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Date:   Tue, 25 May 2021 13:33:08 +0530
From:   rajeevny@codeaurora.org
To:     Doug Anderson <dianders@chromium.org>
Cc:     Rob Herring <robh@kernel.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>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Lyude Paul <lyude@redhat.com>,
        "Lankhorst, Maarten" <maarten.lankhorst@intel.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Andrzej Hajda <a.hajda@samsung.com>
Subject: Re: [v3 1/2] dt-bindings: backlight: add DisplayPort aux backlight
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Hi,

On 18-05-2021 01:51, Doug Anderson wrote:
> 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

Thank you, for all your suggestions.

I have posted the new patch series (v4) on top of the above series. 
Please check:
https://lore.kernel.org/dri-devel/1621927831-29471-1-git-send-email-rajeevny@codeaurora.org/


Thanks,
Rajeev