Documentation: iio: Document ad7606 driver

The Analog Devices Inc. AD7606 (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: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-3-654faf1ae08c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

2 files changed, 145 insertions, 0 deletions

diff --git a/Documentation/iio/ad7606.rst b/Documentation/iio/ad7606.rst
new file mode 100644
index 000000000000..930199e03c67
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+.. SPDX-License-Identifier: GPL-2.0-only
+
+=============
+AD7606 driver
+=============
+
+ADC driver for Analog Devices Inc. AD7606 and similar devices. The module name
+is ``ad7606``.
+
+Supported devices
+=================
+
+The following chips are supported by this driver:
+
+* `AD7605 <https://www.analog.com/en/products/ad7605.html>`_
+* `AD7606 <https://www.analog.com/en/products/ad7606.html>`_
+* `AD7606B <https://www.analog.com/en/products/ad7606b.html>`_
+* `AD7616 <https://www.analog.com/en/products/ad7616.html>`_
+
+Supported features
+==================
+
+SPI wiring modes
+----------------
+
+These ADCs can output data on several SDO lines (1/2/4/8). The driver
+currently supports only 1 SDO line.
+
+Parallel wiring mode
+--------------------
+
+There is also a parallel interface, with 16 lines (that can be reduced to 8 in
+byte mode). The parallel interface is selected by declaring the device as
+platform in the device tree (with no io-backends node defined, see below).
+
+IIO-backend mode
+----------------
+
+This mode allows to reach the best sample rates, but it requires an external
+hardware (eg HDL or APU) to handle the low level communication.
+The backend mode is enabled when through the definition of the "io-backends"
+property in the device tree.
+
+The reference configuration for the current implementation of IIO-backend mode
+is the HDL reference provided by ADI:
+https://wiki.analog.com/resources/eval/user-guides/ad7606x-fmc/hdl
+
+This implementation embeds an IIO-backend compatible IP (adi-axi-adc) and a PWM
+connected to the conversion trigger pin.
+
+.. code-block::
+
+    +---+                                       +----------------------------
+    |   |               +-------+               |AD76xx
+    | A |  controls     |       |               |
+    | D |-------------->|  PWM  |-------------->| cnvst
+    | 7 |               |       |               |
+    | 6 |               +-------+               |
+    | 0 | controls  +-----------+-----------+   |
+    | 6 |---------->|           |           |<--| frstdata
+    |   |           | Backend   |  Backend  |<--| busy
+    | D |           | Driver    |           |   |
+    | R |           |           |           |-->| clk
+    | I |  requests |+---------+| DMA       |   |
+    | V |----------->|  Buffer ||<----      |<=>| DATA
+    | E |           |+---------+|           |   |
+    | R |           +-----------+-----------+   |
+    |   |-------------------------------------->| reset/configuration gpios
+    +---+                                       +-----------------------------
+
+
+Software and hardware modes
+---------------------------
+
+While all the AD7606/AD7616 series parts can be configured using GPIOs, some of
+them can be configured using register.
+
+The chips that support software mode have more values available for configuring
+the device, as well as more settings, and allow to control the range and
+calibration per channel.
+
+The following settings are available per channel in software mode:
+ - Scale
+
+Also, there is a broader choice of oversampling ratios in software mode.
+
+Conversion triggering
+---------------------
+
+The conversion can be triggered by two distinct ways:
+
+ - A GPIO is connected to the conversion trigger pin, and this GPIO is controlled
+   by the driver directly.  In this configuration, the driver sets back the
+   conversion trigger pin to high as soon as it has read all the conversions.
+
+ - An external source is connected to the conversion trigger pin. In the
+   current implementation, it must be a PWM. In this configuration, the driver
+   does not control directly the conversion trigger pin. Instead, it can
+   control the PWM's frequency. This trigger is enabled only for iio-backend.
+
+Reference voltage
+-----------------
+
+2 possible reference voltage sources are supported:
+
+ - Internal reference (2.5V)
+ - External reference (2.5V)
+
+The source is determined by the device tree. If ``refin-supply`` is present,
+then the external reference is used, otherwise the internal reference is used.
+
+Oversampling
+------------
+
+This family supports oversampling to improve SNR.
+In software mode, the following ratios are available:
+1 (oversampling disabled)/2/4/8/16/32/64/128/256.
+
+Unimplemented features
+----------------------
+
+- 2/4/8 SDO lines
+- CRC indication
+- Calibration
+
+Device buffers
+==============
+
+IIO triggered buffer
+--------------------
+
+This driver supports IIO triggered buffers, with a "built in" trigger, i.e the
+trigger is allocated and linked by the driver, and a new conversion is triggered
+as soon as the samples are transferred, and a timestamp channel is added to make
+up for the potential jitter induced by the delays in the interrupt handling.
+
+IIO backend buffer
+------------------
+
+When IIO backend is used, the trigger is not needed, and the sample rate is
+considered as stable. There is no timestamp channel. The communication is
+delegated to an external logic, called a backend, and the backend's driver
+handles the buffer. When this mode is enabled, the driver cannot control the
+conversion pin, because the busy pin is bound to the backend.
diff --git a/Documentation/iio/index.rst b/Documentation/iio/index.rst
index 099d1a9e9551..074dbbf7ba0a 100644
--- a/Documentation/iio/index.rst
+++ b/Documentation/iio/index.rst
@@ -21,6 +21,7 @@ Industrial I/O Kernel Drivers
    ad4000
    ad4695
    ad7380
+   ad7606
    ad7625
    ad7944
    adis16475