[PATCH v3 3/3] Documentation: ABI: IIO: Re-add sysfs-bus-iio-adc-ad4130

[Marcelo Schmitt]

The ad4130 driver exports in_voltageY-voltageZ_filter_mode and
in_voltage-voltage_filter_mode_available attributes to user space. A
previous patch merged the documentation for those attributes with the
documentation for filter_type/filter_type_available into sysfs-bus-iio.
Filter mode and filter type refer to the same feature which is the digital
filter applied over ADC samples. However, since datasheets use the term
`filter type` and ad4130 driver is the only one using filter_mode,
deprecate the filter_mode ABI in favor of filter_type and keep the docs
separate to avoid confusion and intricate attribute descriptions.

The digital filter helps shape the noise rejection bandwidth and is
supposed to be adjusted according to the signal of interest. Depending on
the particular ADC design, the digital filter configuration may have an
impact on the output data rate (ODR) (which is equivalent to the sampling
frequency for SAR ADCs - `sampling_frequency` ABI), 3dB cutoff frequency of
the filter (`_low_pass_3db_frequency` attributes), or settling time.

Digital filters are a common feature among sigma-delta ADCs and
IIO drivers set digital filters according to different attributes.
ad4130 and ad7779 filter options are supported through dedicated
filter_mode/filter_type attributes in sysfs.
ad7768-1 and ad7173 set the sinc filters according to the
sampling_frequency attribute. No filter_type attribute.
ad7124 sets the digital filter according to a filter_low_pass_3db_frequency
attribute so it doesn't export filter type attributes to user space.

Fixes: 01bb12922b60 ("Documentation: ABI: added filter mode doc in sysfs-bus-iio")
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-bus-iio       |  1 -
 .../ABI/testing/sysfs-bus-iio-adc-ad4130      | 20 +++++++++++++++++++
 MAINTAINERS                                   |  1 +
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130

diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
index ff9cc09360dc..e4fd99c92d81 100644
--- a/Documentation/ABI/testing/sysfs-bus-iio
+++ b/Documentation/ABI/testing/sysfs-bus-iio
@@ -2277,7 +2277,6 @@ Description:
 		representing the sensor unique ID number.
 
 What:		/sys/bus/iio/devices/iio:deviceX/filter_type_available
-What:		/sys/bus/iio/devices/iio:deviceX/in_voltage-voltage_filter_mode_available
 KernelVersion:	6.1
 Contact:	linux-iio@xxxxxxxxxxxxxxx
 Description:
diff --git a/Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130 b/Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130
new file mode 100644
index 000000000000..9c0698ad27f9
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130
@@ -0,0 +1,20 @@
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltage-voltage_filter_mode_available
+KernelVersion:  6.2
+Contact:	linux-iio@xxxxxxxxxxxxxxx
+Description:
+		Reading returns a list with the possible filter modes.
+
+		This ABI is only kept for backwards compatibility and the values
+		returned are identical to filter_type_available attribute
+		documented in Documentation/ABI/testing/sysfs-bus-iio. Please,
+		use filter_type_available to provide filter options for new
+		drivers.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY-voltageZ_filter_mode
+KernelVersion:  6.2
+Contact:	linux-iio@xxxxxxxxxxxxxxx
+Description:
+		This ABI is only kept for backwards compatibility and the values
+		returned are identical to in_voltageY-voltageZ_filter_type
+		attribute documented in Documentation/ABI/testing/sysfs-bus-iio.
+		Please, use in_voltageY-voltageZ_filter_type for new drivers.
diff --git a/MAINTAINERS b/MAINTAINERS
index 98a3c1e46311..28dbbf6d6efa 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1279,6 +1279,7 @@ M:	Cosmin Tanislav <cosmin.tanislav@xxxxxxxxxx>
 L:	linux-iio@xxxxxxxxxxxxxxx
 S:	Supported
 W:	https://ez.analog.com/linux-software-drivers
+F:	Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130
 F:	Documentation/devicetree/bindings/iio/adc/adi,ad4130.yaml
 F:	drivers/iio/adc/ad4130.c
 
-- 
2.47.2