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+=============
+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: