Re: [PATCH v4] DocBook: Add initial documentation for IIO

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On Mon, Aug 3, 2015 at 12:24 PM, Peter Meerwald <pmeerw@xxxxxxxxxx> wrote:
> On Fri, 31 Jul 2015, 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.
>
> comments inline below

Hi Peter,

thanks a lot for your review. I've addressed most of your comments and
will send asap v5.

>
>> Signed-off-by: Daniel Baluta <daniel.baluta@xxxxxxxxx>
>> ---
>>  Documentation/DocBook/Makefile |   2 +-
>>  Documentation/DocBook/iio.tmpl | 702 +++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 703 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..b39b3e9
>> --- /dev/null
>> +++ b/Documentation/DocBook/iio.tmpl
>> @@ -0,0 +1,702 @@
>> +<?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@xxxxxxxxx</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
>
> sorsor devices

:), sensor devices. Got it :).

>
>> +    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>
>
> extra spaces after/before tag, here and elsewhere

Nice catch. Fixed.

>
>> +
>> +!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 faster data transfer and for events information
>
> maybe refer to buffered data transfer?

ok.

>
>> +          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
>
> sampling_frequency is rather specific and not found for all devices;
> it may also appear under events/

Most of the devices I use have a configurable sampling_frequency (e.g
gyro, accels, magnetometers). I think it's not far from true that
sampling_frequency
is a common attribute. Or I could change 'common' to something else?

>
>> +            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),
>
> _RAW vs. _PROCESSED?

Not sure I understand this. PROCESSED because I want to avoid exposing now
scale and offset. Also, it's just an example.

>
>> +          },
>> +      };
>> +
>> +      </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 sensor type there are two
>
> per channel type?

Ok.

>
>> +      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:
>
> this is a bit of a corner case: why is there just one _sampling_frequency
> channel? it could also have been named in_illuminance_sampling_frequency

In fact, the attribute will be called sampling frequency, the field name is
wrong it should by info_mask_shared_by_all :).

>
>> +      <itemizedlist>
>> +      <listitem>
>> +      <filename>/sys/bus/iio/iio:deviceX/in_intensity_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>, number of data samples contained by the
>> +        buffer.
>
> wording is not very clean; length is is the buffer length/capacity, not
> the number of samples currently in the buffer

Oh, I see. The same wording is in the ABI file:

>
>> +      </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 bits of data
>
> number of data bits
>
>> +        </listitem>
>> +        <listitem><emphasis>storagebits</emphasis> is the space (after padding)
>> +          that it occupies in the buffer.
>
> is the number of bits (after padding) that...
>
>> +        </listitem>
>> +        <listitem>
>> +        <emphasis>shift</emphasis> if specified, is the shift that needs
>> +          to be a applied prior to masking out unused bits
>
> that needs to be applied -- delete a
>
>> +        </listitem>
>> +        <listitem>
>> +        <emphasis>repeat</emphasis>, specifies the number of real/storage bits
>
> what is real? -- undefined at this point;
> "real/storage" doesn't make it clear if the padded or unpadded data bits
> are repeated -- I think the later
> maybe: "specifies the number of unpadded data repetitions"
>
>> +        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>
>> +      The userspace applications will interpret data samples read from buffer
>
> user space -- inconsistent
> maybe "A user space application will..."
> the buffer -- the
>
>> +      as two byte little endian signed data, that needs a 4 bits right
>> +      shift before masking out the only 12 valid bits of real data.
>
> 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 and  Z axis */
>
> two spaces before Z
> maybe: "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 relative order in which
>
> why relative?
> it is simply the order
>
>> +    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>
>> +    It is important to realize that the scan_index does not define the
>> +    absolute position in the buffer. E.g. a channel with the scan_index = 3
>> +    will not be at offset 3 bytes or 3 words, but rather will be placed in the
>> +    buffer after any channel with a scan_index lower than 3 and before
>> +    any channel with a scan_index larger than 3.
>
> I'd drop the paragraph above, this is just confusing; better mention that
> there are padding rules (e.g. for the timestamp channel) and it follows
> that the scan_index is not a byte offset into the buffer
>
>> +    Furthermore the scan indices do not have to be consecutive. E.g. A
>> +    channel spec array that defines 3 channels with the indices 1, 2 and 3 is
>> +    just as valid as a channel spec that uses the indices 100, 200, 300. The
>> +    relative order of the channels will be the same.
>> +    </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 reading a specific file in sysfs). A
>
> writing a specific file
>
>> +      trigger may initialize data capture for a number of sensors and
>
> initiate -- not initialize
>
>> +      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 triggered is registered with
>
> an IIO trigger
>
>> +          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 to for association with a device.
>
> used for association
>
>> +            </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
>> +            trigger's name in the<filename>current_trigger</filename> file.
>
> the trigger's name -- the
> the <filename> -- add space before tag
>
>> +        </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, buffer, timestamp);
>
> buf -- not buffer
>
>> +
>> +        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 buffer configuration
>
> the buffer configuration -- the
>
>> +    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
>> +-->
>>
>
> --
>
> Peter Meerwald
> +43-664-2444418 (mobile)
> --
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