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SubjectRe: [v3 1/2] dt-bindings: backlight: add DisplayPort aux backlight
On 30-04-2021 02:33, Doug Anderson wrote:
> 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

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@intel.com/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/20210426111116.4lc3ekxjugjr3oho@maple.lan/

Thanks,
Rajeev

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