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Message-ID: <alpine.DEB.2.02.1508031035001.9767@pmeerw.net>
Date: Mon, 3 Aug 2015 11:24:01 +0200 (CEST)
From: Peter Meerwald <pmeerw@...erw.net>
To: Daniel Baluta <daniel.baluta@...el.com>
cc: jic23@...nel.org, corbet@....net, knaack.h@....de, lars@...afoo.de,
linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] DocBook: Add initial documentation for IIO
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
> Signed-off-by: Daniel Baluta <daniel.baluta@...el.com>
> ---
> 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@...el.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
sorsor devices
> + 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
> +
> +!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?
> + 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/
> + 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?
> + },
> + };
> +
> + </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?
> + 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
> + <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
> + </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 = &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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