linux-kernel - Re: [RFCv2 PATCH 3/4] dt-bindings: gpio: add Wiegand GPIO driver dt documentation

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Date:   Thu, 6 Oct 2022 10:34:08 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Martin Zaťovič <m.zatovic1@...il.com>,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        linus.walleij@...aro.org, brgl@...ev.pl,
        gregkh@...uxfoundation.org, jeffrey.l.hugo@...il.com,
        andersson@...nel.org, Michael.Srba@...nam.cz, saravanak@...gle.com,
        mani@...nel.org, hemantk@...eaurora.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-gpio@...r.kernel.org
Subject: Re: [RFCv2 PATCH 3/4] dt-bindings: gpio: add Wiegand GPIO driver dt
 documentation

On 06/10/2022 10:23, Krzysztof Kozlowski wrote:
> On 05/10/2022 16:57, Martin Zaťovič wrote:
>> The Wiegand GPIO driver uses two GPIO lines to transmit data -
>> data-hi and data-lo. These lines need to be defined in the
>> devicetree, otherwise the driver will not probe successfully.
>>
>> Signed-off-by: Martin Zaťovič <m.zatovic1@...il.com>
>> ---
>>  .../bindings/gpio/gpio-wiegand.yaml           | 53 +++++++++++++++++++
>>  1 file changed, 53 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/gpio/gpio-wiegand.yaml
>>
>> diff --git a/Documentation/devicetree/bindings/gpio/gpio-wiegand.yaml b/Documentation/devicetree/bindings/gpio/gpio-wiegand.yaml
>> new file mode 100644
>> index 000000000000..3b235667ae17
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/gpio/gpio-wiegand.yaml
>> @@ -0,0 +1,53 @@
>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/gpio/gpio-wiegand.yaml#
>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: Wiegand GPIO controller
>> +
>> +description: |
>> +  Wiegand GPIO controller running under Wiegand bus.
> 
> GPIO controllers need "gpio-controller" property, so this seems to be
> something else.
> 
>> +
>> +maintainers:
>> +  - Martin Zaťovič <m.zatovic1@...il.com>
>> +
>> +properties:
>> +  $nodename:
>> +    pattern: "^wiegand-gpio@[0-9a-f]+$"
> 
> No need to enforce node name, unless this is for a class of devices. But
> then why "gpio" not just "wiegand"?
> 
>> +
>> +  compatible:
>> +    const: wiegand,wiegand-gpio
>> +
>> +  data-hi-gpios:
>> +    description: GPIO spec for data-hi line to use
>> +    maxItems: 1
>> +
>> +  data-lo-gpios:
>> +    description: GPIO spec for data-lo line to use
>> +    maxItems: 1
>> +
>> +required:
>> +  - compatible
>> +  - data-hi-gpios
>> +  - data-lo-gpios
>> +
>> +additionalProperties: false
>> +
>> +examples:
>> +  - |
>> +    #include <dt-bindings/gpio/gpio.h>
>> +
>> +    wiegand {
>> +        compatible = "wiegand";
>> +
>> +        wiegand-gpio {
> 
> I have troubles understanding this. The "wiegand" node is the bus,
> right? Then what is "wiegand-gpio"? GPIO controller? Then why it is not
> marked as GPIO controller? What GPIOs does it control?
> 
>> +            compatible = "wiegand,wiegand-gpio";
>> +            pinctrl-names = "default";
>> +            pinctrl-0 = <&pinctrl_uart2_wiegand>;
>> +            data-hi-gpios = <&gpio2 7 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
>> +            data-lo-gpios = <&gpio2 6 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
> 
> Aren't these properties of the bus, not the device?

So this looks like specific implementation of Wiegand bus - a Wiegand
bus controller. If it is correct, it should not be represented as child
of a bus... because this is a bus. IOW, just like SPI or I2C controllers
(why is this one different?), the bus is defined by SPI controller:

wiegand {
	compatible = "wiegand,wiegand-gpio";
	data-hi-gpios = <&gpio2 7 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
	data-lo-gpios = <&gpio2 6 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;

	child-device, e.g. some-card {
		compatible = "foo,bar";
		// more properties of the device
	};
};

Best regards,
Krzysztof