linux-kernel - Re: Supporting a Device with Switchable Current/Voltage Measurement

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Date: Thu, 2 May 2024 16:05:45 +0200
From: Peter Rosin <peda@...ntia.se>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 João Paulo Gonçalves <jpaulo.silvagoncalves@...il.com>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
 joao.goncalves@...adex.com
Subject: Re: Supporting a Device with Switchable Current/Voltage Measurement

2024-05-02 at 15:49, Peter Rosin wrote:
> Since you appear to need to change both the gpio pin and the io-channel, the
> mux isn't a perfect fit. The closest you can get with the current code is to
> create a gpio mux, I think. You would then use that mux twice to fan out both
> io-channels, but only expose the "left leg" on the first fan-out and only the
> "right leg" on the other. Something like this (untested, probably riddled with
> errors, use salt etc etc):
> 
> rcs: raw-current-sense {
> 	compatible = "current-sense-shunt";
> 	io-channels = <&adc 0>;
> 	io-channel-name = "raw-current";
> 	#io-channel-cells = <1>;
> 
> 	shunt-resistor-micro-ohms = <3300000>;
> };
> 
> rvs: raw-voltage-sense {
> 	compatible = "voltage-divider";
> 	io-channels = <&adc 1>;
> 	io-channel-name = "raw-voltage";
> 	#io-channel-cells = <1>;
> 
> 	output-ohms = <22>;
> 	full-ohms = <222>;
> };
> 
> mux: gpio-mux {
> 	compatible = "gpio-mux";
> 	#mux-control-cells = <0>;
> 
> 	gpios-mux = <&main_gpio0 29 GPIO_ACTIVE_HIGH>;
> };
> 
> current-sense {
> 	compatible = "io-channel-mux";
> 	io-channels = <&rcs 0>;
> 	io-channel-names = "parent";
> 
> 	mux-controls = <&mux>;
> 
> 	channels = "current", "";
> };
> 
> voltage-sense {
> 	compatible = "io-channel-mux";
> 	io-channels = <&rvs 0>;
> 	io-channel-names = "parent";
> 
> 	mux-controls = <&mux>;
> 
> 	channels = "", "voltage";
> };
> 
> What the mux solves is exclusion, so that the gpio pin is locked while
> measurement is made on either current-sense or voltage-sense.
> 
> However, the channels from the raw-{current,voltage}-sense nodes are exposed
> to user space, and it will be possible to make "raw" measurements without
> regard to how the gpio pin is set. That will of course not yield the desired
> results, but is also a user error and might not be a big problem?

I just realized that it's also possible to do this "the other way around". Maybe
that makes more sense?

Cheers,
Peter

mux: gpio-mux {
	compatible = "gpio-mux";
	#mux-control-cells = <0>;

	gpios-mux = <&main_gpio0 29 GPIO_ACTIVE_HIGH>;
};

rcs: raw-current-sense {
	compatible = "io-channel-mux";
	io-channels = <&adc 0>;
	io-channel-names = "parent";
	#io-channel-cells = <1>;

	mux-controls = <&mux>;

	channels = "raw-current", "";
};

rvs: raw-voltage-sense {
	compatible = "io-channel-mux";
	io-channels = <&adc 1>;
	io-channel-names = "parent";
	#io-channel-cells = <1>;

	mux-controls = <&mux>;

	channels = "", "raw-voltage";
};

current-sense {
	compatible = "current-sense-shunt";
	io-channels = <&rcs 0>;
	io-channel-name = "current";

	shunt-resistor-micro-ohms = <3300000>;
};

voltage-sense {
	compatible = "voltage-divider";
	io-channels = <&rvs 1>;
	io-channel-name = "voltage";

	output-ohms = <22>;
	full-ohms = <222>;
};

Cheers,
Peter