Subject: [PATCH v5 02/11] dt-bindings: hwmon: Add Apple System Management Controller hwmon schema

From: James Calligeros <[email-masked]>
Date: Wed, 12 Nov 2025 21:16:48 +1000

Apple Silicon devices integrate a vast array of sensors, monitoring
current, power, temperature, and voltage across almost every part of
the system. The sensors themselves are all connected to the System
Management Controller (SMC). The SMC firmware exposes the data
reported by these sensors via its standard FourCC-based key-value
API. The SMC is also responsible for monitoring and controlling any
fans connected to the system, exposing them in the same way.

For reasons known only to Apple, each device exposes its sensors with
an almost totally unique set of keys. This is true even for devices
which share an SoC. An M1 Mac mini, for example, will report its core
temperatures on different keys to an M1 MacBook Pro. Worse still, the
SMC does not provide a way to enumerate the available keys at runtime,
nor do the keys follow any sort of reasonable or consistent naming
rules that could be used to deduce their purpose. We must therefore
know which keys are present on any given device, and which function
they serve, ahead of time.

Add a schema so that we can describe the available sensors for a given
Apple Silicon device in the Devicetree.

Reviewed-by: Neal Gompa <[email-masked]>
Reviewed-by: Rob Herring (Arm) <[email-masked]>
Signed-off-by: James Calligeros <[email-masked]>
---
 .../bindings/hwmon/apple,smc-hwmon.yaml  | 86 +++++++++++++++++++++++++
 .../bindings/mfd/apple,smc.yaml          | 36 +++++++++++
 MAINTAINERS                              |  1 +
 3 files changed, 123 insertions(+)

diff --git a/Documentation/devicetree/bindings/hwmon/apple,smc-hwmon.yaml b/Documentation/devicetree/bindings/hwmon/apple,smc-hwmon.yaml
new file mode 100644
index 000000000000..2eec317bc4b3
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwmon/apple,smc-hwmon.yaml
@@ -0,0 +1,86 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/hwmon/apple,smc-hwmon.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Apple SMC Hardware Monitoring
+
+description:
+  Apple's System Management Controller (SMC) exposes a vast array of
+  hardware monitoring sensors, including temperature probes, current and
+  voltage sense, power meters, and fan speeds. It also provides endpoints
+  to manually control the speed of each fan individually. Each Apple
+  Silicon device exposes a different set of endpoints via SMC keys. This
+  is true even when two machines share an SoC. The CPU core temperature
+  sensor keys on an M1 Mac mini are different to those on an M1 MacBook
+  Pro, for example.
+
+maintainers:
+  - James Calligeros <[email-masked]>
+
+$defs:
+  sensor:
+    type: object
+
+    properties:
+      apple,key-id:
+        $ref: /schemas/types.yaml#/definitions/string
+        pattern: "^[A-Za-z0-9]{4}$"
+        description: The SMC FourCC key of the desired sensor.
+          Must match the node's suffix.
+
+      label:
+        description: Human-readable name for the sensor
+
+    required:
+      - apple,key-id
+
+properties:
+  compatible:
+    const: apple,smc-hwmon
+
+patternProperties:
+  "^current-[A-Za-z0-9]{4}$":
+    $ref: "#/$defs/sensor"
+    unevaluatedProperties: false
+
+  "^fan-[A-Za-z0-9]{4}$":
+    $ref: "#/$defs/sensor"
+    unevaluatedProperties: false
+
+    properties:
+      apple,fan-minimum:
+        $ref: /schemas/types.yaml#/definitions/string
+        pattern: "^[A-Za-z0-9]{4}$"
+        description: SMC key containing the fan's minimum speed
+
+      apple,fan-maximum:
+        $ref: /schemas/types.yaml#/definitions/string
+        pattern: "^[A-Za-z0-9]{4}$"
+        description: SMC key containing the fan's maximum speed
+
+      apple,fan-target:
+        $ref: /schemas/types.yaml#/definitions/string
+        pattern: "^[A-Za-z0-9]{4}$"
+        description: Writeable endpoint for setting desired fan speed
+
+      apple,fan-mode:
+        $ref: /schemas/types.yaml#/definitions/string
+        pattern: "^[A-Za-z0-9]{4}$"
+        description: Writeable key to enable/disable manual fan control
+
+
+  "^power-[A-Za-z0-9]{4}$":
+    $ref: "#/$defs/sensor"
+    unevaluatedProperties: false
+
+  "^temperature-[A-Za-z0-9]{4}$":
+    $ref: "#/$defs/sensor"
+    unevaluatedProperties: false
+
+  "^voltage-[A-Za-z0-9]{4}$":
+    $ref: "#/$defs/sensor"
+    unevaluatedProperties: false
+
+additionalProperties: false
diff --git a/Documentation/devicetree/bindings/mfd/apple,smc.yaml b/Documentation/devicetree/bindings/mfd/apple,smc.yaml
index 0410e712c900..34ce048619f5 100644
--- a/Documentation/devicetree/bindings/mfd/apple,smc.yaml
+++ b/Documentation/devicetree/bindings/mfd/apple,smc.yaml
@@ -49,6 +49,9 @@ properties:
   rtc:
     $ref: /schemas/rtc/apple,smc-rtc.yaml
 
+  hwmon:
+    $ref: /schemas/hwmon/apple,smc-hwmon.yaml
+
 additionalProperties: false
 
 required:
@@ -89,5 +92,38 @@ examples:
           nvmem-cells = <&rtc_offset>;
           nvmem-cell-names = "rtc_offset";
        };
+
+        hwmon {
+          compatible = "apple,smc-hwmon";
+
+          current-ID0R {
+            apple,key-id = "ID0R";
+            label = "AC Input Current";
+          };
+
+          fan-F0Ac {
+            apple,key-id = "F0Ac";
+            apple,fan-minimum = "F0Mn";
+            apple,fan-maximum = "F0Mx";
+            apple,fan-target = "F0Tg";
+            apple,fan-mode = "F0Md";
+            label = "Fan 1";
+          };
+
+          power-PSTR {
+            apple,key-id = "PSTR";
+            label = "Total System Power";
+          };
+
+          temperature-TW0P {
+            apple,key-id = "TW0P";
+            label = "WiFi/BT Module Temperature";
+          };
+
+          voltage-VD0R {
+            apple,key-id = "VD0R";
+            label = "AC Input Voltage";
+          };
+        };
       };
     };
diff --git a/MAINTAINERS b/MAINTAINERS
index 51942a9a9b43..6e5e219c5fe6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2422,6 +2422,7 @@ F:	Documentation/devicetree/bindings/cpufreq/apple,cluster-cpufreq.yaml
 F:	Documentation/devicetree/bindings/dma/apple,admac.yaml
 F:	Documentation/devicetree/bindings/gpio/apple,smc-gpio.yaml
 F:	Documentation/devicetree/bindings/gpu/apple,agx.yaml
+F:	Documentation/devicetree/bindings/hwmon/apple,smc-hwmon.yaml
 F:	Documentation/devicetree/bindings/i2c/apple,i2c.yaml
 F:	Documentation/devicetree/bindings/input/touchscreen/apple,z2-multitouch.yaml
 F:	Documentation/devicetree/bindings/interrupt-controller/apple,*

-- 
2.51.2