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

[Doug Anderson <dianders@chromium.org>]

Hi,

On Tue, May 11, 2021 at 4:17 PM Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Tue, May 11, 2021 at 11:12 AM <rajeevny@codeaurora.org> wrote:
> >
> > On 01-05-2021 03:08, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Fri, Apr 30, 2021 at 8:10 AM <rajeevny@codeaurora.org> wrote:
> > >>
> > >> 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/
> > >
> > > 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.
> > >
> >
> > To implement the first suggestion, I can think of the following way
> > to get the "struct drm_dp_aux" in the panel_simple_probe function:
> >
> > - Create a new panel-simple DT property "ddc-aux-bus", a phandle to the
> > platform device that implements the AUX channel.
> >
> > - Create a global list of drm_dp_aux in drm_dp_helper.c. Initialize list
> > head
> > in drm_dp_aux_init(), add the drm_dp_aux onto the list in
> > drm_dp_aux_register().
> > Similarly, remove the drm_dp_aux from list in drm_dp_aux_unregister().
> >
> > - Create a new function of_drm_find_dp_aux_by_node() to get the expected
> > drm_dp_aux from this global list.
> >
> > Please let me know your views on this implementation.
>
> BTW: a bunch of discussion today on IRC:
>
> https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2021-05-11&show_html=true

To finish adding breadcrumbs to those playing at home, v7 of my patch
series should get the AUX channel to the panel device:

https://lore.kernel.org/r/20210517200907.1459182-1-dianders@chromium.org/

Assuming that people don't hate that solution it may be a good things
for you to build your patches atop. :-)

-Doug