Re: [PATCH v2 02/11] dt-bindings: hwmon: Add Apple System Management Controller hwmon schema

Janne Grunau
2025-09-25T20:49:28Z

On Fri, Aug 29, 2025 at 11:40:57AM -0500, Rob Herring wrote:
> On Wed, Aug 27, 2025 at 09:22:36PM +1000, James Calligeros wrote:
> > 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.
> >
> > Signed-off-by: James Calligeros <jcalligeros99@gmail.com>
> > ---
> > .../bindings/hwmon/apple,smc-hwmon.yaml | 132 +++++++++++++++++++++++++
> > .../bindings/mfd/apple,smc.yaml | 36 +++++++
> > MAINTAINERS | 1 +
> > 3 files changed, 169 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 0000000000000000000000000000000000000000..08cc4f55f3a41ca8b3b428088f96240266fa42e8
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/hwmon/apple,smc-hwmon.yaml
> > @@ -0,0 +1,132 @@
>
> This should be something like this:
>
> "^current-[A-Za-z0-9]{4}$":
> $ref: "#/$defs/sensor"
> unevaluatedProperties: false
>
> With the $defs/sensor being:
>
> $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
> - label
>
> Though in general, 'label' should never be required being just for human
> convenience.

That does not sound as it would be compatible with skipping nodes in the
driver if the node misses label. The driver could of course fall back
to create a hwmon sensors without labels. I looks to me it would be a
stretch to call the presence of the labels human convenience.

Janne