Re: [v3 1/2] dt-bindings: backlight: add DisplayPort aux backlight

[rajeevny]

On 30-04-2021 02:33, Doug Anderson wrote:
> Hi,
>
> On Thu, Apr 29, 2021 at 11:04 AM Rob Herring <robh@xxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > > ---
> > >  .../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@xxxxxxxxxxxxxx>
> > > +
> > > +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@xxxxxxxxxxxxxxx/
>
> I think Rajeev needs to rework everything anyway as per:
>
> https://lore.kernel.org/r/87zgxl5qar.fsf@xxxxxxxxx
>
> ...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

It was not a good idea to create a separate backlight driver and use
ddc-i2c-bus to get access to DP aux. I am working to move the code
to the panel driver and to utilize the new DRM helper functions
(drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to 
the
"struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property
to give the panel access to the DDC bus and is currently being used to
get the EDID from the panel. Can I use the same ddc bus i2c_adapter to 
get
the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the
i2c_adapter of ddc bus (maybe I didn't understand the suggestion 
correctly),
and, it turned out, the way I have implemented is not the right way [3].
So, I am afraid to use the same method in the panel driver.


[1] https://lore.kernel.org/dri-devel/871rb5bcf9.fsf@xxxxxxxxx/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3] 
https://lore.kernel.org/dri-devel/20210426111116.4lc3ekxjugjr3oho@xxxxxxxxx/

Thanks,
Rajeev