolor.yaml#
>>
>>     unevaluatedProperties: false
> 
> OK.
> 
>>> +
>>> +    properties:
>>> +      color-index:
>>> +        description: |
>>> +          A 3-item array specifying color of each components in this LED. It
>>> +          should be one of the LED_COLOR_ID_* prefixed definitions from the
>>> +          header include/dt-bindings/leds/common.h. Defaults to
>>> +          <LED_COLOR_ID_GREEN LED_COLOR_ID_RED LED_COLOR_ID_BLUE>
>>> +          if unspecified.
>>> +        $ref: /schemas/types.yaml#/definitions/uint32-array
>>> +        minItems: 3
>>
>> Drop minItems.... but see comment below:
>>
>>> +        maxItems: 3
>>
>> Why this is different than other multi-color LEDs? I would expect here
>> children with common.yaml.
> 
> WS2812B is a single LED package with 3 diodes and a microcontroller.
> Each LED package has 3 colors. The original chip comes with GRB
> color while there are some clones with RGB arrangement instead.
> The LED chain can be really long so I'd like to simplify the binding
> by using a single property to override the only variable, color, here.

OK, that makes sense.

> 
>>
>>> +
>>> +      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
>>
>> What controls the intensity? Don't you have PWM there?
> 
> The LED takes 3-byte brightness value of each color. This property is used to
> specify the default multi_intensity field for the multi-color LED. The final
> brightness value is calculated with led_mc_calc_color_components like this:
> 
> mcled_cdev->subled_info[i].brightness = brightness *
> mcled_cdev->subled_info[i].intensity / led_cdev->max_brightness;
> 
> The LED chip takes exactly 8 bits for the brightness (max_brightness = 255
> which can't be changed.), so according to the formula above the maximum
> intensity should be 255.

So this is brightness of each color... why insisting on calling it
differently?

Best regards,
Krzysztof