iio: core: Introduce _zeropoint for differential channels

Address an ABI gap for device where the offset of both lines in a
differential pair may be controlled so as to allow a wider range of
inputs, but without having any direct effect of the differential
measurement.

_offset cannot be used as to remain in line with existing usage,
userspace would be expected to apply it as (_raw + _offset) * _scale
whereas _zeropoint is not. i.e. If we were computing the differential
in software it would be.
((postive_raw + _zeropoint) - (negative_raw + zeropoint) + _offset) * _scale
= ((postive_raw - negative_raw) + _offset) * _scale
= (differential_raw + _offset) * _scale

Similarly calibbias is expected to tweak the measurement seen, not
the adjust the two lines of the differential pair.

Needed for in_capacitanceX-capacitanceY_zeropoint for the
AD7746 CDC driver.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-12-jic23@kernel.org

Documentation/ABI/testing/sysfs-bus-iio

@@ -207,6 +207,25 @@ Description:
 		is required is a consistent labeling.  Units after application
 		of scale and offset are nanofarads.
 
+What:		/sys/.../iio:deviceX/in_capacitanceY-capacitanceZ_zeropoint
+KernelVersion:	6.1
+Contact:	linux-iio@vger.kernel.org
+Description:
+		For differential channels, this an offset that is applied
+		equally to both inputs. As the reading is of the difference
+		between the two inputs, this should not be applied to the _raw
+		reading by userspace (unlike _offset) and unlike calibbias
+		it does not affect the differential value measured because
+		the effect of _zeropoint cancels out across the two inputs
+		that make up the differential pair. It's purpose is to bring
+		the individual signals, before the differential is measured,
+		within the measurement range of the device. The naming is
+		chosen because if the separate inputs that make the
+		differential pair are drawn on a graph in their
+		_raw  units, this is the value that the zero point on the
+		measurement axis represents. It is expressed with the
+		same scaling as _raw.
+
 What:		/sys/bus/iio/devices/iio:deviceX/in_temp_raw
 What:		/sys/bus/iio/devices/iio:deviceX/in_tempX_raw
 What:		/sys/bus/iio/devices/iio:deviceX/in_temp_x_raw

drivers/iio/industrialio-core.c

@@ -168,6 +168,7 @@ static const char * const iio_chan_info_postfix[] = {
 	[IIO_CHAN_INFO_OVERSAMPLING_RATIO] = "oversampling_ratio",
 	[IIO_CHAN_INFO_THERMOCOUPLE_TYPE] = "thermocouple_type",
 	[IIO_CHAN_INFO_CALIBAMBIENT] = "calibambient",
+	[IIO_CHAN_INFO_ZEROPOINT] = "zeropoint",
 };
 /**
  * iio_device_id() - query the unique ID for the device

include/linux/iio/types.h

@@ -63,6 +63,7 @@ enum iio_chan_info_enum {
 	IIO_CHAN_INFO_OVERSAMPLING_RATIO,
 	IIO_CHAN_INFO_THERMOCOUPLE_TYPE,
 	IIO_CHAN_INFO_CALIBAMBIENT,
+	IIO_CHAN_INFO_ZEROPOINT,
 };
 
 #endif /* _IIO_TYPES_H_ */