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@xxxxxxxxx> --- Documentation/DocBook/Makefile | 2 +- Documentation/DocBook/iio.tmpl | 701 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 702 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..0d696e2 --- /dev/null +++ b/Documentation/DocBook/iio.tmpl @@ -0,0 +1,701 @@ +<?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 very much directed at low sample rate sensors used in + applications such as fan speed control and 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. It is also a common + use case to have combo 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 faster 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 sensor type there are 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/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. + </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 + </listitem> + <listitem><emphasis>storagebits</emphasis> is the space (after padding) + that it occupies in the buffer. + </listitem> + <listitem> + <emphasis>shift</emphasis> if specified, is the shift that needs + to be a applied prior to masking out unused bits + </listitem> + <listitem> + <emphasis>repeat</emphasis>, specifies the number of real/storage bits + 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 + 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. + </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 */ + } + </programlisting> + </para> + <para> + Here <emphasis> scan_index </emphasis> defines the relative order in which + the enabled channels are placed inside the buffer, a channel with a lower + scan_index will be placed before a channel with a higher index. Each + channels needs to have a unique scan_index. + </para> + <para> + It is important to realize that the scan_index does not define the + absolution 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. + 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 + trigger may initialize 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/triggerX</filename>, + this file is created once an IIO triggered is registered with + the IIO core and corresponds to trigger with index X. 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. + </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. + </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]; + + /* 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); + + iio_trigger_notify_done(trigger); + } + + /* 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 + 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 usually does little + processing (as 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 +--> -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
