Industrial I/O buffers

Industrial I/O buffers
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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
      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 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.