linux-next/Documentation/DocBook/iio.tmpl

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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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<book id="iioid">
  <bookinfo>
    <title>Industrial I/O driver developer's guide </title>

    <authorgroup>
      <author>
        <firstname>Daniel</firstname>
        <surname>Baluta</surname>
        <affiliation>
          <address>
            <email>daniel.baluta@intel.com</email>
          </address>
        </affiliation>
      </author>
    </authorgroup>

    <copyright>
      <year>2015</year>
      <holder>Intel Corporation</holder>
    </copyright>

    <legalnotice>
      <para>
        This documentation is free software; you can redistribute
        it and/or modify it under the terms of the GNU General Public
        License version 2.
      </para>
    </legalnotice>
  </bookinfo>

  <toc></toc>

  <chapter id="intro">
    <title>Introduction</title>
    <para>
      The main purpose of the Industrial I/O subsystem (IIO) is to provide
      support for devices that in some sense perform either analog-to-digital
      conversion (ADC) or digital-to-analog conversion (DAC) or both. The aim
      is to fill the gap between the somewhat similar hwmon and input
      subsystems.
      Hwmon is directed at low sample rate sensors used to monitor and
      control the system itself, like fan speed control or temperature
      measurement. Input is, as its name suggests, focused on human interaction
      input devices (keyboard, mouse, touchscreen). In some cases there is
      considerable overlap between these and IIO.
  </para>
  <para>
    Devices that fall into this category include:
    <itemizedlist>
      <listitem>
        analog to digital converters (ADCs)
      </listitem>
      <listitem>
        accelerometers
      </listitem>
      <listitem>
        capacitance to digital converters (CDCs)
      </listitem>
      <listitem>
        digital to analog converters (DACs)
      </listitem>
      <listitem>
        gyroscopes
      </listitem>
      <listitem>
        inertial measurement units (IMUs)
      </listitem>
      <listitem>
        color and light sensors
      </listitem>
      <listitem>
        magnetometers
      </listitem>
      <listitem>
        pressure sensors
      </listitem>
      <listitem>
        proximity sensors
      </listitem>
      <listitem>
        temperature sensors
      </listitem>
    </itemizedlist>
    Usually these sensors are connected via SPI or I2C. A common use case of the
    sensors devices is to have combined functionality (e.g. light plus proximity
    sensor).
  </para>
  </chapter>
  <chapter id='iiosubsys'>
    <title>Industrial I/O core</title>
    <para>
      The Industrial I/O core offers:
      <itemizedlist>
        <listitem>
         a unified framework for writing drivers for many different types of
         embedded sensors.
        </listitem>
        <listitem>
         a standard interface to user space applications manipulating sensors.
        </listitem>
      </itemizedlist>
      The implementation can be found under <filename>
      drivers/iio/industrialio-*</filename>
  </para>
  <sect1 id="iiodevice">
    <title> Industrial I/O devices </title>

!Finclude/linux/iio/iio.h iio_dev
!Fdrivers/iio/industrialio-core.c iio_device_alloc
!Fdrivers/iio/industrialio-core.c iio_device_free
!Fdrivers/iio/industrialio-core.c iio_device_register
!Fdrivers/iio/industrialio-core.c iio_device_unregister

    <para>
      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.
    </para>
    <para>
        There are two ways for a user space application to interact
        with an IIO driver.
      <itemizedlist>
        <listitem>
          <filename>/sys/bus/iio/iio:deviceX/</filename>, this
          represents a hardware sensor and groups together the data
          channels of the same chip.
        </listitem>
        <listitem>
          <filename>/dev/iio:deviceX</filename>, character device node
          interface used for buffered data transfer and for events information
          retrieval.
        </listitem>
      </itemizedlist>
    </para>
    A typical IIO driver will register itself as an I2C or SPI driver and will
    create two routines, <function> probe </function> and <function> remove
    </function>. At <function>probe</function>:
    <itemizedlist>
    <listitem>call <function>iio_device_alloc</function>, which allocates memory
      for an IIO device.
    </listitem>
    <listitem> initialize IIO device fields with driver specific information
              (e.g. device name, device channels).
    </listitem>
    <listitem>call <function> iio_device_register</function>, this registers the
      device with the IIO core. After this call the device is ready to accept
      requests from user space applications.
    </listitem>
    </itemizedlist>
      At <function>remove</function>, we free the resources allocated in
      <function>probe</function> in reverse order:
    <itemizedlist>
    <listitem><function>iio_device_unregister</function>, unregister the device
      from the IIO core.
    </listitem>
    <listitem><function>iio_device_free</function>, free the memory allocated
      for the IIO device.
    </listitem>
    </itemizedlist>

    <sect2 id="iioattr"> <title> IIO device sysfs interface </title>
      <para>
        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
        <filename>/sys/bus/iio/iio:deviceX/ </filename> directory.
        Common attributes are:
        <itemizedlist>
          <listitem><filename>name</filename>, description of the physical
            chip.
          </listitem>
          <listitem><filename>dev</filename>, shows the major:minor pair
            associated with <filename>/dev/iio:deviceX</filename> node.
          </listitem>
          <listitem><filename>sampling_frequency_available</filename>,
            available discrete set of sampling frequency values for
            device.
          </listitem>
      </itemizedlist>
      Available standard attributes for IIO devices are described in the
      <filename>Documentation/ABI/testing/sysfs-bus-iio </filename> file
      in the Linux kernel sources.
      </para>
    </sect2>
    <sect2 id="iiochannel"> <title> IIO device channels </title>
!Finclude/linux/iio/iio.h iio_chan_spec structure.
      <para>
        An IIO device channel is a representation of a data channel. An
        IIO device can have one or multiple channels. For example:
        <itemizedlist>
          <listitem>
          a thermometer sensor has one channel representing the
          temperature measurement.
          </listitem>
          <listitem>
          a light sensor with two channels indicating the measurements in
          the visible and infrared spectrum.
          </listitem>
          <listitem>
          an accelerometer can have up to 3 channels representing
          acceleration on X, Y and Z axes.
          </listitem>
        </itemizedlist>
      An IIO channel is described by the <type> struct iio_chan_spec
      </type>. A thermometer driver for the temperature sensor in the
      example above would have to describe its channel as follows:
      <programlisting>
      static const struct iio_chan_spec temp_channel[] = {
          {
              .type = IIO_TEMP,
              .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
          },
      };

      </programlisting>
      Channel sysfs attributes exposed to userspace are specified in
      the form of <emphasis>bitmasks</emphasis>. Depending on their
      shared info, attributes can be set in one of the following masks:
      <itemizedlist>
      <listitem><emphasis>info_mask_separate</emphasis>, attributes will
        be specific to this channel</listitem>
      <listitem><emphasis>info_mask_shared_by_type</emphasis>,
        attributes are shared by all channels of the same type</listitem>
      <listitem><emphasis>info_mask_shared_by_dir</emphasis>, attributes
        are shared by all channels of the same direction </listitem>
      <listitem><emphasis>info_mask_shared_by_all</emphasis>,
        attributes are shared by all channels</listitem>
      </itemizedlist>
      When there are multiple data channels per channel type we have two
      ways to distinguish between them:
      <itemizedlist>
      <listitem> set <emphasis> .modified</emphasis> field of <type>
        iio_chan_spec</type> to 1. Modifiers are specified using
        <emphasis>.channel2</emphasis> field of the same
        <type>iio_chan_spec</type> 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.
      </listitem>
      <listitem> set <emphasis>.indexed </emphasis> field of
        <type>iio_chan_spec</type> to 1. In this case the channel is
        simply another instance with an index specified by the
        <emphasis>.channel</emphasis> field.
      </listitem>
      </itemizedlist>
      Here is how we can make use of the channel's modifiers:
      <programlisting>
      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),
          },

      }
      </programlisting>
      This channel's definition will generate two separate sysfs files
      for raw data retrieval:
      <itemizedlist>
      <listitem>
      <filename>/sys/bus/iio/iio:deviceX/in_intensity_ir_raw</filename>
      </listitem>
      <listitem>
      <filename>/sys/bus/iio/iio:deviceX/in_intensity_both_raw</filename>
      </listitem>
      </itemizedlist>
      one file for processed data:
      <itemizedlist>
      <listitem>
      <filename>/sys/bus/iio/iio:deviceX/in_illuminance_input
      </filename>
      </listitem>
      </itemizedlist>
      and one shared sysfs file for sampling frequency:
      <itemizedlist>
      <listitem>
      <filename>/sys/bus/iio/iio:deviceX/sampling_frequency.
      </filename>
      </listitem>
      </itemizedlist>
      </para>
      <para>
      Here is how we can make use of the channel's indexing:
      <programlisting>
      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),
          },
      }
      </programlisting>
      This will generate two separate attributes files for raw data
      retrieval:
      <itemizedlist>
      <listitem>
        <filename>/sys/bus/iio/devices/iio:deviceX/in_voltage0_raw</filename>,
          representing voltage measurement for channel 0.
      </listitem>
      <listitem>
        <filename>/sys/bus/iio/devices/iio:deviceX/in_voltage1_raw</filename>,
          representing voltage measurement for channel 1.
      </listitem>
      </itemizedlist>
      </para>
    </sect2>
  </sect1>

  <sect1 id="iiobuffer"> <title> Industrial I/O buffers </title>
!Finclude/linux/iio/buffer.h iio_buffer
!Edrivers/iio/industrialio-buffer.c

    <para>
    The Industrial I/O core offers a way for continuous data capture
    based on a trigger source. Multiple data channels can be read at once
    from <filename>/dev/iio:deviceX</filename> character device node,
    thus reducing the CPU load.
    </para>

    <sect2 id="iiobuffersysfs">
    <title>IIO buffer sysfs interface </title>
    <para>
      An IIO buffer has an associated attributes directory under <filename>
      /sys/bus/iio/iio:deviceX/buffer/</filename>. Here are the existing
      attributes:
      <itemizedlist>
      <listitem>
      <emphasis>length</emphasis>, the total number of data samples
      (capacity) that can be stored by the buffer.
      </listitem>
      <listitem>
        <emphasis>enable</emphasis>, activate buffer capture.
      </listitem>
      </itemizedlist>

    </para>
    </sect2>
    <sect2 id="iiobuffersetup"> <title> IIO buffer setup </title>
      <para>The meta information associated with a channel reading
        placed in a buffer is called a <emphasis> scan element </emphasis>.
        The important bits configuring scan elements are exposed to
        userspace applications via the <filename>
        /sys/bus/iio/iio:deviceX/scan_elements/</filename> directory. This
        file contains attributes of the following form:
      <itemizedlist>
      <listitem><emphasis>enable</emphasis>, used for enabling a channel.
        If and only if its attribute is non zero, then a triggered capture
        will contain data samples for this channel.
      </listitem>
      <listitem><emphasis>type</emphasis>, description of the scan element
        data storage within the buffer and hence the form in which it is
        read from user space. Format is <emphasis>
        [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] </emphasis>.
        <itemizedlist>
        <listitem> <emphasis>be</emphasis> or <emphasis>le</emphasis>, specifies
          big or little endian.
        </listitem>
        <listitem>
        <emphasis>s </emphasis>or <emphasis>u</emphasis>, specifies if
          signed (2's complement) or unsigned.
        </listitem>
        <listitem><emphasis>bits</emphasis>, is the number of valid data
          bits.
        </listitem>
        <listitem><emphasis>storagebits</emphasis>, is the number of bits
          (after padding) that it occupies in the buffer.
        </listitem>
        <listitem>
        <emphasis>shift</emphasis>, if specified, is the shift that needs
          to be applied prior to masking out unused bits.
        </listitem>
        <listitem>
        <emphasis>repeat</emphasis>, specifies the number of bits/storagebits
        repetitions. When the repeat element is 0 or 1, then the repeat
        value is omitted.
        </listitem>
        </itemizedlist>
      </listitem>
      </itemizedlist>
      For example, a driver for a 3-axis accelerometer with 12 bit
      resolution where data is stored in two 8-bits registers as
      follows:
      <programlisting>
        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
      +---+---+---+---+---+---+---+---+

        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
      +---+---+---+---+---+---+---+---+
      </programlisting>

      will have the following scan element type for each axis:
      <programlisting>
      $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
      le:s12/16>>4
      </programlisting>
      A user space application will interpret data samples read from the
      buffer as two byte little endian signed data, that needs a 4 bits
      right shift before masking out the 12 valid bits of data.
    </para>
    <para>
      For implementing buffer support a driver should initialize the following
      fields in <type>iio_chan_spec</type> definition:
      <programlisting>
          struct iio_chan_spec {
              /* other members */
              int scan_index
              struct {
                  char sign;
                  u8 realbits;
                  u8 storagebits;
                  u8 shift;
                  u8 repeat;
                  enum iio_endian endianness;
              } scan_type;
          };
      </programlisting>
      The driver implementing the accelerometer described above will
      have the following channel definition:
      <programlisting>
      struct struct iio_chan_spec accel_channels[] = {
          {
            .type = IIO_ACCEL,
            .modified = 1,
            .channel2 = IIO_MOD_X,
            /* other stuff here */
            .scan_index = 0,
            .scan_type = {
              .sign = 's',
              .realbits = 12,
              .storagebits = 16,
              .shift = 4,
              .endianness = IIO_LE,
            },
        }
        /* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
         * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
         */
    }
    </programlisting>
    </para>
    <para>
    Here <emphasis> scan_index </emphasis> defines the order in which
    the enabled channels are placed inside the buffer. Channels with a lower
    scan_index will be placed before channels with a higher index. Each
    channel needs to have a unique scan_index.
    </para>
    <para>
    Setting scan_index to -1 can be used to indicate that the specific
    channel does not support buffered capture. In this case no entries will
    be created for the channel in the scan_elements directory.
    </para>
    </sect2>
  </sect1>

  <sect1 id="iiotrigger"> <title> Industrial I/O triggers  </title>
!Finclude/linux/iio/trigger.h iio_trigger
!Edrivers/iio/industrialio-trigger.c
    <para>
      In many situations it is useful for a driver to be able to
      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 = &amp;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/buffer/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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