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

Rob Herring
2025-09-25T21:43:36Z

On Thu, Sep 25, 2025 at 3:49 PM Janne Grunau <j@jannau.net> wrote:
>
> 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.

The driver absolutely should.

> I looks to me it would be a
> stretch to call the presence of the labels human convenience.

Then it is an abuse of 'label". "label" is supposed to be literally
that. Matching a sticker on a port of a device.

If you need to associate a sensor with some other piece of h/w, then
that should be via a phandle or something.

Rob