forked from Minki/linux
aab978772a
This is intended to help developers faster find their way inside the Industrial I/O core and reduce time spent on IIO drivers development. Signed-off-by: Daniel Baluta <daniel.baluta@intel.com> Acked-by: Crt Mori <cmo@melexis.com> Reviewed-by: Lars-Peter Clausen <lars@metafoo.de> Signed-off-by: Jonathan Cameron <jic23@kernel.org>
698 lines
26 KiB
XML
698 lines
26 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="iioid">
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<bookinfo>
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<title>Industrial I/O driver developer's guide </title>
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<authorgroup>
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<author>
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<firstname>Daniel</firstname>
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<surname>Baluta</surname>
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<affiliation>
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<address>
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<email>daniel.baluta@intel.com</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2015</year>
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<holder>Intel Corporation</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License version 2.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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The main purpose of the Industrial I/O subsystem (IIO) is to provide
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support for devices that in some sense perform either analog-to-digital
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conversion (ADC) or digital-to-analog conversion (DAC) or both. The aim
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is to fill the gap between the somewhat similar hwmon and input
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subsystems.
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Hwmon is directed at low sample rate sensors used to monitor and
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control the system itself, like fan speed control or temperature
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measurement. Input is, as its name suggests, focused on human interaction
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input devices (keyboard, mouse, touchscreen). In some cases there is
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considerable overlap between these and IIO.
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</para>
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<para>
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Devices that fall into this category include:
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<itemizedlist>
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<listitem>
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analog to digital converters (ADCs)
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</listitem>
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<listitem>
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accelerometers
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</listitem>
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<listitem>
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capacitance to digital converters (CDCs)
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</listitem>
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<listitem>
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digital to analog converters (DACs)
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</listitem>
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<listitem>
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gyroscopes
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</listitem>
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<listitem>
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inertial measurement units (IMUs)
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</listitem>
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<listitem>
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color and light sensors
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</listitem>
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<listitem>
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magnetometers
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</listitem>
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<listitem>
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pressure sensors
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</listitem>
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<listitem>
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proximity sensors
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</listitem>
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<listitem>
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temperature sensors
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</listitem>
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</itemizedlist>
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Usually these sensors are connected via SPI or I2C. A common use case of the
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sensors devices is to have combined functionality (e.g. light plus proximity
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sensor).
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</para>
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</chapter>
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<chapter id='iiosubsys'>
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<title>Industrial I/O core</title>
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<para>
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The Industrial I/O core offers:
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<itemizedlist>
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<listitem>
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a unified framework for writing drivers for many different types of
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embedded sensors.
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</listitem>
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<listitem>
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a standard interface to user space applications manipulating sensors.
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</listitem>
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</itemizedlist>
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The implementation can be found under <filename>
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drivers/iio/industrialio-*</filename>
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</para>
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<sect1 id="iiodevice">
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<title> Industrial I/O devices </title>
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!Finclude/linux/iio/iio.h iio_dev
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!Fdrivers/iio/industrialio-core.c iio_device_alloc
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!Fdrivers/iio/industrialio-core.c iio_device_free
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!Fdrivers/iio/industrialio-core.c iio_device_register
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!Fdrivers/iio/industrialio-core.c iio_device_unregister
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<para>
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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
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device then we will show how a device driver makes use of an IIO
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device.
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</para>
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<para>
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There are two ways for a user space application to interact
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with an IIO driver.
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<itemizedlist>
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<listitem>
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<filename>/sys/bus/iio/iio:deviceX/</filename>, this
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represents a hardware sensor and groups together the data
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channels of the same chip.
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</listitem>
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<listitem>
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<filename>/dev/iio:deviceX</filename>, character device node
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interface used for buffered data transfer and for events information
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retrieval.
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</listitem>
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</itemizedlist>
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</para>
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A typical IIO driver will register itself as an I2C or SPI driver and will
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create two routines, <function> probe </function> and <function> remove
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</function>. At <function>probe</function>:
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<itemizedlist>
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<listitem>call <function>iio_device_alloc</function>, which allocates memory
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for an IIO device.
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</listitem>
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<listitem> initialize IIO device fields with driver specific information
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(e.g. device name, device channels).
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</listitem>
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<listitem>call <function> iio_device_register</function>, this registers the
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device with the IIO core. After this call the device is ready to accept
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requests from user space applications.
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</listitem>
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</itemizedlist>
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At <function>remove</function>, we free the resources allocated in
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<function>probe</function> in reverse order:
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<itemizedlist>
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<listitem><function>iio_device_unregister</function>, unregister the device
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from the IIO core.
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</listitem>
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<listitem><function>iio_device_free</function>, free the memory allocated
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for the IIO device.
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</listitem>
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</itemizedlist>
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<sect2 id="iioattr"> <title> IIO device sysfs interface </title>
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<para>
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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
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with index X, attributes can be found under
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<filename>/sys/bus/iio/iio:deviceX/ </filename> directory.
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Common attributes are:
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<itemizedlist>
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<listitem><filename>name</filename>, description of the physical
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chip.
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</listitem>
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<listitem><filename>dev</filename>, shows the major:minor pair
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associated with <filename>/dev/iio:deviceX</filename> node.
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</listitem>
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<listitem><filename>sampling_frequency_available</filename>,
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available discrete set of sampling frequency values for
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device.
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</listitem>
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</itemizedlist>
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Available standard attributes for IIO devices are described in the
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<filename>Documentation/ABI/testing/sysfs-bus-iio </filename> file
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in the Linux kernel sources.
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</para>
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</sect2>
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<sect2 id="iiochannel"> <title> IIO device channels </title>
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!Finclude/linux/iio/iio.h iio_chan_spec structure.
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<para>
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An IIO device channel is a representation of a data channel. An
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IIO device can have one or multiple channels. For example:
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<itemizedlist>
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<listitem>
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a thermometer sensor has one channel representing the
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temperature measurement.
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</listitem>
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<listitem>
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a light sensor with two channels indicating the measurements in
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the visible and infrared spectrum.
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</listitem>
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<listitem>
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an accelerometer can have up to 3 channels representing
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acceleration on X, Y and Z axes.
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</listitem>
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</itemizedlist>
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An IIO channel is described by the <type> struct iio_chan_spec
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</type>. A thermometer driver for the temperature sensor in the
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example above would have to describe its channel as follows:
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<programlisting>
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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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</programlisting>
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Channel sysfs attributes exposed to userspace are specified in
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the form of <emphasis>bitmasks</emphasis>. Depending on their
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shared info, attributes can be set in one of the following masks:
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<itemizedlist>
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<listitem><emphasis>info_mask_separate</emphasis>, attributes will
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be specific to this channel</listitem>
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<listitem><emphasis>info_mask_shared_by_type</emphasis>,
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attributes are shared by all channels of the same type</listitem>
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<listitem><emphasis>info_mask_shared_by_dir</emphasis>, attributes
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are shared by all channels of the same direction </listitem>
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<listitem><emphasis>info_mask_shared_by_all</emphasis>,
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attributes are shared by all channels</listitem>
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</itemizedlist>
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When there are multiple data channels per channel type we have two
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ways to distinguish between them:
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<itemizedlist>
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<listitem> set <emphasis> .modified</emphasis> field of <type>
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iio_chan_spec</type> to 1. Modifiers are specified using
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<emphasis>.channel2</emphasis> field of the same
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<type>iio_chan_spec</type> structure and are used to indicate a
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physically unique characteristic of the channel such as its direction
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or spectral response. For example, a light sensor can have two channels,
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one for infrared light and one for both infrared and visible light.
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</listitem>
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<listitem> set <emphasis>.indexed </emphasis> field of
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<type>iio_chan_spec</type> to 1. In this case the channel is
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simply another instance with an index specified by the
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<emphasis>.channel</emphasis> field.
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</listitem>
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</itemizedlist>
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Here is how we can make use of the channel's modifiers:
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<programlisting>
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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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</programlisting>
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This channel's definition will generate two separate sysfs files
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for raw data retrieval:
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<itemizedlist>
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<listitem>
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<filename>/sys/bus/iio/iio:deviceX/in_intensity_ir_raw</filename>
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</listitem>
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<listitem>
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<filename>/sys/bus/iio/iio:deviceX/in_intensity_both_raw</filename>
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</listitem>
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</itemizedlist>
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one file for processed data:
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<itemizedlist>
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<listitem>
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<filename>/sys/bus/iio/iio:deviceX/in_illuminance_input
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</filename>
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</listitem>
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</itemizedlist>
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and one shared sysfs file for sampling frequency:
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<itemizedlist>
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<listitem>
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<filename>/sys/bus/iio/iio:deviceX/sampling_frequency.
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</filename>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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Here is how we can make use of the channel's indexing:
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<programlisting>
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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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</programlisting>
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This will generate two separate attributes files for raw data
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retrieval:
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<itemizedlist>
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<listitem>
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<filename>/sys/bus/iio/devices/iio:deviceX/in_voltage0_raw</filename>,
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representing voltage measurement for channel 0.
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</listitem>
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<listitem>
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<filename>/sys/bus/iio/devices/iio:deviceX/in_voltage1_raw</filename>,
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representing voltage measurement for channel 1.
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</listitem>
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</itemizedlist>
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</para>
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</sect2>
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</sect1>
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<sect1 id="iiobuffer"> <title> Industrial I/O buffers </title>
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!Finclude/linux/iio/buffer.h iio_buffer
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!Edrivers/iio/industrialio-buffer.c
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<para>
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The Industrial I/O core offers a way for continuous data capture
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based on a trigger source. Multiple data channels can be read at once
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from <filename>/dev/iio:deviceX</filename> character device node,
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thus reducing the CPU load.
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</para>
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<sect2 id="iiobuffersysfs">
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<title>IIO buffer sysfs interface </title>
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<para>
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An IIO buffer has an associated attributes directory under <filename>
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/sys/bus/iio/iio:deviceX/buffer/</filename>. Here are the existing
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attributes:
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<itemizedlist>
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<listitem>
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<emphasis>length</emphasis>, the total number of data samples
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(capacity) that can be stored by the buffer.
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</listitem>
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<listitem>
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<emphasis>enable</emphasis>, activate buffer capture.
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</listitem>
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</itemizedlist>
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</para>
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</sect2>
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<sect2 id="iiobuffersetup"> <title> IIO buffer setup </title>
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<para>The meta information associated with a channel reading
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placed in a buffer is called a <emphasis> scan element </emphasis>.
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The important bits configuring scan elements are exposed to
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userspace applications via the <filename>
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/sys/bus/iio/iio:deviceX/scan_elements/</filename> directory. This
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file contains attributes of the following form:
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<itemizedlist>
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<listitem><emphasis>enable</emphasis>, used for enabling a channel.
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If and only if its attribute is non zero, then a triggered capture
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will contain data samples for this channel.
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</listitem>
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<listitem><emphasis>type</emphasis>, description of the scan element
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data storage within the buffer and hence the form in which it is
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read from user space. Format is <emphasis>
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[be|le]:[s|u]bits/storagebitsXrepeat[>>shift] </emphasis>.
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<itemizedlist>
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<listitem> <emphasis>be</emphasis> or <emphasis>le</emphasis>, specifies
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big or little endian.
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</listitem>
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<listitem>
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<emphasis>s </emphasis>or <emphasis>u</emphasis>, specifies if
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signed (2's complement) or unsigned.
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</listitem>
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<listitem><emphasis>bits</emphasis>, is the number of valid data
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bits.
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</listitem>
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<listitem><emphasis>storagebits</emphasis>, is the number of bits
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(after padding) that it occupies in the buffer.
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</listitem>
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<listitem>
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<emphasis>shift</emphasis>, if specified, is the shift that needs
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to be applied prior to masking out unused bits.
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</listitem>
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<listitem>
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<emphasis>repeat</emphasis>, specifies the number of bits/storagebits
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repetitions. When the repeat element is 0 or 1, then the repeat
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value is omitted.
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</listitem>
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</itemizedlist>
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</listitem>
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</itemizedlist>
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For example, a driver for a 3-axis accelerometer with 12 bit
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resolution where data is stored in two 8-bits registers as
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follows:
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<programlisting>
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7 6 5 4 3 2 1 0
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+---+---+---+---+---+---+---+---+
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|D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
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+---+---+---+---+---+---+---+---+
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7 6 5 4 3 2 1 0
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+---+---+---+---+---+---+---+---+
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|D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
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+---+---+---+---+---+---+---+---+
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</programlisting>
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will have the following scan element type for each axis:
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<programlisting>
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$ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
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le:s12/16>>4
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</programlisting>
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A user space application will interpret data samples read from the
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buffer as two byte little endian signed data, that needs a 4 bits
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right shift before masking out the 12 valid bits of data.
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</para>
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<para>
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For implementing buffer support a driver should initialize the following
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fields in <type>iio_chan_spec</type> definition:
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<programlisting>
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struct iio_chan_spec {
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/* other members */
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int scan_index
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struct {
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char sign;
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u8 realbits;
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u8 storagebits;
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u8 shift;
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u8 repeat;
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enum iio_endian endianness;
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} scan_type;
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};
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</programlisting>
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The driver implementing the accelerometer described above will
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have the following channel definition:
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<programlisting>
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struct struct iio_chan_spec accel_channels[] = {
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{
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.type = IIO_ACCEL,
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.modified = 1,
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.channel2 = IIO_MOD_X,
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/* other stuff here */
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.scan_index = 0,
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.scan_type = {
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.sign = 's',
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.realbits = 12,
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.storgebits = 16,
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.shift = 4,
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.endianness = IIO_LE,
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},
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}
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/* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
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* and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
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*/
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}
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</programlisting>
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</para>
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<para>
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Here <emphasis> scan_index </emphasis> defines the order in which
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the enabled channels are placed inside the buffer. Channels with a lower
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scan_index will be placed before channels with a higher index. Each
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channel needs to have a unique scan_index.
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</para>
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<para>
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Setting scan_index to -1 can be used to indicate that the specific
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channel does not support buffered capture. In this case no entries will
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be created for the channel in the scan_elements directory.
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</para>
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</sect2>
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</sect1>
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<sect1 id="iiotrigger"> <title> Industrial I/O triggers </title>
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!Finclude/linux/iio/trigger.h iio_trigger
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!Edrivers/iio/industrialio-trigger.c
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<para>
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In many situations it is useful for a driver to be able to
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capture data based on some external event (trigger) as opposed
|
|
to periodically polling for data. An IIO trigger can be provided
|
|
by a device driver that also has an IIO device based on hardware
|
|
generated events (e.g. data ready or threshold exceeded) or
|
|
provided by a separate driver from an independent interrupt
|
|
source (e.g. GPIO line connected to some external system, timer
|
|
interrupt or user space writing a specific file in sysfs). A
|
|
trigger may initiate data capture for a number of sensors and
|
|
also it may be completely unrelated to the sensor itself.
|
|
</para>
|
|
|
|
<sect2 id="iiotrigsysfs"> <title> IIO trigger sysfs interface </title>
|
|
There are two locations in sysfs related to triggers:
|
|
<itemizedlist>
|
|
<listitem><filename>/sys/bus/iio/devices/triggerY</filename>,
|
|
this file is created once an IIO trigger is registered with
|
|
the IIO core and corresponds to trigger with index Y. Because
|
|
triggers can be very different depending on type there are few
|
|
standard attributes that we can describe here:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<emphasis>name</emphasis>, trigger name that can be later
|
|
used for association with a device.
|
|
</listitem>
|
|
<listitem>
|
|
<emphasis>sampling_frequency</emphasis>, some timer based
|
|
triggers use this attribute to specify the frequency for
|
|
trigger calls.
|
|
</listitem>
|
|
</itemizedlist>
|
|
</listitem>
|
|
<listitem>
|
|
<filename>/sys/bus/iio/devices/iio:deviceX/trigger/</filename>, this
|
|
directory is created once the device supports a triggered
|
|
buffer. We can associate a trigger with our device by writing
|
|
the trigger's name in the <filename>current_trigger</filename> file.
|
|
</listitem>
|
|
</itemizedlist>
|
|
</sect2>
|
|
|
|
<sect2 id="iiotrigattr"> <title> IIO trigger setup</title>
|
|
|
|
<para>
|
|
Let's see a simple example of how to setup a trigger to be used
|
|
by a driver.
|
|
|
|
<programlisting>
|
|
struct iio_trigger_ops trigger_ops = {
|
|
.set_trigger_state = sample_trigger_state,
|
|
.validate_device = sample_validate_device,
|
|
}
|
|
|
|
struct iio_trigger *trig;
|
|
|
|
/* first, allocate memory for our trigger */
|
|
trig = iio_trigger_alloc(dev, "trig-%s-%d", name, idx);
|
|
|
|
/* setup trigger operations field */
|
|
trig->ops = &trigger_ops;
|
|
|
|
/* now register the trigger with the IIO core */
|
|
iio_trigger_register(trig);
|
|
</programlisting>
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="iiotrigsetup"> <title> IIO trigger ops</title>
|
|
!Finclude/linux/iio/trigger.h iio_trigger_ops
|
|
<para>
|
|
Notice that a trigger has a set of operations attached:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<function>set_trigger_state</function>, switch the trigger on/off
|
|
on demand.
|
|
</listitem>
|
|
<listitem>
|
|
<function>validate_device</function>, function to validate the
|
|
device when the current trigger gets changed.
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
<sect1 id="iiotriggered_buffer">
|
|
<title> Industrial I/O triggered buffers </title>
|
|
<para>
|
|
Now that we know what buffers and triggers are let's see how they
|
|
work together.
|
|
</para>
|
|
<sect2 id="iiotrigbufsetup"> <title> IIO triggered buffer setup</title>
|
|
!Edrivers/iio/industrialio-triggered-buffer.c
|
|
!Finclude/linux/iio/iio.h iio_buffer_setup_ops
|
|
|
|
|
|
<para>
|
|
A typical triggered buffer setup looks like this:
|
|
<programlisting>
|
|
const struct iio_buffer_setup_ops sensor_buffer_setup_ops = {
|
|
.preenable = sensor_buffer_preenable,
|
|
.postenable = sensor_buffer_postenable,
|
|
.postdisable = sensor_buffer_postdisable,
|
|
.predisable = sensor_buffer_predisable,
|
|
};
|
|
|
|
irqreturn_t sensor_iio_pollfunc(int irq, void *p)
|
|
{
|
|
pf->timestamp = iio_get_time_ns();
|
|
return IRQ_WAKE_THREAD;
|
|
}
|
|
|
|
irqreturn_t sensor_trigger_handler(int irq, void *p)
|
|
{
|
|
u16 buf[8];
|
|
int i = 0;
|
|
|
|
/* read data for each active channel */
|
|
for_each_set_bit(bit, active_scan_mask, masklength)
|
|
buf[i++] = sensor_get_data(bit)
|
|
|
|
iio_push_to_buffers_with_timestamp(indio_dev, buf, timestamp);
|
|
|
|
iio_trigger_notify_done(trigger);
|
|
return IRQ_HANDLED;
|
|
}
|
|
|
|
/* setup triggered buffer, usually in probe function */
|
|
iio_triggered_buffer_setup(indio_dev, sensor_iio_polfunc,
|
|
sensor_trigger_handler,
|
|
sensor_buffer_setup_ops);
|
|
</programlisting>
|
|
</para>
|
|
The important things to notice here are:
|
|
<itemizedlist>
|
|
<listitem><function> iio_buffer_setup_ops</function>, the buffer setup
|
|
functions to be called at predefined points in the buffer configuration
|
|
sequence (e.g. before enable, after disable). If not specified, the
|
|
IIO core uses the default <type>iio_triggered_buffer_setup_ops</type>.
|
|
</listitem>
|
|
<listitem><function>sensor_iio_pollfunc</function>, the function that
|
|
will be used as top half of poll function. It should do as little
|
|
processing as possible, because it runs in interrupt context. The most
|
|
common operation is recording of the current timestamp and for this reason
|
|
one can use the IIO core defined <function>iio_pollfunc_store_time
|
|
</function> function.
|
|
</listitem>
|
|
<listitem><function>sensor_trigger_handler</function>, the function that
|
|
will be used as bottom half of the poll function. This runs in the
|
|
context of a kernel thread and all the processing takes place here.
|
|
It usually reads data from the device and stores it in the internal
|
|
buffer together with the timestamp recorded in the top half.
|
|
</listitem>
|
|
</itemizedlist>
|
|
</sect2>
|
|
</sect1>
|
|
</chapter>
|
|
<chapter id='iioresources'>
|
|
<title> Resources </title>
|
|
IIO core may change during time so the best documentation to read is the
|
|
source code. There are several locations where you should look:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<filename>drivers/iio/</filename>, contains the IIO core plus
|
|
and directories for each sensor type (e.g. accel, magnetometer,
|
|
etc.)
|
|
</listitem>
|
|
<listitem>
|
|
<filename>include/linux/iio/</filename>, contains the header
|
|
files, nice to read for the internal kernel interfaces.
|
|
</listitem>
|
|
<listitem>
|
|
<filename>include/uapi/linux/iio/</filename>, contains files to be
|
|
used by user space applications.
|
|
</listitem>
|
|
<listitem>
|
|
<filename>tools/iio/</filename>, contains tools for rapidly
|
|
testing buffers, events and device creation.
|
|
</listitem>
|
|
<listitem>
|
|
<filename>drivers/staging/iio/</filename>, contains code for some
|
|
drivers or experimental features that are not yet mature enough
|
|
to be moved out.
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
Besides the code, there are some good online documentation sources:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<ulink url="http://marc.info/?l=linux-iio"> Industrial I/O mailing
|
|
list </ulink>
|
|
</listitem>
|
|
<listitem>
|
|
<ulink url="http://wiki.analog.com/software/linux/docs/iio/iio">
|
|
Analog Device IIO wiki page </ulink>
|
|
</listitem>
|
|
<listitem>
|
|
<ulink url="https://fosdem.org/2015/schedule/event/iiosdr/">
|
|
Using the Linux IIO framework for SDR, Lars-Peter Clausen's
|
|
presentation at FOSDEM </ulink>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</chapter>
|
|
</book>
|
|
|
|
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|
|
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|
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