docs: iio: add documentation for device buffers

Add documentation for IIO device buffers describing buffer
attributes and how data is structured in buffers using
scan elements.
Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Link: https://lore.kernel.org/r/20240221085848.991413-3-ramona.gradinariu@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/iio_devbuf.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
+Industrial IIO device buffers
+=============================
+
+1. Overview
+===========
+
+The Industrial I/O core offers a way for continuous data capture based on a
+trigger source. Multiple data channels can be read at once from
+``/dev/iio:deviceX`` character device node, thus reducing the CPU load.
+
+Devices with buffer support feature an additional sub-directory in the
+``/sys/bus/iio/devices/iio:deviceX/`` directory hierarchy, called bufferY, where
+Y defaults to 0, for devices with a single buffer.
+
+2. Buffer attributes
+====================
+
+An IIO buffer has an associated attributes directory under
+``/sys/bus/iio/iio:deviceX/bufferY/``. The attributes are described below.
+
+``length``
+----------
+
+Read / Write attribute which states the total number of data samples (capacity)
+that can be stored by the buffer.
+
+``enable``
+----------
+
+Read / Write attribute which starts / stops the buffer capture. This file should
+be written last, after length and selection of scan elements. Writing a non-zero
+value may result in an error, such as EINVAL, if, for example, an unsupported
+combination of channels is given.
+
+``watermark``
+-------------
+
+Read / Write positive integer attribute specifying the maximum number of scan
+elements to wait for.
+
+Poll will block until the watermark is reached.
+
+Blocking read will wait until the minimum between the requested read amount or
+the low watermark is available.
+
+Non-blocking read will retrieve the available samples from the buffer even if
+there are less samples than the watermark level. This allows the application to
+block on poll with a timeout and read the available samples after the timeout
+expires and thus have a maximum delay guarantee.
+
+Data available
+--------------
+
+Read-only attribute indicating the bytes of data available in the buffer. In the
+case of an output buffer, this indicates the amount of empty space available to
+write data to. In the case of an input buffer, this indicates the amount of data
+available for reading.
+
+Scan elements
+-------------
+
+The meta information associated with a channel data placed in a buffer is called
+a scan element. The scan elements attributes are presented below.
+
+**_en**
+
+Read / Write attribute used for enabling a channel. If and only if its value
+is non-zero, then a triggered capture will contain data samples for this
+channel.
+
+**_index**
+
+Read-only unsigned integer attribute specifying the position of the channel in
+the buffer. Note these are not dependent on what is enabled and may not be
+contiguous. Thus for userspace to establish the full layout these must be used
+in conjunction with all _en attributes to establish which channels are present,
+and the relevant _type attributes to establish the data storage format.
+
+**_type**
+
+Read-only attribute containing the description of the scan element data storage
+within the buffer and hence the form in which it is read from userspace. Format
+is [be|le]:[s|u]bits/storagebits[Xrepeat][>>shift], where:
+
+- **be** or **le** specifies big or little-endian.
+- **s** or **u** specifies if signed (2's complement) or unsigned.
+- **bits** is the number of valid data bits.
+- **storagebits** is the number of bits (after padding) that it occupies in the
+  buffer.
+- **repeat** specifies the number of bits/storagebits repetitions. When the
+  repeat element is 0 or 1, then the repeat value is omitted.
+- **shift** if specified, is the shift that needs to be applied prior to
+  masking out unused bits.
+
+For example, a driver for a 3-axis accelerometer with 12-bit resolution where
+data is stored in two 8-bit registers is as follows::
+
+          7   6   5   4   3   2   1   0
+        +---+---+---+---+---+---+---+---+
+        |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
+        +---+---+---+---+---+---+---+---+
+
+          7   6   5   4   3   2   1   0
+        +---+---+---+---+---+---+---+---+
+        |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
+        +---+---+---+---+---+---+---+---+
+
+will have the following scan element type for each axis:
+
+.. code-block:: bash
+
+        $ cat /sys/bus/iio/devices/iio:device0/buffer0/in_accel_y_type
+        le:s12/16>>4
+
+A userspace application will interpret data samples read from the buffer as
+two-byte little-endian signed data, that needs a 4 bits right shift before
+masking out the 12 valid bits of data.
+
+It is also worth mentioning that the data in the buffer will be naturally
+aligned, so the userspace application has to handle the buffers accordingly.
+
+Take for example, a driver with four channels with the following description:
+- channel0: index: 0, type: be:u16/16>>0
+- channel1: index: 1, type: be:u32/32>>0
+- channel2: index: 2, type: be:u32/32>>0
+- channel3: index: 3, type: be:u64/64>>0
+
+If all channels are enabled, the data will be aligned in the buffer as follows::
+
+          0-1   2   3   4-7  8-11  12  13  14  15  16-23   -> buffer byte number
+        +-----+---+---+-----+-----+---+---+---+---+-----+
+        |CHN_0|PAD|PAD|CHN_1|CHN_2|PAD|PAD|PAD|PAD|CHN_3|  -> buffer content
+        +-----+---+---+-----+-----+---+---+---+---+-----+
+
+If only channel0 and channel3 are enabled, the data will be aligned in the
+buffer as follows::
+
+          0-1   2   3   4   5   6   7  8-15    -> buffer byte number
+        +-----+---+---+---+---+---+---+-----+
+        |CHN_0|PAD|PAD|PAD|PAD|PAD|PAD|CHN_3|  -> buffer content
+        +-----+---+---+---+---+---+---+-----+
+
+Typically the buffered data is found in raw format (unscaled with no offset
+applied), however there are corner cases in which the buffered data may be found
+in a processed form. Please note that these corner cases are not addressed by
+this documentation.
+
+Please see ``Documentation/ABI/testing/sysfs-bus-iio`` for a complete
+description of the attributes.

Documentation/iio/index.rst

@@ -8,6 +8,7 @@ Industrial I/O
    :maxdepth: 1
 
    iio_configfs
+   iio_devbuf
 
 Industrial I/O Kernel Drivers
 =============================