Re: [PATCH v2 1/2] dt-bindings: display: bridge: Add GPIO display mux binding

From: Pin-yen Lin
Date: Tue Feb 07 2023 - 05:31:23 EST

Hi Laurent,

On Tue, Feb 7, 2023 at 6:25 PM Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 07, 2023 at 06:07:44PM +0800, Pin-yen Lin wrote:
> > On Wed, Jan 18, 2023 at 4:17 AM Rob Herring wrote:
> > > On Mon, Jan 16, 2023 at 07:08:19PM +0800, Pin-yen Lin wrote:
> > > > From: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> > > >
> > > > Add bindings for Generic GPIO mux driver.
> > > >
> > > > Signed-off-by: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> > > > Signed-off-by: Pin-yen Lin <treapking@xxxxxxxxxxxx>
> > > > ---
> > > >
> > > > Changes in v2:
> > > > - Referenced existing dt-binding schemas from graph.yaml
> > > > - Added ddc-i2c-bus into the bindings
> > > >
> > > > .../bindings/display/bridge/gpio-mux.yaml | 95 +++++++++++++++++++
> > > > 1 file changed, 95 insertions(+)
> > > > create mode 100644 Documentation/devicetree/bindings/display/bridge/gpio-mux.yaml
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/display/bridge/gpio-mux.yaml b/Documentation/devicetree/bindings/display/bridge/gpio-mux.yaml
> > > > new file mode 100644
> > > > index 000000000000..da29ba078f05
> > > > --- /dev/null
> > > > +++ b/Documentation/devicetree/bindings/display/bridge/gpio-mux.yaml
> > > > @@ -0,0 +1,95 @@
> > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > +%YAML 1.2
> > > > +---
> > > > +$id: http://devicetree.org/schemas/display/bridge/gpio-mux.yaml#
> > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > +
> > > > +title: Generic display mux (1 input, 2 outputs)
> > > > +
> > > > +maintainers:
> > > > + - Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> > > > +
> > > > +description: |
> > > > + This bindings describes a simple display (e.g. HDMI) mux, that has 1
> > > > + input, and 2 outputs. The mux status is controlled by hardware, and
> > > > + its status is read back using a GPIO.
> > > > +
> > > > +properties:
> > > > + compatible:
> > > > + const: gpio-display-mux
> > > > +
> > > > + detect-gpios:
> > > > + maxItems: 1
> > > > + description: GPIO that indicates the active output
> > >
> > > What are we detecting? That implies an input, but this is selecting the
> > > output path, right? Or what does 'mux status is controlled by hardware'
> > > mean exactly? Something else? That does not sound very generic.
> >
> > The GPIO (or any kind of MUX) is an input that indicates where the
> > output should go. The actual "output selection" procedure is done in
> > the driver. That is, the driver monitors this GPIO and selects the
> > output path accordingly. In our use case, the GPIO is reported by the
> > embedded controller on the device.
> >
> > [1] listed other similar bridges that can leverage this driver, so we
> > called this driver "generic".
> >
> > [1]: https://lore.kernel.org/all/CAJMQK-jGw8kJFNjoHjeZUL+3NCiOS2hgGERnAnMwNsL_cm_J=Q@xxxxxxxxxxxxxx/
> >
> > > In any case, we have a common mux binding so any kind of mux control
> > > could be used here, not just GPIO. Then you can make this just a generic
> > > display mux.
> >
> > Thanks for sharing this, I'll update the binding in the next version.
> >
> > > > +
> > > > + ddc-i2c-bus:
> > > > + description: phandle link to the I2C controller used for DDC EDID probing
> > > > + $ref: /schemas/types.yaml#/definitions/phandle
> > >
> > > This belongs in the connector node(s).
> >
> > The HDMI bridge before the MUX doesn't (and doesn't have to) know that
> > its next bridge is a MUX. We put it here so that the HDMI bridge can
> > parse the phandle and get the bus node.
>
> How does that work, does the HDMI encoder driver parse the ddc-i2c-bus
> property of the next DT node in the OF graph ?

Yes. In our use case, mtk_hdmi.c[2] checks the remote node of its
output port to get the bus phandle. sun4i_hdmi_enc.c[3] seems to use a
similar approach as well.

[2]: https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/mediatek/mtk_hdmi.c#L1500
[3]: https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c#L240

Regards,
Pin-yen
>
> > > > +
> > > > + ports:
> > > > + $ref: /schemas/graph.yaml#/properties/ports
> > > > +
> > > > + properties:
> > > > + port@0:
> > > > + $ref: /schemas/graph.yaml#/properties/port
> > > > + description: |
> > > > + Video port for input.
> > > > +
> > > > + port@1:
> > > > + $ref: /schemas/graph.yaml#/properties/port
> > > > + description: |
> > > > + 2 video ports for output.
> > > > + The reg value in the endpoints matches the GPIO status: when
> > > > + GPIO is asserted, endpoint with reg value <1> is selected.
> > > > +
> > > > + required:
> > > > + - port@0
> > > > + - port@1
> > > > +
> > > > +required:
> > > > + - compatible
> > > > + - detect-gpios
> > > > + - ports
> > > > +
> > > > +unevaluatedProperties: false
> > > > +
> > > > +examples:
> > > > + - |
> > > > + #include <dt-bindings/gpio/gpio.h>
> > > > + hdmi_mux: hdmi_mux {
> > > > + compatible = "gpio-display-mux";
> > > > + detect-gpios = <&pio 36 GPIO_ACTIVE_HIGH>;
> > > > + pinctrl-names = "default";
> > > > + pinctrl-0 = <&hdmi_mux_pins>;
> > > > + ddc-i2c-bus = <&hdmiddc0>;
> > > > +
> > > > + ports {
> > > > + #address-cells = <1>;
> > > > + #size-cells = <0>;
> > > > +
> > > > + port@0 { /* input */
> > > > + reg = <0>;
> > > > +
> > > > + hdmi_mux_in: endpoint {
> > > > + remote-endpoint = <&hdmi0_out>;
> > > > + };
> > > > + };
> > > > +
> > > > + port@1 { /* output */
> > > > + reg = <1>;
> > > > +
> > > > + #address-cells = <1>;
> > > > + #size-cells = <0>;
> > > > +
> > > > + hdmi_mux_out_anx: endpoint@0 {
> > > > + reg = <0>;
> > > > + remote-endpoint = <&dp_bridge_in>;
> > > > + };
> > > > +
> > > > + hdmi_mux_out_hdmi: endpoint@1 {
> > > > + reg = <1>;
> > > > + remote-endpoint = <&hdmi_connector_in>;
> > > > + };
> > > > + };
> > > > + };
> > > > + };
>
> --
> Regards,
>
> Laurent Pinchart