[v3 1/2] dt-bindings: backlight: add DisplayPort aux backlight
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Tue May 11 22:33:50 UTC 2021
Hi Rajeevny,
On Tue, May 11, 2021 at 11:41:57PM +0530, rajeevny at codeaurora.org wrote:
> On 01-05-2021 03:08, Doug Anderson wrote:
> > On Fri, Apr 30, 2021 at 8:10 AM <rajeevny at codeaurora.org> wrote:
> >> On 30-04-2021 02:33, Doug Anderson wrote:
> >> > On Thu, Apr 29, 2021 at 11:04 AM Rob Herring <robh at 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 at 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 at 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.
> >>
> >> 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.
>
> Below is the summary of the changes in drm dp helper:
>
> ---
>
> // drm_dp_helper.h
>
> struct drm_dp_aux {
> ...
> struct list_head list;
> ...
> }
>
> // drm_dp_helper.c
>
> static DEFINE_MUTEX(dp_aux_lock);
> static LIST_HEAD(dp_aux_list);
>
> static void drm_dp_aux_add(struct drm_dp_aux *aux)
> {
> mutex_lock(&dp_aux_lock);
> list_add_tail(&aux->list, &dp_aux_list);
> mutex_unlock(&dp_aux_lock);
> }
>
> static void drm_dp_aux_remove(struct drm_dp_aux *aux)
> {
> mutex_lock(&dp_aux_lock);
> list_del_init(&aux->list);
> mutex_unlock(&dp_aux_lock);
> }
>
> #ifdef CONFIG_OF
> struct drm_dp_aux *of_drm_find_dp_aux_by_node(struct device_node *np)
> {
> struct drm_dp_aux *aux;
> mutex_lock(&dp_aux_lock);
>
> list_for_each_entry(aux, &dp_aux_list, list) {
> if (aux->dev->of_node == np) {
> mutex_unlock(&dp_aux_lock);
> return aux;
> }
> }
>
> mutex_unlock(&dp_aux_lock);
> return NULL;
> }
> EXPORT_SYMBOL(of_drm_find_dp_aux_by_node);
> #endif
>
>
> int drm_dp_aux_init(struct drm_dp_aux *aux)
> {
> INIT_LIST_HEAD(&aux->list);
> ...
> }
>
> int drm_dp_aux_register(struct drm_dp_aux *aux)
> {
> ...
> drm_dp_aux_add(aux);
>
> return 0;
> }
>
> void drm_dp_aux_unregister(struct drm_dp_aux *aux)
> {
> drm_dp_aux_remove(aux);
> ...
> }
Overall this seems like a good approach, but there's one unanswered
question: what happens if drm_dp_aux_unregister() is called while a
panel holds a reference to it ? The drm_dp_aux instances likely need to
be reference-counted.
> > 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.
--
Regards,
Laurent Pinchart
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