======= Buffers

struct iio_buffer — general buffer structure
:c:func:iio_validate_scan_mask_onehot — Validates that exactly one channel is selected
:c:func:iio_buffer_get — Grab a reference to the buffer
:c:func:iio_buffer_put — Release the reference to the buffer

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 :file:/dev/iio:device{X} character device node, thus reducing the CPU load.

IIO buffer sysfs interface

An IIO buffer has an associated attributes directory under :file:/sys/bus/iio/iio:device{X}/buffer/*. Here are some of the existing attributes:

:file:length, the total number of data samples (capacity) that can be stored by the buffer.
:file:enable, activate buffer capture.

IIO buffer setup

The meta information associated with a channel reading placed in a buffer is called a scan element. The important bits configuring scan elements are exposed to userspace applications via the :file:/sys/bus/iio/iio:device{X}/scan_elements/ directory. This directory contains attributes of the following form:

:file:enable, used for enabling a channel. If and only if its attribute is non zero, then a triggered capture will contain data samples for this channel.
:file:index, the scan_index of the channel.
:file:type, description of the scan element data storage within the buffer and hence the form in which it is read from user space. Format is [be|le]:[s|u]bits/storagebits[Xrepeat][>>shift] .
- 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-bits registers 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::

  $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
  le:s12/16>>4

A user space 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.

For implementing buffer support a driver should initialize the following fields in iio_chan_spec definition::

struct iio_chan_spec { /* other members */ int scan_index struct { char sign; u8 realbits; u8 storagebits; u8 shift; u8 repeat; enum iio_endian endianness; } scan_type; };

The driver implementing the accelerometer described above will have the following channel definition::

struct iio_chan_spec accel_channels[] = { { .type = IIO_ACCEL, .modified = 1, .channel2 = IIO_MOD_X, /* other stuff here / .scan_index = 0, .scan_type = { .sign = 's', .realbits = 12, .storagebits = 16, .shift = 4, .endianness = IIO_LE, }, } / similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1) * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis */ }

Here scan_index defines the order in which the enabled channels are placed inside the buffer. Channels with a lower scan_index will be placed before channels with a higher index. Each channel needs to have a unique scan_index.

Setting scan_index to -1 can be used to indicate that the specific channel does not support buffered capture. In this case no entries will be created for the channel in the scan_elements directory.

More details

.. kernel-doc:: include/linux/iio/buffer.h .. kernel-doc:: drivers/iio/industrialio-buffer.c :export: