docs: iio: new docs for ad7944 driver

This adds a new page to document how to use the ad7944 ADC driver.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240322-mainline-ad7944-doc-v2-2-0923d35d5596@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/ad7944.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+
+=============
+AD7944 driver
+=============
+
+ADC driver for Analog Devices Inc. AD7944 and similar devices. The module name
+is ``ad7944``.
+
+
+Supported devices
+=================
+
+The following chips are supported by this driver:
+
+* `AD7944 <https://www.analog.com/AD7944>`_
+* `AD7985 <https://www.analog.com/AD7985>`_
+* `AD7986 <https://www.analog.com/AD7986>`_
+
+
+Supported features
+==================
+
+SPI wiring modes
+----------------
+
+The driver currently supports two of the many possible SPI wiring configurations.
+
+CS mode, 3-wire, without busy indicator
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block::
+
+                                         +-------------+
+                    +--------------------| CS          |
+                    v                    |             |
+    VIO   +--------------------+         |     HOST    |
+     |    |        CNV         |         |             |
+     +--->| SDI   AD7944   SDO |-------->| SDI         |
+          |        SCK         |         |             |
+          +--------------------+         |             |
+                    ^                    |             |
+                    +--------------------| SCLK        |
+                                         +-------------+
+
+To select this mode in the device tree, set the ``adi,spi-mode`` property to
+``"single"`` and omit the ``cnv-gpios`` property.
+
+CS mode, 4-wire, without busy indicator
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block::
+
+                                         +-------------+
+     +-----------------------------------| CS          |
+     |                                   |             |
+     |              +--------------------| GPIO        |
+     |              v                    |             |
+     |    +--------------------+         |     HOST    |
+     |    |        CNV         |         |             |
+     +--->| SDI   AD7944   SDO |-------->| SDI         |
+          |        SCK         |         |             |
+          +--------------------+         |             |
+                    ^                    |             |
+                    +--------------------| SCLK        |
+                                         +-------------+
+
+To select this mode in the device tree, omit the ``adi,spi-mode`` property and
+provide the ``cnv-gpios`` property.
+
+Reference voltage
+-----------------
+
+All 3 possible reference voltage sources are supported:
+
+- Internal reference
+- External 1.2V reference and internal buffer
+- External reference
+
+The source is determined by the device tree. If ``ref-supply`` is present, then
+the external reference is used. If ``refin-supply`` is present, then the internal
+buffer is used. If neither is present, then the internal reference is used.
+
+Unimplemented features
+----------------------
+
+- ``BUSY`` indication
+- ``TURBO`` mode
+- Daisy chain mode
+
+
+Device attributes
+=================
+
+There are two types of ADCs in this family, pseudo-differential and fully
+differential. The channel name is different depending on the type of ADC.
+
+Pseudo-differential ADCs
+------------------------
+
+AD7944 and AD7985 are pseudo-differential ADCs and have the following attributes:
+
++---------------------------------------+--------------------------------------------------------------+
+| Attribute                             | Description                                                  |
++=======================================+==============================================================+
+| ``in_voltage0_raw``                   | Raw ADC voltage value (*IN+* referenced to ground sense).    |
++---------------------------------------+--------------------------------------------------------------+
+| ``in_voltage0_scale``                 | Scale factor to convert raw value to mV.                     |
++---------------------------------------+--------------------------------------------------------------+
+
+Fully-differential ADCs
+-----------------------
+
+AD7986 is a fully-differential ADC and has the following attributes:
+
++---------------------------------------+--------------------------------------------------------------+
+| Attribute                             | Description                                                  |
++=======================================+==============================================================+
+| ``in_voltage0-voltage1_raw``          | Raw ADC voltage value (*IN+* - *IN-*).                       |
++---------------------------------------+--------------------------------------------------------------+
+| ``in_voltage0-voltage1_scale``        | Scale factor to convert raw value to mV.                     |
++---------------------------------------+--------------------------------------------------------------+
+
+
+Device buffers
+==============
+
+This driver supports IIO triggered buffers.
+
+See :doc:`iio_devbuf` for more information.

Documentation/iio/index.rst

@@ -16,6 +16,7 @@ Industrial I/O Kernel Drivers
 .. toctree::
    :maxdepth: 1
 
+   ad7944
    adis16475
    bno055
    ep93xx_adc

MAINTAINERS

@@ -448,6 +448,7 @@ R:	David Lechner <dlechner@baylibre.com>
 S:	Supported
 W:	https://ez.analog.com/linux-software-drivers
 F:	Documentation/devicetree/bindings/iio/adc/adi,ad7944.yaml
+F:	Documentation/iio/ad7944.rst
 F:	drivers/iio/adc/ad7944.c
 
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