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iio: core: Introduce _zeropoint for differential channels

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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
This commit is contained in:
Jonathan Cameron 2022-06-26 13:29:32 +01:00
parent 431e9147b4
commit 5c64990b99
3 changed files with 21 additions and 0 deletions
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19
Documentation/ABI/testing/sysfs-bus-iio
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@ -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

1
drivers/iio/industrialio-core.c
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@ -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

1
include/linux/iio/types.h
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@ -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_ */