Re: #thermal-sensor-cells under protocol@15

On 7/28/26 12:23, Sudeep Holla wrote:
> On Tue, Jul 28, 2026 at 11:53:41AM +0200, Laszlo Ersek wrote:

>> And so describing protocol@15 in the Device Tree, without a
>> #thermal-sensor-cells property, looks like a valid use case.
>>
>
> No, unless you go back in time and find a replacement for
> #thermal-sensor-cells and replace it w/o breaking any compatibility.
>
>> Am I wrong?
>>
>
> We need some property cell to be set to 1 to read the SCMI IDs. For
> backward compatibility reasons, it will remain #thermal-sensor-cells.

(1) Are you saying that the property "#thermal-sensor-cells", under
protocol@15, is actually a *misnomer* today, and it is in fact a
*generic* specification of the Sensor ID width -- regardless of any
referring stanzas elsewhere in the Device Tree (of which there may be
none, or some may be thermal-UNrelated)?

(2) If so, would you accept a patch for
"Documentation/devicetree/bindings/firmware/arm,scmi.yaml" that points
out the above fact in a "description" yaml key?

| diff --git a/Documentation/devicetree/bindings/firmware/arm,scmi.yaml b/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
| index d06cca9273c4..78cc26ce0458 100644
| --- a/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
| +++ b/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
| @@ -223,20 +223,25 @@ properties:
|    protocol@15:
|      $ref: '#/$defs/protocol-node'
|      unevaluatedProperties: false
|
|      properties:
|        reg:
|          const: 0x15
|
|        '#thermal-sensor-cells':
|          const: 1
| +        description:
| +          The width of SCMI Sensor IDs, expressed as a number of 32-bit cells.
| +          (The property name is an historical artifact, and counts nowadays as
| +          a misnomer. The property is no longer specific to anything
| +          "thermal".)
|
|      required:
|        - '#thermal-sensor-cells'
|
|    protocol@16:
|      $ref: '#/$defs/protocol-node'
|      unevaluatedProperties: false
|
|      properties:
|        reg:

(3) Where exactly does the kernel consume "#thermal-sensor-cells" in the
proposed (general) sense?

I can only find matches for the property name in
"drivers/thermal/thermal_of.c" and
"drivers/thermal/ti-soc-thermal/ti-thermal-common.c", but those appear
like thermal drivers, not general SCMI stuff. AIUI,
of_thermal_zone_find() in the former consults "#thermal-sensor-cells" in
the "provider" (such as SCMI protocol@15) to figure out the expected
argument cell count in the "thermal-sensors" list.

But that implies that, if there ever was another (non-thermal) driver
that wanted to reference SCMI sensor identifiers similarly, it would
also have to consult "#thermal-sensor-cells", for the argument cell
count. Which seems to support the idea that "#thermal-sensor-cells"
counts as a misnomer today. Is that right?

>> IMPORTANT NOTICE: [...]
>
> Deleted now, please fix it for next time.

My bad, should be fixed now. (I've tested it with another external
addressee.)

Thank you,
Laszlo