Re: [PATCH 1/2] dt-bindings: i2c: Add i2c-shared-gpio

[Rob Herring <robh@kernel.org>]

On Thu, May 07, 2026 at 08:17:10PM +0200, Markus Stockhausen wrote:
> Document the driver for bitbanged gpio I2C busses
> with shared SCL lines.
> 
> Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
> ---
>  .../bindings/i2c/i2c-gpio-shared.yaml         | 115 ++++++++++++++++++
>  1 file changed, 115 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/i2c/i2c-gpio-shared.yaml
> 
> diff --git a/Documentation/devicetree/bindings/i2c/i2c-gpio-shared.yaml b/Documentation/devicetree/bindings/i2c/i2c-gpio-shared.yaml
> new file mode 100644
> index 000000000000..7db344821e2f
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/i2c/i2c-gpio-shared.yaml
> @@ -0,0 +1,115 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/i2c/i2c-gpio-shared.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Multiple GPIO bitbanged I2C buses with shared SCL
> +
> +maintainers:
> +  - Markus Stockhausen <markus.stockhausen@gmx.de>
> +
> +description:
> +  Bitbanging I2C bus driver that supports multiple independent I2C buses
> +  sharing a single SCL line. Each child node represents one I2C bus with
> +  its own SDA line. The shared SCL line is driven by the parent node.
> +  A mutex serializes access so that only one bus transfers at a time.

Unless it's a h/w mutex, that doesn't belong in the binding. Just define 
the requirement, not how it might be implemented.

This is basically a mux, so you should leverage i2c-mux.yaml. Maybe 
there is driver infrastructure you can leverage too.

> +
> +select:
> +  properties:
> +    compatible:
> +      contains:
> +        const: i2c-gpio-shared
> +  required:
> +    - compatible

You don't need 'select'.

> +
> +properties:
> +  compatible:
> +    const: i2c-gpio-shared
> +
> +  scl-gpios:
> +    maxItems: 1
> +    description:
> +      GPIO used for the shared SCL signal. Must be configured as
> +      open-drain. All child buses share this single clock line.
> +
> +  i2c-gpio-shared,scl-output-only:
> +    type: boolean
> +    description:
> +      If present, SCL is treated as output only and clock stretching
> +      by devices is not supported.
> +
> +  i2c-gpio-shared,timeout-ms:
> +    description:
> +      Bus timeout in milliseconds. If not specified, defaults to 100 ms.

These 2 properties don't appear to be specific in any way to this 
particular h/w.

> +
> +  "#address-cells":
> +    const: 1
> +
> +  "#size-cells":
> +    const: 0
> +
> +patternProperties:
> +  "^i2c@[0-9a-f]+$":
> +    $ref: /schemas/i2c/i2c-controller.yaml#
> +    unevaluatedProperties: false
> +
> +    properties:
> +      reg:
> +        maxItems: 1
> +        description:
> +          Bus index used to identify this child bus. Must be unique among
> +          siblings and match the node unit address.
> +
> +      sda-gpios:
> +        maxItems: 1
> +        description:
> +          GPIO used for the SDA signal of this I2C bus. Must be
> +          configured as open-drain.

I would move this to the parent and make it multiple entries. 

> +
> +      i2c-gpio-shared,delay-us:
> +        default: 5
> +        description:
> +          Delay in microseconds between signal transitions for this bus.
> +          Controls the I2C clock frequency. Defaults to 5 us (~100 kHz).

We already have 'clock-frequency' to define the bus freq.

> +
> +      i2c-gpio-shared,sda-output-only:
> +        type: boolean
> +        description:
> +          If present, SDA is treated as output only. No acknowledgment
> +          or read data from devices can be received on this bus.
> +
> +    required:
> +      - reg
> +      - sda-gpios
> +
> +required:
> +  - compatible
> +  - scl-gpios
> +  - "#address-cells"
> +  - "#size-cells"
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    #include <dt-bindings/gpio/gpio.h>
> +
> +    i2c-gpio-shared {
> +        compatible = "i2c-gpio-shared";
> +        scl-gpios = <&gpio1 31 (GPIO_ACTIVE_HIGH | GPIO_OPEN_DRAIN)>;
> +        #address-cells = <1>;
> +        #size-cells = <0>;
> +
> +        i2c@0 {
> +            reg = <0>;
> +            sda-gpios = <&gpio1 6 (GPIO_ACTIVE_HIGH | GPIO_OPEN_DRAIN)>;
> +            i2c-gpio-shared,delay-us = <2>;
> +        };
> +
> +        i2c@1 {
> +            reg = <1>;
> +            sda-gpios = <&gpio1 7 (GPIO_ACTIVE_HIGH | GPIO_OPEN_DRAIN)>;
> +            i2c-gpio-shared,delay-us = <2>;
> +        };
> +    };
> -- 
> 2.54.0
>