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

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

[Vincent Jardin via B4 Relay]

From: Vincent Jardin <vjardin@free.fr>

Since commit
  3aa74796cfd0 ("hwmon: (pmbus) Register with thermal for PSC_TEMPERATURE")
the PMBus core calls devm_thermal_of_zone_register() for every
temperature input, so the devices of this binding can be referenced as
thermal sensors.

By code review, it should work for the MPQ and MPM device bindings.
They register a single page with PMBUS_HAVE_TEMP through
pmbus_do_probe(), it means that one device has one
temperature input, temp1.

The sensor id is the hwmon index of the input (temp1 is 1), so one cell
is needed.

Signed-off-by: Vincent Jardin <vjardin@free.fr>
---
 Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml b/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml
index d7a32fb3b7af..36760d89f665 100644
--- a/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml
+++ b/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml
@@ -34,6 +34,9 @@ properties:
     maximum: 4095
     default: 706
 
+  "#thermal-sensor-cells":
+    const: 1
+
 required:
   - compatible
   - reg

---
base-commit: 4781ca52761e666cf18b591e6bb0478396c90320
change-id: 20260930-for-upstream-dt-mpq8785-thermal-sensor-cells-c77fa6da5406

Best regards,
-- 
Vincent Jardin <vjardin@free.fr>

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] dt-bindings: incorrect #thermal-sensor-cells constraint for single-sensor device
--

commit e3f74468edb24a9bcc5d029cc615555210a46a78
Author: Vincent Jardin <vjardin@free.fr>

dt-bindings: hwmon: pmbus: mps,mpq8785: add #thermal-sensor-cells

This patch adds the #thermal-sensor-cells property to the mpq8785 binding
schema to allow referencing the device as a thermal sensor. It sets the
cell count to 1 to match the 1-based sensor index used internally by the
PMBus core.

> diff --git a/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml b/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml
> --- a/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml
> +++ b/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpq8785.yaml
[ ... ]
> @@ -34,6 +34,9 @@ properties:
>      maximum: 4095
>      default: 706
>  
> +  "#thermal-sensor-cells":
> +    const: 1

[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.

Would it be better to map a Devicetree ID of 0 to the internal hwmon index 1
within the driver code itself, rather than requiring device tree authors to
pass a factually incorrect dummy index in their thermal-sensors phandle?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260930-for-upstream-dt-mpq8785-thermal-sensor-cells-v1-1-6573c765eef8@free.fr?part=1

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

[Vincent Jardin]

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

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

[Guenter Roeck]

On Wed, Sep 30, 2026 at 07:27:43PM +0200, Vincent Jardin wrote:
> 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.
> 

Yes, that is a bug. Please send a patch. I'd prefer the index
to be handled in the call to devm_thermal_of_zone_register(),
though, with an added comment explaining that the temperature sensor
index in the PMBus core starts with 1 and the thermal core expects
a 0-based index.

Thanks,
Guenter