Chapter 2. Industrial I/O core
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Chapter 2. Industrial I/O core

Table of Contents

The Industrial I/O core offers:

a unified framework for writing drivers for many different types of embedded sensors.
a standard interface to user space applications manipulating sensors.

The implementation can be found under drivers/iio/industrialio-*

Industrial I/O devices

struct iio_dev — industrial I/O device
iio_device_alloc — allocate an iio_dev from a driver
iio_device_free — free an iio_dev from a driver
iio_device_register — register a device with the IIO subsystem
iio_device_unregister — unregister a device from the IIO subsystem
IIO device sysfs interface
IIO device channels

An IIO device usually corresponds to a single hardware sensor and it provides all the information needed by a driver handling a device. Let's first have a look at the functionality embedded in an IIO device then we will show how a device driver makes use of an IIO device.

There are two ways for a user space application to interact with an IIO driver.

/sys/bus/iio/iio:deviceX/, this represents a hardware sensor and groups together the data channels of the same chip.
/dev/iio:deviceX, character device node interface used for buffered data transfer and for events information retrieval.

A typical IIO driver will register itself as an I2C or SPI driver and will create two routines, probe and

 remove

. At probe:

call iio_device_alloc, which allocates memory for an IIO device.
initialize IIO device fields with driver specific information (e.g. device name, device channels).
call iio_device_register, this registers the device with the IIO core. After this call the device is ready to accept requests from user space applications.

At remove, we free the resources allocated in probe in reverse order:

iio_device_unregister, unregister the device from the IIO core.
iio_device_free, free the memory allocated for the IIO device.

IIO device sysfs interface

Attributes are sysfs files used to expose chip info and also allowing applications to set various configuration parameters. For device with index X, attributes can be found under /sys/bus/iio/iio:deviceX/ directory. Common attributes are:

name, description of the physical chip.
dev, shows the major:minor pair associated with /dev/iio:deviceX node.
sampling_frequency_available, available discrete set of sampling frequency values for device.

Available standard attributes for IIO devices are described in the Documentation/ABI/testing/sysfs-bus-iio file in the Linux kernel sources.

IIO device channels

struct iio_chan_spec — specification of a single channel

An IIO device channel is a representation of a data channel. An IIO device can have one or multiple channels. For example:

a thermometer sensor has one channel representing the temperature measurement.
a light sensor with two channels indicating the measurements in the visible and infrared spectrum.
an accelerometer can have up to 3 channels representing acceleration on X, Y and Z axes.

An IIO channel is described by the struct iio_chan_spec . A thermometer driver for the temperature sensor in the example above would have to describe its channel as follows:

      static const struct iio_chan_spec temp_channel[] = {
          {
              .type = IIO_TEMP,
              .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
          },
      };

Channel sysfs attributes exposed to userspace are specified in the form of bitmasks. Depending on their shared info, attributes can be set in one of the following masks:

info_mask_separate, attributes will be specific to this channel
info_mask_shared_by_type, attributes are shared by all channels of the same type
info_mask_shared_by_dir, attributes are shared by all channels of the same direction
info_mask_shared_by_all, attributes are shared by all channels

When there are multiple data channels per channel type we have two ways to distinguish between them:

set .modified field of iio_chan_spec to 1. Modifiers are specified using .channel2 field of the same iio_chan_spec structure and are used to indicate a physically unique characteristic of the channel such as its direction or spectral response. For example, a light sensor can have two channels, one for infrared light and one for both infrared and visible light.
set .indexed field of iio_chan_spec to 1. In this case the channel is simply another instance with an index specified by the .channel field.

Here is how we can make use of the channel's modifiers:

      static const struct iio_chan_spec light_channels[] = {
          {
              .type = IIO_INTENSITY,
              .modified = 1,
              .channel2 = IIO_MOD_LIGHT_IR,
              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
              .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
          },
          {
              .type = IIO_INTENSITY,
              .modified = 1,
              .channel2 = IIO_MOD_LIGHT_BOTH,
              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
              .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
          },
          {
              .type = IIO_LIGHT,
              .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
              .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
          },

      }

This channel's definition will generate two separate sysfs files for raw data retrieval:

/sys/bus/iio/iio:deviceX/in_intensity_ir_raw
/sys/bus/iio/iio:deviceX/in_intensity_both_raw

one file for processed data:

/sys/bus/iio/iio:deviceX/in_illuminance_input

and one shared sysfs file for sampling frequency:

/sys/bus/iio/iio:deviceX/sampling_frequency.

Here is how we can make use of the channel's indexing:

      static const struct iio_chan_spec light_channels[] = {
          {
              .type = IIO_VOLTAGE,
              .indexed = 1,
              .channel = 0,
              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
          },
          {
              .type = IIO_VOLTAGE,
              .indexed = 1,
              .channel = 1,
              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
          },
      }

This will generate two separate attributes files for raw data retrieval:

/sys/bus/iio/devices/iio:deviceX/in_voltage0_raw, representing voltage measurement for channel 0.
/sys/bus/iio/devices/iio:deviceX/in_voltage1_raw, representing voltage measurement for channel 1.