Message-ID: <20230526140513.6943-1-mike.looijmans@topic.nl>
Date:   Fri, 26 May 2023 16:05:12 +0200
From:   Mike Looijmans <mike.looijmans@...ic.nl>
To:     devicetree@...r.kernel.org, linux-clk@...r.kernel.org
CC:     Mike Looijmans <mike.looijmans@...ic.nl>,
        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: [PATCH 1/2] dt-bindings: clock: Add nvmem-clock

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>

---

Note this patch causes dt_binding_check errors that I need some help
with. It claims "clock-frequency" is a required property, though I don't
want it to be, and it also doesn't like the nvmem entries, claiming they
should match "pinctrl-[0-9]+". Cannot make sense of the error messages...


 .../bindings/clock/nvmem-clock.yaml           | 62 +++++++++++++++++++
 1 file changed, 62 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..2e40df150b59
--- /dev/null
+++ b/Documentation/devicetree/bindings/clock/nvmem-clock.yaml
@@ -0,0 +1,62 @@
+# 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.
+
+  nvmem-cell-names:
+    items:
+      - const: clock-frequency
+      - const: clock-accuracy
+
+  clock-accuracy:
+    $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 = "fixed-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


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