Industrial I/O buffers

struct iio_buffer — general buffer structure
iio_validate_scan_mask_onehot — Validates that exactly one channel is selected
iio_buffer_get — Grab a reference to the buffer
iio_buffer_put — Release the reference to the buffer
IIO buffer sysfs interface
IIO buffer setup

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.

IIO buffer sysfs interface

An IIO buffer has an associated attributes directory under /sys/bus/iio/iio:deviceX/buffer/. Here are the existing attributes:

  • length, the total number of data samples (capacity) that can be stored by the buffer.
  • 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 /sys/bus/iio/iio:deviceX/scan_elements/ directory. This file contains attributes of the following form:

  • 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.
  • 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/storagebitsXrepeat[>>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.
    • shift, if specified, is the shift that needs to be applied prior to masking out unused bits.
    • repeat, specifies the number of bits/storagebits repetitions. When the repeat element is 0 or 1, then the repeat value is omitted.

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

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 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,
              .storgebits = 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.