Re: [PATCH] dt-bindings: hwmon: pmbus: mps,mpq8785: add #thermal-sensor-cells

[Vincent Jardin <vjardin@free.fr>]

Hi Guenter,

On Wed, Sep 30, 2026 at 02:07:21PM +0000, sashiko-bot@kernel.org wrote:
> [Severity: Medium]
> Is it correct to set the cell count constraint to 1 for this hardware?
>
> The Devicetree specification generally mandates that single-sensor devices
> use a cell count of 0, as no index is needed to identify the sensor. By
> forcing this to 1, it appears we might be baking a Linux-specific driver
> quirk (the PMBus core's internal 1-based index) into the OS-agnostic
> hardware description ABI.

M. Bot has a point, and it is a PMBus core question, so I would like
your view before a v2 of this binding.

Since the commit
  3aa74796cfd0 ("hwmon: (pmbus) Register with thermal for PSC_TEMPERATURE")
pmbus_core registers each temperature input using its hwmon index as the
sensor id, so temp1 is sensor 1.

The hwmon core counts from 0 instead:
for example temp1 is sensor 0 for jc42, tmp401, lm90 or adt7470.

With the 1 based id, a single-sensor PMBus device cannot use
#thermal-sensor-cells = <0>: thermal_of matches a phandle without
cells as id 0, and no PMBus sensor has id 0. That is why this patch
uses const 1.

No current DTS references a PMBus device from a thermal zone, and no
PMBus binding allows #thermal-sensor-cells yet.

Aligning PMBus on the hwmon core would mean:

drivers/hwmon/pmbus/pmbus_core.c
static struct pmbus_sensor *pmbus_add_sensor(struct pmbus_data *data,
  ...
 	if (class == PSC_TEMPERATURE && strcmp(type, "input") == 0)
-		pmbus_thermal_add_sensor(data, sensor, seq);
+		pmbus_thermal_add_sensor(data, sensor, seq - 1);

 	return sensor;
 }


If you agree, I will send it, with a v2 of this binding using
const: 0, since each device of mps,mpq8785.yaml has a
single temperature input. Otherwise, we should keep const 1.

Best regards,
  Vincent