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Subject: Re: [PATCH v5] DocBook: Add initial documentation for IIO
To: Daniel Baluta <daniel.baluta@intel.com>, corbet@lwn.net,
        rdunlap@infradead.org, cmo@melexis.com
References: <1438698008-13718-1-git-send-email-daniel.baluta@intel.com>
 <1438698008-13718-2-git-send-email-daniel.baluta@intel.com>
Cc: pmeerw@pmeerw.net, knaack.h@gmx.de, lars@metafoo.de,
        linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
        herbert@gondor.apana.org.au, smueller@chronox.de, mmarek@suse.cz,
        linux-doc@vger.kernel.org, cristina.opriceana@gmail.com
From: Jonathan Cameron <jic23@kernel.org>
Message-ID: <55C5E8D7.6040305@kernel.org>
Date: Sat, 8 Aug 2015 12:32:39 +0100
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On 04/08/15 15:20, Daniel Baluta wrote:
> 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>
Applied to the togreg branch of iio.git.

Lets fine tune this / expand it in place.

Thanks for doing this Daniel, it is great to have this.

Jonathan
> ---
>  Documentation/DocBook/Makefile |   2 +-
>  Documentation/DocBook/iio.tmpl | 697 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 698 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/DocBook/iio.tmpl
> 
> diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile
> index b6a6a2e..9e08606 100644
> --- a/Documentation/DocBook/Makefile
> +++ b/Documentation/DocBook/Makefile
> @@ -15,7 +15,7 @@ DOCBOOKS := z8530book.xml device-drivers.xml \
>  	    80211.xml debugobjects.xml sh.xml regulator.xml \
>  	    alsa-driver-api.xml writing-an-alsa-driver.xml \
>  	    tracepoint.xml drm.xml media_api.xml w1.xml \
> -	    writing_musb_glue_layer.xml crypto-API.xml
> +	    writing_musb_glue_layer.xml crypto-API.xml iio.xml
>  
>  include Documentation/DocBook/media/Makefile
>  
> diff --git a/Documentation/DocBook/iio.tmpl b/Documentation/DocBook/iio.tmpl
> new file mode 100644
> index 0000000..06bb53d
> --- /dev/null
> +++ b/Documentation/DocBook/iio.tmpl
> @@ -0,0 +1,697 @@
> +<?xml version="1.0" encoding="UTF-8"?>
> +<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
> +	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
> +
> +<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,
> +              .storgebits = 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/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>
> +
> +<!--
> +vim: softtabstop=2:shiftwidth=2:expandtab:textwidth=72
> +-->
> 

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