Re-add filter_type/filter_mode

Better document sysfs ABI for ADC digital filter configuration.

Change log v1 -> v2
- Split into 3 patches.
- Re-added sysfs-bus-iio-adc-ad4130 to keep filter_mode separate from filter_type.
- Removed in_voltage-voltage_filter_mode_available from sysfs-bus-iio.
- Added deprecation notes in sysfs-bus-iio-adc-ad4130 asking to use filter_type for new drivers and referencing sysfs-bus-iio.
- Improved commit message with explanation about use of filter type attributes.
- Dropped AD4130 specific filter_type_available options from sysfs-bus-iio.

Link to v1: https://lore.kernel.org/linux-iio/b2132bd3ca1d64cdd8d5afab1f1f33c574718b50.1732901318.git.marcelo.schmitt@analog.com/

Additional specific comments in patches 1 and 3.

Marcelo Schmitt (3):
Documentation: ABI: IIO: Re-add sysfs-bus-iio-adc-ad4130
Documentation: ABI: IIO: Add filter_type documentation
Documentation: ABI: IIO: Drop AD4130 specific filter_type_available
options

 Documentation/ABI/testing/sysfs-bus-iio       | 29 ++++------
 .../ABI/testing/sysfs-bus-iio-adc-ad4130      | 55 +++++++++++++++++++
 MAINTAINERS                                   |  1 +
 3 files changed, 68 insertions(+), 17 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130

base-commit: 9b75dd1b7d6b98699a104c6b1eec0c8817e5fd4b

2.45.2

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.

---
Second and third paragraphs of the body could maybe go into device specific docs as suggested in some previous thread but those drivers don't have docs in Documentation/iio/ and I've been more interested in how filter_type is supported accross IIO to be able to support it appropriatly in new drivers.

 Documentation/ABI/testing/sysfs-bus-iio       |  1 -
 .../ABI/testing/sysfs-bus-iio-adc-ad4130      | 55 +++++++++++++++++++
 MAINTAINERS                                   |  1 +
 3 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-adc-ad4130

index d97c35057a03..1faac5b2ad84 100644
@@ -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:	[email protected]
 Description:

index 98a3c1e46311..28dbbf6d6efa 100644
@@ -1279,6 +1279,7 @@ M:	Cosmin Tanislav <[email protected]>
 L:	[email protected]
 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
 

2.45.2

A previous patch added documentation for filter_type_available attributes. However, the description for the value attribute (filter_type) was missing. Add documentation for filter_type sysfs ABI.

---
 Documentation/ABI/testing/sysfs-bus-iio | 7 +++++++
 1 file changed, 7 insertions(+)

index 1faac5b2ad84..1ddf3d7dd2b3 100644
@@ -2299,6 +2299,13 @@ Description:
 		* "sinc3+pf3" - Sinc3 + device specific Post Filter 3.
 		* "sinc3+pf4" - Sinc3 + device specific Post Filter 4.
 
+What:		/sys/bus/iio/devices/iio:deviceX/filter_type
+KernelVersion:	6.1
+Contact:	[email protected]
+Description:
+		Specifies which filter type apply to the channel. The possible
+		values are given by the filter_type_available attribute.
+
 What:		/sys/.../events/in_proximity_thresh_either_runningperiod
 KernelVersion:	6.6
 Contact:	[email protected]

2.45.2

The only driver exporting filter_type_available attribute is ad7779 and it only supports sinc3 and sinc5 filters. Remove options that are not presented through filter_type_available attribute.

---
Maybe this is not worth it (or desirable?) since the options may come back in the future if new drivers happen to need them.

 Documentation/ABI/testing/sysfs-bus-iio | 21 +++++----------------
 1 file changed, 5 insertions(+), 16 deletions(-)

index 1ddf3d7dd2b3..946c519f8687 100644
@@ -2280,24 +2280,13 @@ What:		/sys/bus/iio/devices/iio:deviceX/filter_type_available
 KernelVersion:	6.1
 Contact:	[email protected]
 Description:
-		Reading returns a list with the possible filter modes. Options
-		for the attribute:
+		Lists all available filter types. Possible options for the
+		attribute:
 
-		* "sinc3" - The digital sinc3 filter. Moderate 1st
-		  conversion time. Good noise performance.
-		* "sinc4" - Sinc 4. Excellent noise performance. Long
-		  1st conversion time.
+		* "sinc3" - The digital sinc3 filter. Moderate 1st conversion
+		  time. Good noise performance.
 		* "sinc5" - The digital sinc5 filter. Excellent noise
-		  performance
-		* "sinc4+sinc1" - Sinc4 + averaging by 8. Low 1st conversion
-		  time.
-		* "sinc3+rej60" - Sinc3 + 60Hz rejection.
-		* "sinc3+sinc1" - Sinc3 + averaging by 8. Low 1st conversion
-		  time.
-		* "sinc3+pf1" - Sinc3 + device specific Post Filter 1.
-		* "sinc3+pf2" - Sinc3 + device specific Post Filter 2.
-		* "sinc3+pf3" - Sinc3 + device specific Post Filter 3.
-		* "sinc3+pf4" - Sinc3 + device specific Post Filter 4.
+		  performance.
 
 What:		/sys/bus/iio/devices/iio:deviceX/filter_type
 KernelVersion:	6.1

2.45.2

I would be tempted to add filter_type{,_available} attributes to the ad4130 driver that mirrors the existing filter_mode{,_available} attributes. We can't remove filter_mode since that would break things, but having filter_type in addition to that would mean than userspace tools could standardize on filter_type and not have to make a special exception for the different naming on ad4130.

Then we wouldn't need to delete the extra items here and we wouldn't have to repeat the docs for sysfs-bus-iio-adc-ad4130. Those docs can just have the deprecation paragraph and mention that it returns values identical to the filter_type attributes for backwards compatibility.

Good idea. Given we always allow any attribute to change any other, having duplicates for ABI backwards compatibility is fine.

Thanks,

Jonathan

If ad7779 is the only driver with filter_type{,available} attributes while ad4130 and admv8818 both have filter_mode{,available} attributes, shouldn't we be deprecating filter_type and promoting filter_mode?

(Looks like I might have got that backwards in my previous reply. :-/)

Oh, I had only grepped the adc dir and I didn't noticed admv8818 also had filter_mode.
Hmm, don't know, though still biased towards filter_type.

- The admv8818 filter_mode_available ABI exports different values (auto, manual, bypass).
- AD7768-1, ADAQ7768-1, AD7768/AD7768-4, AD7124-4, and AD4170 datasheets refer to sinc filters as filter types (though not all of those designs are supported in IIO or have filter attributes).
- The AD4130 datasheets also refers to the digital filter as filter type in some occasions.

But some datasheets seem to use filter mode and filter type interchangeably so I don't know. The admv8818 use case is valid and is both low and high-pass filtering while the ADCs sinc digital filters act like low-pass filter. Maybe even provide a different name for ADC digital filtering? digital_filter_type (or digital_filter_mode)?.

Thanks for pointing that out.

Ah, I didn't look that far, so yes, I agree this is a good reason to standardize on filter_type instead.