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From:   Doug Anderson <dianders@chromium.org>
Date:   Thu, 29 Apr 2021 14:03:18 -0700
Message-ID: <CAD=FV=V-kdySH5Pp-Fb-PRYk60Ha_UOTXJHcvMp+uV3P1oo7Uw@mail.gmail.com>
Subject: Re: [v3 1/2] dt-bindings: backlight: add DisplayPort aux backlight
To:     Rob Herring <robh@kernel.org>
Cc:     Rajeev Nandan <rajeevny@codeaurora.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Rob Clark <robdclark@gmail.com>, mkrishn@codeaurora.org,
        Kalyan Thota <kalyan_t@codeaurora.org>,
        "Kristian H. Kristensen" <hoegsberg@chromium.org>,
        Abhinav Kumar <abhinavk@codeaurora.org>,
        Sean Paul <seanpaul@chromium.org>
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Precedence: bulk

Hi,

On Thu, Apr 29, 2021 at 11:04 AM Rob Herring <robh@kernel.org> wrote:
>
> On Mon, Apr 26, 2021 at 11:29:15AM +0530, Rajeev Nandan wrote:
> > Add bindings for DisplayPort aux backlight driver.
> >
> > Changes in v2:
> > - New
> >
> > Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>
> > ---
> >  .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
> >  1 file changed, 49 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml b/Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml
> > new file mode 100644
> > index 00000000..0fa8bf0
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml
> > @@ -0,0 +1,49 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: DisplayPort aux backlight driver bindings
> > +
> > +maintainers:
> > +  - Rajeev Nandan <rajeevny@codeaurora.org>
> > +
> > +description:
> > +  Backlight driver to control the brightness over DisplayPort aux channel.
> > +
> > +allOf:
> > +  - $ref: common.yaml#
> > +
> > +properties:
> > +  compatible:
> > +    const: dp-aux-backlight
> > +
> > +  ddc-i2c-bus:
> > +    $ref: /schemas/types.yaml#/definitions/phandle
> > +    description:
> > +      A phandle to the system I2C controller connected to the DDC bus used
> > +      for the DisplayPort AUX channel.
> > +
> > +  enable-gpios:
> > +    maxItems: 1
> > +    description: GPIO specifier for backlight enable pin.
> > +
> > +  max-brightness: true
> > +
> > +required:
> > +  - compatible
> > +  - ddc-i2c-bus
> > +
> > +additionalProperties: false
> > +
> > +examples:
> > +  - |
> > +    backlight {
> > +        compatible = "dp-aux-backlight";
> > +        ddc-i2c-bus = <&sn65dsi86_bridge>;
> > +        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;
>
> So the DDC bus is connected to a backlight and also a panel? This
> binding is not reflecting the h/w, but rather what you want for some
> driver.
>
> There's only one thing here and that's an eDP panel which supports
> backlight control via DP aux channel. You can figure all that out from
> the panel's compatible and/or reading the EDID.
>
> You might also be interested in this thread:
>
> https://lore.kernel.org/lkml/YIKsDtjcIHGNvW0u@orome.fritz.box/

I think Rajeev needs to rework everything anyway as per:

https://lore.kernel.org/r/87zgxl5qar.fsf@intel.com

...but you're right that it makes sense not to model the backlight as
a separate node in the device tree. The panel driver can handle
setting up the backlight.

-Doug