[PATCH v2 11/17] iio: core: Introduce _inputoffset for differential channels

[Jonathan Cameron <jic23@kernel.org>]

From: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

Needed for in_capacitanceX-capacitanceY_inputoffset for the
AD7746 CDC driver.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

---
I''m not entirely happy with the naming on this one, so if anyone
has a better idea I'm definitely open to it!
---
 Documentation/ABI/testing/sysfs-bus-iio | 13 +++++++++++++
 drivers/iio/industrialio-core.c         |  1 +
 include/linux/iio/types.h               |  1 +
 3 files changed, 15 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
index d3a0c0ef8948..132eb4e849f9 100644
--- a/Documentation/ABI/testing/sysfs-bus-iio
+++ b/Documentation/ABI/testing/sysfs-bus-iio
@@ -204,6 +204,19 @@ Description:
 		is required is a consistent labeling.  Units after application
 		of scale and offset are nanofarads.
 
+What:		/sys/.../iio:deviceX/in_capacitableY-capacitanceZ_inputoffset
+KernelVersion:	5.19
+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 _inputoffset cancels out across the two inputs
+		that make up the differential pair. 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
diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 6b401f63805f..ad1c2354502d 100644
--- a/drivers/iio/industrialio-core.c
+++ b/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_INPUTOFFSET] = "inputoffset",
 };
 /**
  * iio_device_id() - query the unique ID for the device
diff --git a/include/linux/iio/types.h b/include/linux/iio/types.h
index a7aa91f3a8dc..c7f79a251a14 100644
--- a/include/linux/iio/types.h
+++ b/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_INPUTOFFSET,
 };
 
 #endif /* _IIO_TYPES_H_ */
-- 
2.36.1