Documentation: iio: Document ad4695 driver

The Analog Devices Inc. AD4695 (and similar chips) are complex ADCs that
will benefit from a detailed driver documentation.

This documents the current features supported by the driver.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/ad4695.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+
+=============
+AD4695 driver
+=============
+
+ADC driver for Analog Devices Inc. AD4695 and similar devices. The module name
+is ``ad4695``.
+
+
+Supported devices
+=================
+
+The following chips are supported by this driver:
+
+* `AD4695 <https://www.analog.com/AD4695>`_
+* `AD4696 <https://www.analog.com/AD4696>`_
+* `AD4697 <https://www.analog.com/AD4697>`_
+* `AD4698 <https://www.analog.com/AD4698>`_
+
+
+Supported features
+==================
+
+SPI wiring modes
+----------------
+
+The driver currently supports the following SPI wiring configuration:
+
+4-wire mode
+^^^^^^^^^^^
+
+In this mode, CNV and CS are tied together and there is a single SDO line.
+
+.. code-block::
+
+    +-------------+         +-------------+
+    |          CS |<-+------| CS          |
+    |         CNV |<-+      |             |
+    |     ADC     |         |     HOST    |
+    |             |         |             |
+    |         SDI |<--------| SDO         |
+    |         SDO |-------->| SDI         |
+    |        SCLK |<--------| SCLK        |
+    +-------------+         +-------------+
+
+To use this mode, in the device tree, omit the ``cnv-gpios`` and
+``spi-rx-bus-width`` properties.
+
+Channel configuration
+---------------------
+
+Since the chip supports multiple ways to configure each channel, this must be
+described in the device tree based on what is actually wired up to the inputs.
+
+There are three typical configurations:
+
+An ``INx`` pin is used as the positive input with the ``REFGND``, ``COM`` or
+the next ``INx`` pin as the negative input.
+
+Pairing with REFGND
+^^^^^^^^^^^^^^^^^^^
+
+Each ``INx`` pin can be used as a pseudo-differential input in conjunction with
+the ``REFGND`` pin. The device tree will look like this:
+
+.. code-block::
+
+    channel@0 {
+        reg = <0>; /* IN0 */
+    };
+
+If no other channel properties are needed (e.g. ``adi,no-high-z``), the channel
+node can be omitted entirely.
+
+This will appear on the IIO bus as the ``voltage0`` channel. The processed value
+(*raw × scale*) will be the voltage present on the ``IN0`` pin relative to
+``REFGND``. (Offset is always 0 when pairing with ``REFGND``.)
+
+Pairing with COM
+^^^^^^^^^^^^^^^^
+
+Each ``INx`` pin can be used as a pseudo-differential input in conjunction with
+the ``COM`` pin. The device tree will look like this:
+
+.. code-block::
+
+    com-supply = <&vref_div_2>;
+
+    channel@1 {
+        reg = <1>; /* IN1 */
+        common-mode-channel = <AD4695_COMMON_MODE_COM>;
+        bipolar;
+    };
+
+This will appear on the IIO bus as the ``voltage1`` channel. The processed value
+(*(raw + offset) × scale*) will be the voltage measured on the ``IN1`` pin
+relative to ``REFGND``. (The offset is determined by the ``com-supply`` voltage.)
+
+The macro comes from:
+
+.. code-block::
+
+    #include <dt-bindings/iio/adi,ad4695.h>
+
+Pairing two INx pins
+^^^^^^^^^^^^^^^^^^^^
+
+An even-numbered ``INx`` pin and the following odd-numbered ``INx`` pin can be
+used as a pseudo-differential input. The device tree for using ``IN2`` as the
+positive input and ``IN3`` as the negative input will look like this:
+
+.. code-block::
+
+    in3-supply = <&vref_div_2>;
+
+    channel@2 {
+        reg = <2>; /* IN2 */
+        common-mode-channel = <3>; /* IN3 */
+        bipolar;
+    };
+
+This will appear on the IIO bus as the ``voltage2`` channel. The processed value
+(*(raw + offset) × scale*) will be the voltage measured on the ``IN1`` pin
+relative to ``REFGND``. (Offset is determined by the ``in3-supply`` voltage.)
+
+VCC supply
+----------
+
+The chip supports being powered by an external LDO via the ``VCC`` input or an
+internal LDO via the ``LDO_IN`` input. The driver looks at the device tree to
+determine which is being used. If ``ldo-supply`` is present, then the internal
+LDO is used. If ``vcc-supply`` is present, then the external LDO is used and
+the internal LDO is disabled.
+
+Reference voltage
+-----------------
+
+The chip supports an external reference voltage via the ``REF`` input or an
+internal buffered reference voltage via the ``REFIN`` input. The driver looks
+at the device tree to determine which is being used. If ``ref-supply`` is
+present, then the external reference voltage is used and the internal buffer is
+disabled. If ``refin-supply`` is present, then the internal buffered reference
+voltage is used.
+
+Unimplemented features
+----------------------
+
+- Additional wiring modes
+- Buffered reads
+- Threshold events
+- Oversampling
+- Gain/offset calibration
+- GPIO support
+- CRC support

Documentation/iio/index.rst

@@ -18,6 +18,7 @@ Industrial I/O Kernel Drivers
 .. toctree::
    :maxdepth: 1
 
+   ad4695
    ad7380
    ad7944
    adis16475

MAINTAINERS

@@ -1231,6 +1231,7 @@ L:	linux-iio@vger.kernel.org
 S:	Supported
 W:	https://ez.analog.com/linux-software-drivers
 F:	Documentation/devicetree/bindings/iio/adc/adi,ad4695.yaml
+F:	Documentation/iio/ad4695.rst
 F:	drivers/iio/adc/ad4695.c
 F:	include/dt-bindings/iio/adi,ad4695.h