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

[Doug Anderson]

Hi,

On Fri, Apr 30, 2021 at 8:10 AM <rajeevny@xxxxxxxxxxxxxx> wrote:
>
> 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/

So it's definitely up to maintainers, not me. ...but I guess I would
have expected something like a new property called "ddc-aux-bus". Then
you'd have to create a new API call called something like
"of_find_ddc_aux_adapter_by_node()" that would allow you to find it.

I guess an alternate way to solve this (I'm not totally sure whether
it's better or worse) would be to add a function that would walk up
the chain of parent bridges and ask them for a pointer to the aux bus.
I definitely haven't thought it all the way through, but I'd imagine
something like drm_bridge_chain_get_ddc_aux(). This is _probably_
better than adding the "ddc-aux-bus" property but it assumes that the
aux bus is provided by one of our parents. Hrm, looking at this
briefly, though, I'm not sure how to do it. It doesn't seem possible
to get the parent bridges from the panel structure. Even if you assume
that your parent is wrapping you with a panel_bridge it still doesn't
seem possible?

This probably needs more drm-expertise.

-Doug