Re: [PATCH 2/2] schemas: i2c: Introduce I2C bus extensions

[Ayush Singh <ayush@beagleboard.org>]

On 4/1/25 13:40, Herve Codina wrote:

> An I2C bus can be wired to the connector and allows an add-on board to
> connect additional I2C devices to this bus.
>
> Those additional I2C devices could be described as sub-nodes of the I2C
> bus controller node however for hotplug connectors described via device
> tree overlays there is additional level of indirection, which is needed
> to decouple the overlay and the base tree:
>
>    --- base device tree ---
>
>    i2c1: i2c@abcd0000 {
>        compatible = "xyz,i2c-ctrl";
>        i2c-bus-extension@0 {
>            i2c-bus = <&i2c_ctrl>;
>        };
>        ...
>    };
>
>    i2c5: i2c@cafe0000 {
>        compatible = "xyz,i2c-ctrl";
>        i2c-bus-extension@0 {
>            i2c-bus = <&i2c-sensors>;
>        };
>        ...
>    };
>
>    connector {
>        i2c_ctrl: i2c-ctrl {
>            i2c-parent = <&i2c1>;
>            #address-cells = <1>;
>            #size-cells = <0>;
>        };
>
>        i2c-sensors {
>            i2c-parent = <&i2c5>;
>            #address-cells = <1>;
>            #size-cells = <0>;
>        };
>    };
>
>    --- device tree overlay ---
>
>    ...
>    // This node will overlay on the i2c-ctrl node of the base tree
>    i2c-ctrl {
>        eeprom@50 { compatible = "atmel,24c64"; ... };
>    };
>    ...
>
>    --- resulting device tree ---
>
>    i2c1: i2c@abcd0000 {
>        compatible = "xyz,i2c-ctrl";
>        i2c-bus-extension@0 {
>            i2c-bus = <&i2c_ctrl>;
>        };
>        ...
>    };
>
>    i2c5: i2c@cafe0000 {
>        compatible = "xyz,i2c-ctrl";
>        i2c-bus-extension@0 {
>            i2c-bus = <&i2c-sensors>;
>        };
>        ...
>    };
>
>    connector {
>        i2c-ctrl {
>            i2c-parent = <&i2c1>;
>            #address-cells = <1>;
>            #size-cells = <0>;
>
>            eeprom@50 { compatible = "atmel,24c64"; ... };
>        };
>
>        i2c-sensors {
>            i2c-parent = <&i2c5>;
>            #address-cells = <1>;
>            #size-cells = <0>;
>        };
>    };
>
> Here i2c-ctrl (same goes for i2c-sensors) represent the part of I2C bus
> that is on the hot-pluggable add-on. On hot-plugging it will physically
> connect to the I2C adapter on the base board. Let's call the 'i2c-ctrl'
> node an "extension node".
>
> In order to decouple the overlay from the base tree, the I2C adapter
> (i2c@abcd0000) and the extension node (i2c-ctrl) are separate nodes.
>
> The extension node is linked to the I2C bus controller in two ways. The
> first one with the i2c-bus-extension available in I2C controller
> sub-node and the second one with the i2c-parent property available in
> the extension node itself.
>
> The purpose of those two links is to provide the link in both direction
> from the I2C controller to the I2C extension and from the I2C extension
> to the I2C controller.
>
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---
>   dtschema/schemas/i2c/i2c-controller.yaml | 67 ++++++++++++++++++++++++
>   1 file changed, 67 insertions(+)
>
> diff --git a/dtschema/schemas/i2c/i2c-controller.yaml b/dtschema/schemas/i2c/i2c-controller.yaml
> index 018d266..509b581 100644
> --- a/dtschema/schemas/i2c/i2c-controller.yaml
> +++ b/dtschema/schemas/i2c/i2c-controller.yaml
> @@ -30,6 +30,13 @@ properties:
>       minimum: 1
>       maximum: 5000000
>   
> +  i2c-parent:
> +    $ref: /schemas/types.yaml#/definitions/phandle
> +    description:
> +      In case of an I2C bus extension, reference to the I2C bus controller
> +      this extension is connected to. In other word, reference the I2C bus
> +      controller on the fixed side that drives the bus extension.
> +
>     i2c-scl-falling-time-ns:
>       description:
>         Number of nanoseconds the SCL signal takes to fall; t(f) in the I2C
> @@ -159,6 +166,25 @@ allOf:
>           - i2c-scl-has-clk-low-timeout
>   
>   patternProperties:
> +  'i2c-bus-extension@[0-9a-f]+$':
> +    type: object
> +    description:
> +      An I2C bus extension connected to an I2C bus. Those extensions allow to
> +      decouple I2C busses when they are wired to connectors.
> +
> +    properties:
> +      reg:
> +        maxItems: 1
> +
> +      i2c-bus:
> +        $ref: /schemas/types.yaml#/definitions/phandle
> +        description:
> +          Reference to the extension bus.
> +
> +    required:
> +      - reg
> +      - i2c-bus
> +
>     '@[0-9a-f]+$':
>       type: object
>   
> @@ -221,3 +247,44 @@ dependentRequired:
>     i2c-digital-filter-width-ns: [ i2c-digital-filter ]
>   
>   additionalProperties: true
> +
> +examples:
> +  # I2C bus extension example involving an I2C bus controller and a connector.
> +  #
> +  #  +--------------+     +-------------+     +-------------+
> +  #  | i2c@abcd0000 |     |  Connector  |     | Addon board |
> +  #  |    (i2c1)    +-----+ (i2c-addon) +-----+ (device@10) |
> +  #  |              |     |             |     |             |
> +  #  +--------------+     +-------------+     +-------------+
> +  #
> +  # The i2c1 I2C bus is wired from a I2C controller to a connector. It is
> +  # identified at connector level as i2c-addon bus.
> +  # An addon board can be connected to this connector and connects a device
> +  # (device@10) to this i2c-addon extension bus.
> +  - |
> +    i2c1: i2c@abcd0000 {
> +        compatible = "xyz,i2c-ctrl";
> +        reg = <0xabcd0000 0x100>;
> +        #address-cells = <1>;
> +        #size-cells = <0>;
> +
> +        i2c-bus-extension@0 {
> +            reg = <0>;
> +            i2c-bus = <&i2c_addon>;
> +        };
> +    };
> +
> +    connector {
> +        i2c_addon: i2c-addon {
> +            i2c-parent = <&i2c1>;
> +            #address-cells = <1>;
> +            #size-cells = <0>;
> +
> +            device@10 {
> +                compatible = "xyz,foo";
> +                reg = <0x10>;
> +            };
> +        };
> +    };
> +
> +...


Reviewed-by: Ayush Singh <ayush@beagleboard.org>