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

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 22 Jan 2026 15:23:06 +0100
Andreas Kemnade <andreas@kemnade.info> wrote:

> 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.

That's always been a little controversial for the iio-hwmon
but I had quite a few discussions of the years of how to describe
this and no one ever came up with anything that worked other than
maybe pushing it to configfs and making it a usespace script problem.

Jonathan

> 
> Regards,
> Andreas
>