linux-kernel - Re: [PATCH v2 1/2] dt-bindings: clock: Add nvmem-clock

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Message-ID: <20230526-wok-thwarting-90b7170d9e99@spud>
Date:   Fri, 26 May 2023 19:47:13 +0100
From:   Conor Dooley <conor@...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>,
        Rob Herring <robh+dt@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: clock: Add nvmem-clock

Hey Mike,

I have no comments about whether this is a good idea or not, I'll leave
that Stephen, Krzysztof etc..

On Fri, May 26, 2023 at 04:38:06PM +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 v2:
> Changed "fixed-clock" into "nvmem-clock" in dts example
> Add minItems:1 to nvmem-cell-names
> 
>  .../bindings/clock/nvmem-clock.yaml           | 63 +++++++++++++++++++
>  1 file changed, 63 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/clock/nvmem-clock.yaml
> 
> diff --git a/Documentation/devicetree/bindings/clock/nvmem-clock.yaml b/Documentation/devicetree/bindings/clock/nvmem-clock.yaml
> new file mode 100644
> index 000000000000..af96a5e9372d
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/clock/nvmem-clock.yaml
> @@ -0,0 +1,63 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/clock/nvmem-clock.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Simple fixed-rate clock source from NVMEM
> +
> +maintainers:
> +  - Mike Looijmans <mike.looijmans@...ic.nl>
> +
> +description:
> +  Provides a clock rate from NVMEM. Typical usage is that the factory places a
> +  crystal on the board and writes the rate into an EEPROM or EFUSE. If some math
> +  is required, one can add a fixed-factor clock using this clock as input.
> +
> +properties:
> +  compatible:
> +    const: nvmem-clock
> +
> +  "#clock-cells":
> +    const: 0
> +
> +  nvmem-cells:
> +    minItems: 1
> +    maxItems: 2
> +    description:
> +      Reads clock-frequency and optionally 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.

How is that size communicated to the driver? Some nvmem magic just takes
care of it?

> +
> +  nvmem-cell-names:
> +    minItems: 1
> +    items:
> +      - const: clock-frequency
> +      - const: clock-accuracy
> +
> +  clock-accuracy:

I think the common pattern is to add a unit suffix for things like
this, which I guess would be -ppb?

Cheers,
Conor.

> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    description:
> +      accuracy of clock in ppb (parts per billion). Alternative for providing
> +      this through nvmem, the nvmem provided value takes precedence.
> +
> +  clock-output-names:
> +    maxItems: 1
> +
> +required:
> +  - compatible
> +  - "#clock-cells"
> +  - nvmem-cells
> +  - nvmem-cell-names
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    xtal {
> +      compatible = "nvmem-clock";
> +      #clock-cells = <0>;
> +      nvmem-cells = <&efuse_xtal_freq>;
> +      nvmem-cell-names = "clock-frequency";
> +    };
> +...
> -- 
> 2.17.1
> 
> 
> Met vriendelijke groet / kind regards,
> 
> Mike Looijmans
> System Expert
> 
> 
> TOPIC Embedded Products B.V.
> Materiaalweg 4, 5681 RJ Best
> The Netherlands
> 
> T: +31 (0) 499 33 69 69
> E: mike.looijmans@...icproducts.com
> W: www.topic.nl
> 
> Please consider the environment before printing this e-mail

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