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

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Message-ID: <20260122152306.6211995e@kemnade.info>
Date: Thu, 22 Jan 2026 15:23:06 +0100
From: Andreas Kemnade <andreas@...nade.info>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Mark Brown <broonie@...nel.org>, jdelvare@...e.com, lgirdwood@...il.com,
 linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org, Alistair Francis
 <alistair@...stair23.me>, "linux-iio@...r.kernel.org"
 <linux-iio@...r.kernel.org>, Jonathan Cameron <jic23@...nel.org>
Subject: In-kernel hwmon read: (was: Re: [PATCH RFC 1/2] hwmon: (sy7636a)
 fix races during probe of mfd subdevices)

On Wed, 24 Sep 2025 12:16:14 -0700
Guenter Roeck <linux@...ck-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