linux-kernel - Re: In-kernel hwmon read:

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Message-ID: <2565161d-4393-4f2d-bd09-83a25515aad1@roeck-us.net>
Date: Sun, 1 Feb 2026 08:29:36 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Andreas Kemnade <andreas@...nade.info>
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: Re: In-kernel hwmon read:

On 1/22/26 06:23, Andreas Kemnade wrote:
> 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.
> 

b) _is_ the hwmon->iio bridge. Add HWMON_C_REGISTER_IIO,
and register as appropriate with iio if it is set. Then add
HWMON_C_REGISTER_IIO to all drivers needing it.

Not sure though how that would look like in devicetree for
in-kernel use.

Guenter

> 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
>