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Date:   Fri, 26 May 2023 19:47:13 +0100
From:   Conor Dooley <conor@kernel.org>
To:     Mike Looijmans <mike.looijmans@topic.nl>
Cc:     devicetree@vger.kernel.org, linux-clk@vger.kernel.org,
        Conor Dooley <conor+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Rob Herring <robh+dt@kernel.org>,
        Stephen Boyd <sboyd@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: clock: Add nvmem-clock
Message-ID: <20230526-wok-thwarting-90b7170d9e99@spud>
References: <1b153bce-a66a-45ee-a5c6-963ea6fb1c82.949ef384-8293-46b8-903f-40a477c056ae.2167d5ad-7e99-4eb9-a313-030fc7a7d546@emailsignatures365.codetwo.com>
 <20230526143807.10164-1-mike.looijmans@topic.nl>
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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.
>=20
> 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.
>=20
> Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
>=20
> ---
>=20
> Changes in v2:
> Changed "fixed-clock" into "nvmem-clock" in dts example
> Add minItems:1 to nvmem-cell-names
>=20
>  .../bindings/clock/nvmem-clock.yaml           | 63 +++++++++++++++++++
>  1 file changed, 63 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/clock/nvmem-clock.y=
aml
>=20
> diff --git a/Documentation/devicetree/bindings/clock/nvmem-clock.yaml b/D=
ocumentation/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@topic.nl>
> +
> +description:
> +  Provides a clock rate from NVMEM. Typical usage is that the factory pl=
aces a
> +  crystal on the board and writes the rate into an EEPROM or EFUSE. If s=
ome math
> +  is required, one can add a fixed-factor clock using this clock as inpu=
t.
> +
> +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 prov=
iding
> +      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 =3D "nvmem-clock";
> +      #clock-cells =3D <0>;
> +      nvmem-cells =3D <&efuse_xtal_freq>;
> +      nvmem-cell-names =3D "clock-frequency";
> +    };
> +...
> --=20
> 2.17.1
>=20
>=20
> Met vriendelijke groet / kind regards,
>=20
> Mike Looijmans
> System Expert
>=20
>=20
> TOPIC Embedded Products B.V.
> Materiaalweg 4, 5681 RJ Best
> The Netherlands
>=20
> T: +31 (0) 499 33 69 69
> E: mike.looijmans@topicproducts.com
> W: www.topic.nl
>=20
> Please consider the environment before printing this e-mail

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