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

[Jonathan Cameron <jic23@jic23.retrosnub.co.uk>]

On Sat, 24 Mar 2018 13:53:19 +0000
Jonathan Cameron <jic23@kernel.org> wrote:

> 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-channels and same above.

> > +	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>  
> 
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