Re: [PATCH 2/3] dt-bindings: iio: wrapper: add io-channel-unit-converter

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 19 Mar 2018 18:02:45 +0100
Peter Rosin <peda@axentia.se> wrote:

> Allow linear scaling and modification of the type of an io-channel.
> 
> When an ADC channel measures the midpoint of a voltage divider, the
> interesting voltage is often the voltage over the full resistance
> of the divider. Likewise, measuring the voltage over a resistor is
> often a way to get to the current through it.
> 
> This binding allows description of such hardware which is external
> to the ADC.
> 
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>  .../iio/wrapper/io-channel-unit-converter.txt      | 84 ++++++++++++++++++++++
Hmm. I'm not convinced by the naming really though I can see where you care
coming from as it can effectively use a voltage ADC to measure a current.

Lets see if the devicetree people or anyone else has a suggestion on this.

Could go with AFE as that is how a chip doing this would normally be described.
It's just that here we are doing it in old fashioned resistors...

There are a few unusual elements in here binding wise so definitely looking
for input on the bindings!

Thanks,

Jonathan

>  MAINTAINERS                                        |  6 ++
>  2 files changed, 90 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> 
> diff --git a/Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt b/Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> new file mode 100644
> index 000000000000..23af661abe32
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> @@ -0,0 +1,84 @@
> +I/O channel unit converter bindings
> +
> +Allow linear scaling and modification of the type of an io-channel.
I can certainly conceive that we will have simple non linear cases in future
though they get awfully hard to describe so we can tackle that when it
happens.

> +
> +When an ADC channel measures the midpoint of a voltage divider, the
> +interesting voltage is often the voltage over the full resistance
> +of the divider. Likewise, measuring the voltage over a resistor is
> +often a way to get to the current through it.
> +
> +Required properties:
> +- compatible : "io-channel-unit-converter"
> +- io-channels : Channel node of the parent channel.
> +- io-channel-names : Should be "parent".
> +
> +Optional properties:
> +- numerator : The parent channel scale is multiplied by this value (default 1).
> +- denominator : The parent channel scale is divided by this value (default 1).
> +- type : The type of the wrapped channel is modified to this type. The default
> +	 is to use the same type as the parent channel. Recognized types are:
> +		"voltage"
> +		"current"
> +
> +Example 1:
> +The system voltage is circa 12V, but divided down with a 22/200
> +voltage divider to adjust it to the ADC range.
> +
> +SYSV        ADC       GND
> +  +          +         +
> +  |  .-----. | .----.  |
> +  '--| 200 |-+-| 22 |--'
> +     '-----'   '----'
> +
> +sysv {
> +	compatible = "io-channel-unit-converter";
> +	io-channles = <&maxadc 1>;
> +	io-channel-names = "parent";
> +
> +	/* multiply the ADC voltage by 222/22 to get the system voltage */
> +	numerator = <222>; /* 200 + 22 */
> +	denominator = <22>;
> +}
> +
> +&spi {
> +	maxadc: adc@0 {
> +		compatible = "maxim,max1027";
> +	      	reg = <0>;
> +		#io-channel-cells = <1>;
> +		interrupt-parent = <&gpio5>;
> +		interrupts = <15 IRQ_TYPE_EDGE_RISING>;
> +		spi-max-frequency = <1000000>;
> +	};
> +};
> +
> +Example 2:
> +The system current is measured by measuring the voltage over a
> +3.3 ohm resistor.
> +
> +sysi {
> +	compatible = "io-channel-unit-converter";
> +	io-channles = <&tiadc 0>;
> +	io-channel-names = "parent";
> +
> +	/* divide the ADC voltage by 33/10 (i.e. 3.3) to get current */
> +	numerator = <10>;
> +	denominator = <33>;
> +	type = "current";
> +}
> +
> +&i2c {
> +	tiadc: adc@48 {
> +		compatible = "ti,ads1015";
> +	      	reg = <0x48>;
> +		#io-channel-cells = <1>;
> +
> +		#address-cells = <1>;
> +		#size-cells = <0>;
> +
> +		channel@0 { /* IN0,IN1 differential */
> +			reg = <0>;
> +			ti,gain = <1>;
> +			ti,datarate = <4>;
> +		};
> +	};
> +};
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 96e5503bfb60..5dd555c7b1b0 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6884,6 +6884,12 @@ F:	drivers/staging/iio/
>  F:	include/linux/iio/
>  F:	tools/iio/
>  
> +IIO UNIT CONVERTER
> +M:	Peter Rosin <peda@axentia.se>
> +L:	linux-iio@vger.kernel.org
> +S:	Maintained
> +F:	Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> +
>  IKANOS/ADI EAGLE ADSL USB DRIVER
>  M:	Matthieu Castet <castet.matthieu@free.fr>
>  M:	Stanislaw Gruszka <stf_xl@wp.pl>