docs: iio: add documentation for ad3552r driver

Add documentation for ad3552r driver, needed to describe the high-speed
driver debugfs attributes and shows how the user may use them.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250409-wip-bl-ad3552r-fixes-v5-2-fb429c3a6515@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/ad3552r.rst

@@ -0,0 +1,72 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+==============
+AD3552R driver
+==============
+
+Device driver for Analog Devices Inc. AD35XXR series of DACs. The module name
+is ``ad3552r``.
+With the same module name, two different driver variants are available, the
+``generic spi`` variant, to be used with any classic SPI controllers, and the
+``hs`` (high speed) variant, for an ADI ``axi-dac`` (IP core) based controller
+that allows to reach the maximum sample rate supported from the DACs, using the
+DMA transfer and all the SPI lines available (D/QDSPI)..
+The high speed driver variant is intended to be used with the ``adi-axi-dac``
+backend support enabled, that is enabled by default when the driver is selected.
+
+Supported devices
+=================
+
+* `AD3541R <https://www.analog.com/en/products/ad3541r.html>`_
+* `AD3542R <https://www.analog.com/en/products/ad3542r.html>`_
+* `AD3551R <https://www.analog.com/en/products/ad3551r.html>`_
+* `AD3552R <https://www.analog.com/en/products/ad3552r.html>`_
+
+Wiring connections
+==================
+
+Generic SPI
+-----------
+Use the classic SPI S_CLK/CS/SDO/SDI connection.
+
+High speed (using axi-dac backend)
+----------------------------------
+
+::
+
+    .-----------------.                .-------.
+    |                 |--- D/QSPI -----|       |
+    |   DAC IP CORE   |--- SPI S_CLK --|  DAC  |
+    |                 |--- SPI CS -----|       |
+    |                 |--- LDAC -------|       |
+    |                 |--- RESET ------|       |
+    |_________________|                |_______|
+
+
+High speed features
+===================
+
+Device attributes
+-----------------
+
+The following table shows the ad35xxr related device debug files, found in the
+specific debugfs path ``/sys/kernel/debug/iio/iio:deviceX``.
+
++-----------------------+------------------------------------------------------+
+| Debugfs device files  | Description                                          |
++-----------------------+------------------------------------------------------+
+| data_source           | The used data source, as                             |
+|                       | ``normal``, ``ramp-16bit``, etc.                        |
++-----------------------+------------------------------------------------------+
+| data_source_available | The available data sources.                          |
++-----------------------+------------------------------------------------------+
+
+Usage examples
+--------------
+
+. code-block:: bash
+	root:/sys/bus/iio/devices/iio:device0# cat data_source
+	normal
+	root:/sys/bus/iio/devices/iio:device0# echo -n ramp-16bit > data_source
+	root:/sys/bus/iio/devices/iio:device0# cat data_source
+	ramp-16bit

Documentation/iio/index.rst

@@ -19,6 +19,7 @@ Industrial I/O Kernel Drivers
 .. toctree::
    :maxdepth: 1
 
+   ad3552r
    ad4000
    ad4030
    ad4695

MAINTAINERS

@@ -1306,6 +1306,7 @@ L:	linux-iio@vger.kernel.org
 S:	Supported
 W:	https://ez.analog.com/linux-software-drivers
 F:	Documentation/devicetree/bindings/iio/dac/adi,ad3552r.yaml
+F:	Documentation/iio/ad3552r.rst
 F:	drivers/iio/dac/ad3552r.c
 
 ANALOG DEVICES INC AD4000 DRIVER