xref: /openbmc/linux/Documentation/driver-api/iio/core.rst (revision cbecf716ca618fd44feda6bd9a64a8179d031fc5)

=============
Core elements
=============

The Industrial I/O core offers both a unified framework for writing drivers for
many different types of embedded sensors and a standard interface to user space
applications manipulating sensors. The implementation can be found under
:file:`drivers/iio/industrialio-*`

Industrial I/O Devices
----------------------

* struct iio_dev - industrial I/O device
* iio_device_alloc() - allocate an :c:type:`iio_dev` from a driver
* iio_device_free() - free an :c:type:`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

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.

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

A typical IIO driver will register itself as an :doc:`I2C <../i2c>` or
:doc:`SPI <../spi>` driver and will create two routines, probe and remove.

At probe:

1. Call iio_device_alloc(), which allocates memory for an IIO device.
2. Initialize IIO device fields with driver specific information (e.g.
   device name, device channels).
3. 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:

1. iio_device_unregister(), unregister the device from the IIO core.
2. 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:

* :file:`name`, description of the physical chip.
* :file:`dev`, shows the major:minor pair associated with
  :file:`/dev/iio:deviceX` node.
* :file:`sampling_frequency_available`, available discrete set of sampling
  frequency values for device.
* Available standard attributes for IIO devices are described in the
  :file:`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 :c:type:`iio_chan_spec` to 1. Modifiers are
  specified using **.channel2** field of the same :c:type:`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 :c:type:`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:

* :file:`/sys/bus/iio/iio:device{X}/in_intensity_ir_raw`
* :file:`/sys/bus/iio/iio:device{X}/in_intensity_both_raw`

one file for processed data:

* :file:`/sys/bus/iio/iio:device{X}/in_illuminance_input`

and one shared sysfs file for sampling frequency:

* :file:`/sys/bus/iio/iio:device{X}/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:

* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage0_raw`, representing
  voltage measurement for channel 0.
* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage1_raw`, representing
  voltage measurement for channel 1.

More details
============
.. kernel-doc:: include/linux/iio/iio.h
.. kernel-doc:: drivers/iio/industrialio-core.c
   :export: