In-kernel hwmon read: (was: Re: [PATCH RFC 1/2] hwmon: (sy7636a) fix races during probe of mfd subdevices)

[Andreas Kemnade <andreas@kemnade.info>]

On Wed, 24 Sep 2025 12:16:14 -0700
Guenter Roeck <linux@roeck-us.net> wrote:

> > But what is the hwmon equivalent for
> > devm_fwnode_iio_channel_get_by_name() + iio_read_channel_processed()?
> >   
> 
> Assuming you refer to the exported functions for in-kernel use, so far no one has
> expressed a need for it. The best solution would probably be a hwmon->iio bridge,
> or equivalent functions could be implemented and exported.

So first analyzing the need of such an interface. I think
there is a need for such interface. The need just gets masked by some hacks
to be able to divert to other interfaces.

There is out-of-tree waiting to be upstreamed:
- rockchip_ebc driver: Display on PineNote, uses iio_read_chanel_processed()
                       in combination with an out-of-tree iio temperature
		       driver
- mxc_epdc_drm driver: Display on Kobo/Tolino ebook readers, uses
                       thermal_zone_get_temp() now.

In tree, there is:
- drivers/gpu/drm/tiny/repaper.c
  in combination with
  Documentation/devicetree/bindings/display/repaper.txt

  Code excerpt:
        if (!device_property_read_string(dev, "pervasive,thermal-zone",
                                         &thermal_zone)) {
                epd->thermal = thermal_zone_get_zone_by_name(thermal_zone);

  and thermal_zone_get_temp() to tune refreshes according to panel temperature.



The example in the binding is:
        display_temp: lm75@48 {
                compatible = "lm75b";
                reg = <0x48>;
                #thermal-sensor-cells = <0>;
        };

        thermal-zones {
                display {
                        polling-delay-passive = <0>;
                        polling-delay = <0>;
                        thermal-sensors = <&display_temp>;
                };
        };

[...]
                    pervasive,thermal-zone = "display";


I would prefer to be able to use e.g. pervasive,temperature-sensor = <&display_temp>;
or maybe <&display_temp 0> if there are multiple sensors in the same chip.
so that dtc will tell me when there is a typo and avoid the thermal
zone layer

So what are the options:
a) provide similar logic like iio/inkern.c for hwmon usage.
b) automatically add iio channels during hwmon registration.

looking at the iio_hwmon bridge we already have, it depends on
iio/inkern.c so for a  hwmon_iio bridge we need a) or b) anyways,
so I think a separate bridge device/driver has no advantages.
And having a devicetree node for the bridge would be bad because
we are then describing linux implementation details instead of
the actual hardware in the devicetree.

Regards,
Andreas