Re: Supporting a Device with Switchable Current/Voltage Measurement

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 1 May 2024 20:38:53 -0300
João Paulo Gonçalves <jpaulo.silvagoncalves@gmail.com> wrote:

> Hello all,
> 
> We need to support a hardware that can measure current and voltage on
> the same differential analog input, similar to a multimeter. The mode
> of measurement is controlled by a GPIO switch and goes to different
> ADC inputs depending on the mode. If the switch is enabled, a current
> loop with a shunt is enabled for current measurement; otherwise, voltage
> is measured. From the software point of view, we are considering using
> the iio-rescale driver as a consumer of an ADC IIO parent device. One
> of the problems is that we need to change the mode of measurement at
> runtime, but we are trying to avoid using some userspace "hack". The
> other is that for a minimal solution to enable the mode from boot, we
> can use a gpio-hog and control it with overlays. However,
> still would be better that this was done by the kernel. Do you know
> or have some guidance on how to properly support this in the kernel?
> 
> For the in kernel gpio solution, this is a draft of DT we are thinking:
> 
> current-sense {
>       compatible = "current-sense-shunt";
>       io-channels = <&adc 0>;
>       gpio = <&main_gpio0 29 GPIO_ACTIVE_HIGH>;
>       shunt-resistor-micro-ohms = <3300000>;      
> };
> 
> voltage-sense {
>         compatible = "voltage-divider";
>         io-channels = <&adc 1>;
>         gpio = <&main_gpio0 29 GPIO_ACTIVE_LOW>;
>         output-ohms = <22>;
>         full-ohms = <222>;
> };
> 
> Regards,
> João Paulo Gonçalves
> 
+CC Peter Rosin who wrote all the relevant parts you need I think.
> 

Superficially sounds like you want a mixture of appropriate analog front ends
and a Mux.  I haven't tried the combination but it should be possible to do
something like this with 

An IIO mux via this binding
https://elixir.bootlin.com/linux/v6.9-rc6/source/Documentation/devicetree/bindings/iio/multiplexer/io-channel-mux.yaml
(that includes a gpio-mux example).

Consumed in turn by a pair of AFE devices.

Then you should be able to just read from which ever of the AFE device you want.
A sysfs read from
/sys/bus/iio/devices/iio\:deviceA/in_voltage_raw
will switch the mux to appropriate place then request the
voltage from the iio-mux, which in turn requests it from the ADC IIO driver.

/sys/bus/iio/devices/iio\:deviceB/in_current_raw
switches the mux the other way and otherwise the flow as above.

Jonathan