183 lines
6.8 KiB
ReStructuredText
183 lines
6.8 KiB
ReStructuredText
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=============
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Core elements
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=============
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The Industrial I/O core offers both a unified framework for writing drivers for
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many different types of embedded sensors and a standard interface to user space
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applications manipulating sensors. The implementation can be found under
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:file:`drivers/iio/industrialio-*`
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Industrial I/O Devices
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----------------------
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* struct iio_dev - industrial I/O device
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* iio_device_alloc() - allocate an :c:type:`iio_dev` from a driver
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* iio_device_free() - free an :c:type:`iio_dev` from a driver
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* iio_device_register() - register a device with the IIO subsystem
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* iio_device_unregister() - unregister a device from the IIO
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subsystem
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An IIO device usually corresponds to a single hardware sensor and it
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provides all the information needed by a driver handling a device.
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Let's first have a look at the functionality embedded in an IIO device
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then we will show how a device driver makes use of an IIO device.
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There are two ways for a user space application to interact with an IIO driver.
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1. :file:`/sys/bus/iio/iio:device{X}/`, this represents a hardware sensor
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and groups together the data channels of the same chip.
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2. :file:`/dev/iio:device{X}`, character device node interface used for
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buffered data transfer and for events information retrieval.
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A typical IIO driver will register itself as an :doc:`I2C <../i2c>` or
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:doc:`SPI <../spi>` driver and will create two routines, probe and remove.
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At probe:
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1. Call iio_device_alloc(), which allocates memory for an IIO device.
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2. Initialize IIO device fields with driver specific information (e.g.
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device name, device channels).
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3. Call iio_device_register(), this registers the device with the
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IIO core. After this call the device is ready to accept requests from user
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space applications.
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At remove, we free the resources allocated in probe in reverse order:
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1. iio_device_unregister(), unregister the device from the IIO core.
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2. iio_device_free(), free the memory allocated for the IIO device.
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IIO device sysfs interface
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==========================
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Attributes are sysfs files used to expose chip info and also allowing
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applications to set various configuration parameters. For device with
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index X, attributes can be found under /sys/bus/iio/iio:deviceX/ directory.
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Common attributes are:
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* :file:`name`, description of the physical chip.
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* :file:`dev`, shows the major:minor pair associated with
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:file:`/dev/iio:deviceX` node.
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* :file:`sampling_frequency_available`, available discrete set of sampling
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frequency values for device.
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* Available standard attributes for IIO devices are described in the
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:file:`Documentation/ABI/testing/sysfs-bus-iio` file in the Linux kernel
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sources.
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IIO device channels
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===================
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struct iio_chan_spec - specification of a single channel
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An IIO device channel is a representation of a data channel. An IIO device can
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have one or multiple channels. For example:
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* a thermometer sensor has one channel representing the temperature measurement.
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* a light sensor with two channels indicating the measurements in the visible
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and infrared spectrum.
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* an accelerometer can have up to 3 channels representing acceleration on X, Y
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and Z axes.
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An IIO channel is described by the struct iio_chan_spec.
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A thermometer driver for the temperature sensor in the example above would
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have to describe its channel as follows::
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static const struct iio_chan_spec temp_channel[] = {
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{
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.type = IIO_TEMP,
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.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
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},
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};
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Channel sysfs attributes exposed to userspace are specified in the form of
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bitmasks. Depending on their shared info, attributes can be set in one of the
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following masks:
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* **info_mask_separate**, attributes will be specific to
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this channel
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* **info_mask_shared_by_type**, attributes are shared by all channels of the
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same type
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* **info_mask_shared_by_dir**, attributes are shared by all channels of the same
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direction
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* **info_mask_shared_by_all**, attributes are shared by all channels
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When there are multiple data channels per channel type we have two ways to
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distinguish between them:
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* set **.modified** field of :c:type:`iio_chan_spec` to 1. Modifiers are
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specified using **.channel2** field of the same :c:type:`iio_chan_spec`
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structure and are used to indicate a physically unique characteristic of the
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channel such as its direction or spectral response. For example, a light
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sensor can have two channels, one for infrared light and one for both
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infrared and visible light.
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* set **.indexed** field of :c:type:`iio_chan_spec` to 1. In this case the
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channel is simply another instance with an index specified by the **.channel**
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field.
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Here is how we can make use of the channel's modifiers::
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static const struct iio_chan_spec light_channels[] = {
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{
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.type = IIO_INTENSITY,
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.modified = 1,
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.channel2 = IIO_MOD_LIGHT_IR,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
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.info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
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},
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{
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.type = IIO_INTENSITY,
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.modified = 1,
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.channel2 = IIO_MOD_LIGHT_BOTH,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
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.info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
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},
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{
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.type = IIO_LIGHT,
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.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
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.info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
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},
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}
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This channel's definition will generate two separate sysfs files for raw data
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retrieval:
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* :file:`/sys/bus/iio/iio:device{X}/in_intensity_ir_raw`
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* :file:`/sys/bus/iio/iio:device{X}/in_intensity_both_raw`
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one file for processed data:
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* :file:`/sys/bus/iio/iio:device{X}/in_illuminance_input`
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and one shared sysfs file for sampling frequency:
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* :file:`/sys/bus/iio/iio:device{X}/sampling_frequency`.
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Here is how we can make use of the channel's indexing::
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static const struct iio_chan_spec light_channels[] = {
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{
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.type = IIO_VOLTAGE,
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.indexed = 1,
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.channel = 0,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
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},
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{
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.type = IIO_VOLTAGE,
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.indexed = 1,
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.channel = 1,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
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},
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}
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This will generate two separate attributes files for raw data retrieval:
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* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage0_raw`, representing
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voltage measurement for channel 0.
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* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage1_raw`, representing
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voltage measurement for channel 1.
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More details
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============
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.. kernel-doc:: include/linux/iio/iio.h
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.. kernel-doc:: drivers/iio/industrialio-core.c
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:export:
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