Date:   Tue, 11 May 2021 16:17:59 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Rajeev Nandan <rajeevny@...eaurora.org>
Cc:     Rob Herring <robh@...nel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Rob Clark <robdclark@...il.com>, mkrishn@...eaurora.org,
        Kalyan Thota <kalyan_t@...eaurora.org>,
        "Kristian H. Kristensen" <hoegsberg@...omium.org>,
        Abhinav Kumar <abhinavk@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>,
        "Lankhorst, Maarten" <maarten.lankhorst@...el.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Andrzej Hajda <a.hajda@...sung.com>
Subject: Re: [v3 1/2] dt-bindings: backlight: add DisplayPort aux backlight

Hi,

On Tue, May 11, 2021 at 11:12 AM <rajeevny@...eaurora.org> wrote:
>
> On 01-05-2021 03:08, Doug Anderson wrote:
> > Hi,
> >
> > On Fri, Apr 30, 2021 at 8:10 AM <rajeevny@...eaurora.org> wrote:
> >>
> >> On 30-04-2021 02:33, Doug Anderson wrote:
> >> > Hi,
> >> >
> >> > On Thu, Apr 29, 2021 at 11:04 AM Rob Herring <robh@...nel.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@...eaurora.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@...eaurora.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

-Doug

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