Documentation: Add AD4000 documentation

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/aeee5fd9deccf85beadecf58c9b938b97a3aeba5.1720810545.git.marcelo.schmitt@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/ad4000.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+
+=============
+AD4000 driver
+=============
+
+Device driver for Analog Devices Inc. AD4000 series of ADCs.
+
+Supported devices
+=================
+
+* `AD4000 <https://www.analog.com/AD4000>`_
+* `AD4001 <https://www.analog.com/AD4001>`_
+* `AD4002 <https://www.analog.com/AD4002>`_
+* `AD4003 <https://www.analog.com/AD4003>`_
+* `AD4004 <https://www.analog.com/AD4004>`_
+* `AD4005 <https://www.analog.com/AD4005>`_
+* `AD4006 <https://www.analog.com/AD4006>`_
+* `AD4007 <https://www.analog.com/AD4007>`_
+* `AD4008 <https://www.analog.com/AD4008>`_
+* `AD4010 <https://www.analog.com/AD4010>`_
+* `AD4011 <https://www.analog.com/AD4011>`_
+* `AD4020 <https://www.analog.com/AD4020>`_
+* `AD4021 <https://www.analog.com/AD4021>`_
+* `AD4022 <https://www.analog.com/AD4022>`_
+* `ADAQ4001 <https://www.analog.com/ADAQ4001>`_
+* `ADAQ4003 <https://www.analog.com/ADAQ4003>`_
+
+Wiring connections
+------------------
+
+Devices of the AD4000 series can be connected to the SPI host controller in a
+few different modes.
+
+CS mode, 3-wire turbo mode
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Datasheet "3-wire" mode is what most resembles standard SPI connection which,
+for these devices, comprises of connecting the controller CS line to device CNV
+pin and other SPI lines as usual. This configuration is (misleadingly) called
+"CS Mode, 3-Wire Turbo Mode" connection in datasheets.
+NOTE: The datasheet definition of 3-wire mode for the AD4000 series is NOT the
+same of standard spi-3wire mode.
+This is the only connection mode that allows configuration register access but
+it requires the SPI controller to support the ``SPI_MOSI_IDLE_HIGH`` feature.
+
+Omit the ``adi,sdi-pin`` property in device tree to select this mode.
+
+::
+
+                                         +-------------+
+     + ----------------------------------| SDO         |
+     |                                   |             |
+     |               +-------------------| CS          |
+     |               v                   |             |
+     |    +--------------------+         |     HOST    |
+     |    |        CNV         |         |             |
+     +--->| SDI   AD4000   SDO |-------->| SDI         |
+          |        SCK         |         |             |
+          +--------------------+         |             |
+                    ^                    |             |
+                    +--------------------| SCLK        |
+                                         +-------------+
+
+CS mode, 3-wire, without busy indicator
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Another wiring configuration supported as "3-wire" mode has the SDI pin
+hard-wired to digital input/output interface supply (VIO). In this setup, the
+controller is not required to support ``SPI_MOSI_IDLE_HIGH`` but register access
+is not possible. This connection mode saves one wire and works with any SPI
+controller.
+
+Set the ``adi,sdi-pin`` device tree property to ``"high"`` to select this mode.
+
+::
+
+                                         +-------------+
+                    +--------------------| CS          |
+                    v                    |             |
+    VIO   +--------------------+         |     HOST    |
+     |    |        CNV         |         |             |
+     +--->| SDI   AD4000   SDO |-------->| SDI         |
+          |        SCK         |         |             |
+          +--------------------+         |             |
+                    ^                    |             |
+                    +--------------------| SCLK        |
+                                         +-------------+
+
+Alternatively, a GPIO may be connected to the device CNV pin. This is similar to
+the previous wiring configuration but saves the use of a CS line.
+
+::
+
+                                         +-------------+
+                    +--------------------| GPIO        |
+                    v                    |             |
+    VIO   +--------------------+         |     HOST    |
+     |    |        CNV         |         |             |
+     +--->| SDI   AD4000   SDO |-------->| SDI         |
+          |        SCK         |         |             |
+          +--------------------+         |             |
+                    ^                    |             |
+                    +--------------------| SCLK        |
+                                         +-------------+
+
+CS mode, 4-wire without busy indicator
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In datasheet "4-wire" mode, the controller CS line is connected to the ADC SDI
+pin and a GPIO is connected to the ADC CNV pin. This connection mode may better
+suit scenarios where multiple ADCs can share one CNV trigger.
+
+Set ``adi,sdi-pin`` to ``"cs"`` to select this mode.
+
+
+::
+
+                                         +-------------+
+     + ----------------------------------| CS          |
+     |                                   |             |
+     |               +-------------------| GPIO        |
+     |               v                   |             |
+     |    +--------------------+         |     HOST    |
+     |    |        CNV         |         |             |
+     +--->| SDI   AD4000   SDO |-------->| SDI         |
+          |        SCK         |         |             |
+          +--------------------+         |             |
+                    ^                    |             |
+                    +--------------------| SCLK        |
+                                         +-------------+

Documentation/iio/index.rst

@@ -18,6 +18,7 @@ Industrial I/O Kernel Drivers
 .. toctree::
    :maxdepth: 1
 
+   ad4000
    ad4695
    ad7380
    ad7944

MAINTAINERS

@@ -1220,6 +1220,7 @@ L:	linux-iio@vger.kernel.org
 S:	Supported
 W:	https://ez.analog.com/linux-software-drivers
 F:	Documentation/devicetree/bindings/iio/adc/adi,ad4000.yaml
+F:	Documentation/iio/ad4000.rst
 F:	drivers/iio/adc/ad4000.c
 
 ANALOG DEVICES INC AD4130 DRIVER