docs: iio: new docs for ad7380 driver

This adds a new page to document how to use the ad7380 ADC driver.

Credit: this docs is based on ad7944 docs.
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20240709-ad7380-add-docs-v1-1-458ced3dfcc5@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/ad7380.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+
+=============
+AD7380 driver
+=============
+
+ADC driver for Analog Devices Inc. AD7380 and similar devices. The module name
+is ``ad7380``.
+
+
+Supported devices
+=================
+
+The following chips are supported by this driver:
+
+* `AD7380 <https://www.analog.com/en/products/ad7380.html>`_
+* `AD7381 <https://www.analog.com/en/products/ad7381.html>`_
+* `AD7383 <https://www.analog.com/en/products/ad7383.html>`_
+* `AD7384 <https://www.analog.com/en/products/ad7384.html>`_
+* `AD7380-4 <https://www.analog.com/en/products/ad7380-4.html>`_
+* `AD7381-4 <https://www.analog.com/en/products/ad7381-4.html>`_
+* `AD7383-4 <https://www.analog.com/en/products/ad7383-4.html>`_
+* `AD7384-4 <https://www.analog.com/en/products/ad7384-4.html>`_
+
+
+Supported features
+==================
+
+SPI wiring modes
+----------------
+
+ad738x ADCs can output data on several SDO lines (1/2/4). The driver currently
+supports only 1 SDO line.
+
+Reference voltage
+-----------------
+
+2 possible reference voltage sources are supported:
+
+- Internal reference (2.5V)
+- External reference (2.5V to 3.3V)
+
+The source is determined by the device tree. If ``refio-supply`` is present,
+then the external reference is used, else the internal reference is used.
+
+Oversampling and resolution boost
+---------------------------------
+
+This family supports 2 types of oversampling: normal average and rolling
+average. Only normal average is supported by the driver, as rolling average can
+be achieved by processing a captured data buffer. The following ratios are
+available: 1 (oversampling disabled)/2/4/8/16/32.
+
+When the on-chip oversampling function is enabled the performance of the ADC can
+exceed the default resolution. To accommodate the performance boost achievable,
+it is possible to enable an additional two bits of resolution. Because the
+resolution boost feature can only be enabled when oversampling is enabled and
+oversampling is not as useful without the resolution boost, the driver
+automatically enables the resolution boost if and only if oversampling is
+enabled.
+
+Since the resolution boost feature causes 16-bit chips to now have 18-bit data
+which means the storagebits has to change from 16 to 32 bits, we use the new
+ext_scan_type feature to allow changing the scan_type at runtime. Unfortunately
+libiio does not support it. So when enabling or disabling oversampling, user
+must restart iiod using the following command:
+
+.. code-block:: bash
+
+	root:~# systemctl restart iiod
+
+
+Unimplemented features
+----------------------
+
+- 2/4 SDO lines
+- Rolling average oversampling
+- Power down mode
+- CRC indication
+- Alert
+
+
+Device buffers
+==============
+
+This driver supports IIO triggered buffers.
+
+See :doc:`iio_devbuf` for more information.

Documentation/iio/index.rst

@@ -18,6 +18,7 @@ Industrial I/O Kernel Drivers
 .. toctree::
    :maxdepth: 1
 
+   ad7380
    ad7944
    adis16475
    adis16480

MAINTAINERS

@@ -448,6 +448,7 @@ S:	Supported
 W:	https://wiki.analog.com/resources/tools-software/linux-drivers/iio-adc/ad738x
 W:	https://ez.analog.com/linux-software-drivers
 F:	Documentation/devicetree/bindings/iio/adc/adi,ad7380.yaml
+F:	Documentation/iio/ad7380.rst
 F:	drivers/iio/adc/ad7380.c
 
 AD7877 TOUCHSCREEN DRIVER