Re: [PATCH 1/3 v2] dt-bindings: backlight: Add some common backlight properties

[Daniel Thompson <daniel.thompson@linaro.org>]

On Wed, Aug 12, 2020 at 10:58:48AM +0200, Linus Walleij wrote:
> Let's use a common.yaml include for the backlight like we do with
> the LEDs. The LEDs are inherently incompatible so their bindings
> cannot be reused for backlight.
> 
> Cc: devicetree@vger.kernel.org
> Suggested-by: Sam Ravnborg <sam@ravnborg.org>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
> ChangeLog v1->v2:
> - New patch as suggested by Sam.
> ---
>  .../bindings/leds/backlight/common.yaml       | 42 +++++++++++++++++++
>  1 file changed, 42 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/leds/backlight/common.yaml
> 
> diff --git a/Documentation/devicetree/bindings/leds/backlight/common.yaml b/Documentation/devicetree/bindings/leds/backlight/common.yaml
> new file mode 100644
> index 000000000000..8ae7e3818b0d
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/leds/backlight/common.yaml
> @@ -0,0 +1,42 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/leds/backlight/common.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Common backlight properties
> +
> +maintainers:
> +  - Lee Jones <lee.jones@linaro.org>
> +  - Daniel Thompson <daniel.thompson@linaro.org>
> +  - Jingoo Han <jingoohan1@gmail.com>
> +
> +description: |
> +  Backlight devices provide backlight for different types of graphical
> +  displays. They are typically but not necessarilt implemented using a white
> +  LED powered by a boost converter.
> +
> +properties:
> +  default-on:
> +    description:
> +      The initial state of the backlight can be set to be on with this
> +      property. This is a state applied by the operating system so that the
> +      backlight is always turned on at boot.

Is default-on really a common property? I would describe it as legacy
that emerged when we added the gpio bindings and didn't spell
default-brightness correctly!

Currently I think this is only implemented for GPIO and it is simply
not needed for most hardware. More specifically, for hardware that is
capable of flicker-free handover (bootloader -> kernel) by examining
the hardware state at handover then we don't want a DT property. It is
duplicative and can only result in bad handovers.


Daniel.