linux-kernel - Re: [PATCH v4 1/2] dt-bindings: clock: fixed-clock: Add nvmem support

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Date:   Wed, 7 Jun 2023 16:41:55 -0600
From:   Rob Herring <robh@...nel.org>
To:     Mike Looijmans <mike.looijmans@...ic.nl>
Cc:     devicetree@...r.kernel.org, linux-clk@...r.kernel.org,
        Conor Dooley <conor+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] dt-bindings: clock: fixed-clock: Add nvmem support

On Mon, Jun 05, 2023 at 04:01:48PM +0200, Mike Looijmans wrote:
> Add bindings for a fixed-rate clock that retrieves its rate from an
> NVMEM provider. This allows to store clock settings in EEPROM or EFUSE
> or similar device.
> 
> Component shortages lead to boards being shipped with different clock
> crystals, based on what was available at the time. The clock frequency
> was written to EEPROM at production time. Systems can adapt to a wide
> range of input frequencies using the clock framework, but this required
> us to patch the devicetree at runtime or use some custom driver. This
> provides a more generic solution.
> 
> Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
> 
> ---
> 
> Changes in v4:
> Use proper "if" block and add example dts
> 
> Changes in v3:
> Modify fixed-clock instead of introducing nvmem-clock
> 
> Changes in v2:
> Changed "fixed-clock" into "nvmem-clock" in dts example
> Add minItems:1 to nvmem-cell-names
> 
>  .../bindings/clock/fixed-clock.yaml           | 42 ++++++++++++++++++-
>  1 file changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/clock/fixed-clock.yaml b/Documentation/devicetree/bindings/clock/fixed-clock.yaml
> index b0a4fb8256e2..71a5791da438 100644
> --- a/Documentation/devicetree/bindings/clock/fixed-clock.yaml
> +++ b/Documentation/devicetree/bindings/clock/fixed-clock.yaml
> @@ -12,7 +12,9 @@ maintainers:
>  
>  properties:
>    compatible:
> -    const: fixed-clock
> +    enum:
> +      - fixed-clock
> +      - fixed-clock-nvmem
>  
>    "#clock-cells":
>      const: 0
> @@ -26,11 +28,41 @@ properties:
>    clock-output-names:
>      maxItems: 1
>  
> +  nvmem-cells:
> +    minItems: 1
> +    maxItems: 2
> +    description:
> +      Reads clock-frequency and/or clock-accuracy from an NVMEM provider in
> +      binary native integer format. The size of the NVMEM cell can be 1, 2, 4
> +      or 8 bytes. If the contents of the nvmem are all zeroes or all 0xff, the
> +      value reverts to the one given in the property.
> +
> +  nvmem-cell-names:
> +    minItems: 1
> +    items:
> +      - const: clock-frequency
> +      - const: clock-accuracy
> +
>  required:
>    - compatible
>    - "#clock-cells"
>    - clock-frequency
>  
> +allOf:
> +  - if:
> +      properties:
> +        compatible:
> +          contains:
> +            const: fixed-clock-nvmem
> +    then:
> +      required:
> +        - nvmem-cells
> +        - nvmem-cell-names
> +    else:
> +      properties:
> +        nvmem-cells: false
> +        nvmem-cell-names: false
> +
>  additionalProperties: false
>  
>  examples:
> @@ -41,4 +73,12 @@ examples:
>        clock-frequency = <1000000000>;
>        clock-accuracy = <100>;
>      };
> +  - |
> +    clock {
> +      compatible = "fixed-clock-nvmem";
> +      #clock-cells = <0>;
> +      clock-frequency = <48000000>;

If the freq comes from nvmem, why is this needed?

I think this should probably be a separate schema file as the only 
other thing shared is #clock-cells.

Rob