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Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.

1	<?xml version="1.0" encoding="UTF-8"?>
2	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3		"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
4	
5	<book id="iioid">
6	  <bookinfo>
7	    <title>Industrial I/O driver developer's guide </title>
8	
9	    <authorgroup>
10	      <author>
11	        <firstname>Daniel</firstname>
12	        <surname>Baluta</surname>
13	        <affiliation>
14	          <address>
15	            <email>daniel.baluta@intel.com</email>
16	          </address>
17	        </affiliation>
18	      </author>
19	    </authorgroup>
20	
21	    <copyright>
22	      <year>2015</year>
23	      <holder>Intel Corporation</holder>
24	    </copyright>
25	
26	    <legalnotice>
27	      <para>
28	        This documentation is free software; you can redistribute
29	        it and/or modify it under the terms of the GNU General Public
30	        License version 2.
31	      </para>
32	    </legalnotice>
33	  </bookinfo>
34	
35	  <toc></toc>
36	
37	  <chapter id="intro">
38	    <title>Introduction</title>
39	    <para>
40	      The main purpose of the Industrial I/O subsystem (IIO) is to provide
41	      support for devices that in some sense perform either analog-to-digital
42	      conversion (ADC) or digital-to-analog conversion (DAC) or both. The aim
43	      is to fill the gap between the somewhat similar hwmon and input
44	      subsystems.
45	      Hwmon is directed at low sample rate sensors used to monitor and
46	      control the system itself, like fan speed control or temperature
47	      measurement. Input is, as its name suggests, focused on human interaction
48	      input devices (keyboard, mouse, touchscreen). In some cases there is
49	      considerable overlap between these and IIO.
50	  </para>
51	  <para>
52	    Devices that fall into this category include:
53	    <itemizedlist>
54	      <listitem>
55	        analog to digital converters (ADCs)
56	      </listitem>
57	      <listitem>
58	        accelerometers
59	      </listitem>
60	      <listitem>
61	        capacitance to digital converters (CDCs)
62	      </listitem>
63	      <listitem>
64	        digital to analog converters (DACs)
65	      </listitem>
66	      <listitem>
67	        gyroscopes
68	      </listitem>
69	      <listitem>
70	        inertial measurement units (IMUs)
71	      </listitem>
72	      <listitem>
73	        color and light sensors
74	      </listitem>
75	      <listitem>
76	        magnetometers
77	      </listitem>
78	      <listitem>
79	        pressure sensors
80	      </listitem>
81	      <listitem>
82	        proximity sensors
83	      </listitem>
84	      <listitem>
85	        temperature sensors
86	      </listitem>
87	    </itemizedlist>
88	    Usually these sensors are connected via SPI or I2C. A common use case of the
89	    sensors devices is to have combined functionality (e.g. light plus proximity
90	    sensor).
91	  </para>
92	  </chapter>
93	  <chapter id='iiosubsys'>
94	    <title>Industrial I/O core</title>
95	    <para>
96	      The Industrial I/O core offers:
97	      <itemizedlist>
98	        <listitem>
99	         a unified framework for writing drivers for many different types of
100	         embedded sensors.
101	        </listitem>
102	        <listitem>
103	         a standard interface to user space applications manipulating sensors.
104	        </listitem>
105	      </itemizedlist>
106	      The implementation can be found under <filename>
107	      drivers/iio/industrialio-*</filename>
108	  </para>
109	  <sect1 id="iiodevice">
110	    <title> Industrial I/O devices </title>
111	
112	!Finclude/linux/iio/iio.h iio_dev
113	!Fdrivers/iio/industrialio-core.c iio_device_alloc
114	!Fdrivers/iio/industrialio-core.c iio_device_free
115	!Fdrivers/iio/industrialio-core.c iio_device_register
116	!Fdrivers/iio/industrialio-core.c iio_device_unregister
117	
118	    <para>
119	      An IIO device usually corresponds to a single hardware sensor and it
120	      provides all the information needed by a driver handling a device.
121	      Let's first have a look at the functionality embedded in an IIO
122	      device then we will show how a device driver makes use of an IIO
123	      device.
124	    </para>
125	    <para>
126	        There are two ways for a user space application to interact
127	        with an IIO driver.
128	      <itemizedlist>
129	        <listitem>
130	          <filename>/sys/bus/iio/iio:deviceX/</filename>, this
131	          represents a hardware sensor and groups together the data
132	          channels of the same chip.
133	        </listitem>
134	        <listitem>
135	          <filename>/dev/iio:deviceX</filename>, character device node
136	          interface used for buffered data transfer and for events information
137	          retrieval.
138	        </listitem>
139	      </itemizedlist>
140	    </para>
141	    A typical IIO driver will register itself as an I2C or SPI driver and will
142	    create two routines, <function> probe </function> and <function> remove
143	    </function>. At <function>probe</function>:
144	    <itemizedlist>
145	    <listitem>call <function>iio_device_alloc</function>, which allocates memory
146	      for an IIO device.
147	    </listitem>
148	    <listitem> initialize IIO device fields with driver specific information
149	              (e.g. device name, device channels).
150	    </listitem>
151	    <listitem>call <function> iio_device_register</function>, this registers the
152	      device with the IIO core. After this call the device is ready to accept
153	      requests from user space applications.
154	    </listitem>
155	    </itemizedlist>
156	      At <function>remove</function>, we free the resources allocated in
157	      <function>probe</function> in reverse order:
158	    <itemizedlist>
159	    <listitem><function>iio_device_unregister</function>, unregister the device
160	      from the IIO core.
161	    </listitem>
162	    <listitem><function>iio_device_free</function>, free the memory allocated
163	      for the IIO device.
164	    </listitem>
165	    </itemizedlist>
166	
167	    <sect2 id="iioattr"> <title> IIO device sysfs interface </title>
168	      <para>
169	        Attributes are sysfs files used to expose chip info and also allowing
170	        applications to set various configuration parameters. For device
171	        with index X, attributes can be found under
172	        <filename>/sys/bus/iio/iio:deviceX/ </filename> directory.
173	        Common attributes are:
174	        <itemizedlist>
175	          <listitem><filename>name</filename>, description of the physical
176	            chip.
177	          </listitem>
178	          <listitem><filename>dev</filename>, shows the major:minor pair
179	            associated with <filename>/dev/iio:deviceX</filename> node.
180	          </listitem>
181	          <listitem><filename>sampling_frequency_available</filename>,
182	            available discrete set of sampling frequency values for
183	            device.
184	          </listitem>
185	      </itemizedlist>
186	      Available standard attributes for IIO devices are described in the
187	      <filename>Documentation/ABI/testing/sysfs-bus-iio </filename> file
188	      in the Linux kernel sources.
189	      </para>
190	    </sect2>
191	    <sect2 id="iiochannel"> <title> IIO device channels </title>
192	!Finclude/linux/iio/iio.h iio_chan_spec structure.
193	      <para>
194	        An IIO device channel is a representation of a data channel. An
195	        IIO device can have one or multiple channels. For example:
196	        <itemizedlist>
197	          <listitem>
198	          a thermometer sensor has one channel representing the
199	          temperature measurement.
200	          </listitem>
201	          <listitem>
202	          a light sensor with two channels indicating the measurements in
203	          the visible and infrared spectrum.
204	          </listitem>
205	          <listitem>
206	          an accelerometer can have up to 3 channels representing
207	          acceleration on X, Y and Z axes.
208	          </listitem>
209	        </itemizedlist>
210	      An IIO channel is described by the <type> struct iio_chan_spec
211	      </type>. A thermometer driver for the temperature sensor in the
212	      example above would have to describe its channel as follows:
213	      <programlisting>
214	      static const struct iio_chan_spec temp_channel[] = {
215	          {
216	              .type = IIO_TEMP,
217	              .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
218	          },
219	      };
220	
221	      </programlisting>
222	      Channel sysfs attributes exposed to userspace are specified in
223	      the form of <emphasis>bitmasks</emphasis>. Depending on their
224	      shared info, attributes can be set in one of the following masks:
225	      <itemizedlist>
226	      <listitem><emphasis>info_mask_separate</emphasis>, attributes will
227	        be specific to this channel</listitem>
228	      <listitem><emphasis>info_mask_shared_by_type</emphasis>,
229	        attributes are shared by all channels of the same type</listitem>
230	      <listitem><emphasis>info_mask_shared_by_dir</emphasis>, attributes
231	        are shared by all channels of the same direction </listitem>
232	      <listitem><emphasis>info_mask_shared_by_all</emphasis>,
233	        attributes are shared by all channels</listitem>
234	      </itemizedlist>
235	      When there are multiple data channels per channel type we have two
236	      ways to distinguish between them:
237	      <itemizedlist>
238	      <listitem> set <emphasis> .modified</emphasis> field of <type>
239	        iio_chan_spec</type> to 1. Modifiers are specified using
240	        <emphasis>.channel2</emphasis> field of the same
241	        <type>iio_chan_spec</type> structure and are used to indicate a
242	        physically unique characteristic of the channel such as its direction
243	        or spectral response. For example, a light sensor can have two channels,
244	        one for infrared light and one for both infrared and visible light.
245	      </listitem>
246	      <listitem> set <emphasis>.indexed </emphasis> field of
247	        <type>iio_chan_spec</type> to 1. In this case the channel is
248	        simply another instance with an index specified by the
249	        <emphasis>.channel</emphasis> field.
250	      </listitem>
251	      </itemizedlist>
252	      Here is how we can make use of the channel's modifiers:
253	      <programlisting>
254	      static const struct iio_chan_spec light_channels[] = {
255	          {
256	              .type = IIO_INTENSITY,
257	              .modified = 1,
258	              .channel2 = IIO_MOD_LIGHT_IR,
259	              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
260	              .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
261	          },
262	          {
263	              .type = IIO_INTENSITY,
264	              .modified = 1,
265	              .channel2 = IIO_MOD_LIGHT_BOTH,
266	              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
267	              .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
268	          },
269	          {
270	              .type = IIO_LIGHT,
271	              .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
272	              .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
273	          },
274	
275	      }
276	      </programlisting>
277	      This channel's definition will generate two separate sysfs files
278	      for raw data retrieval:
279	      <itemizedlist>
280	      <listitem>
281	      <filename>/sys/bus/iio/iio:deviceX/in_intensity_ir_raw</filename>
282	      </listitem>
283	      <listitem>
284	      <filename>/sys/bus/iio/iio:deviceX/in_intensity_both_raw</filename>
285	      </listitem>
286	      </itemizedlist>
287	      one file for processed data:
288	      <itemizedlist>
289	      <listitem>
290	      <filename>/sys/bus/iio/iio:deviceX/in_illuminance_input
291	      </filename>
292	      </listitem>
293	      </itemizedlist>
294	      and one shared sysfs file for sampling frequency:
295	      <itemizedlist>
296	      <listitem>
297	      <filename>/sys/bus/iio/iio:deviceX/sampling_frequency.
298	      </filename>
299	      </listitem>
300	      </itemizedlist>
301	      </para>
302	      <para>
303	      Here is how we can make use of the channel's indexing:
304	      <programlisting>
305	      static const struct iio_chan_spec light_channels[] = {
306	          {
307	              .type = IIO_VOLTAGE,
308	              .indexed = 1,
309	              .channel = 0,
310	              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
311	          },
312	          {
313	              .type = IIO_VOLTAGE,
314	              .indexed = 1,
315	              .channel = 1,
316	              .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
317	          },
318	      }
319	      </programlisting>
320	      This will generate two separate attributes files for raw data
321	      retrieval:
322	      <itemizedlist>
323	      <listitem>
324	        <filename>/sys/bus/iio/devices/iio:deviceX/in_voltage0_raw</filename>,
325	          representing voltage measurement for channel 0.
326	      </listitem>
327	      <listitem>
328	        <filename>/sys/bus/iio/devices/iio:deviceX/in_voltage1_raw</filename>,
329	          representing voltage measurement for channel 1.
330	      </listitem>
331	      </itemizedlist>
332	      </para>
333	    </sect2>
334	  </sect1>
335	
336	  <sect1 id="iiobuffer"> <title> Industrial I/O buffers </title>
337	!Finclude/linux/iio/buffer.h iio_buffer
338	!Edrivers/iio/industrialio-buffer.c
339	
340	    <para>
341	    The Industrial I/O core offers a way for continuous data capture
342	    based on a trigger source. Multiple data channels can be read at once
343	    from <filename>/dev/iio:deviceX</filename> character device node,
344	    thus reducing the CPU load.
345	    </para>
346	
347	    <sect2 id="iiobuffersysfs">
348	    <title>IIO buffer sysfs interface </title>
349	    <para>
350	      An IIO buffer has an associated attributes directory under <filename>
351	      /sys/bus/iio/iio:deviceX/buffer/</filename>. Here are the existing
352	      attributes:
353	      <itemizedlist>
354	      <listitem>
355	      <emphasis>length</emphasis>, the total number of data samples
356	      (capacity) that can be stored by the buffer.
357	      </listitem>
358	      <listitem>
359	        <emphasis>enable</emphasis>, activate buffer capture.
360	      </listitem>
361	      </itemizedlist>
362	
363	    </para>
364	    </sect2>
365	    <sect2 id="iiobuffersetup"> <title> IIO buffer setup </title>
366	      <para>The meta information associated with a channel reading
367	        placed in a buffer is called a <emphasis> scan element </emphasis>.
368	        The important bits configuring scan elements are exposed to
369	        userspace applications via the <filename>
370	        /sys/bus/iio/iio:deviceX/scan_elements/</filename> directory. This
371	        file contains attributes of the following form:
372	      <itemizedlist>
373	      <listitem><emphasis>enable</emphasis>, used for enabling a channel.
374	        If and only if its attribute is non zero, then a triggered capture
375	        will contain data samples for this channel.
376	      </listitem>
377	      <listitem><emphasis>type</emphasis>, description of the scan element
378	        data storage within the buffer and hence the form in which it is
379	        read from user space. Format is <emphasis>
380	        [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] </emphasis>.
381	        <itemizedlist>
382	        <listitem> <emphasis>be</emphasis> or <emphasis>le</emphasis>, specifies
383	          big or little endian.
384	        </listitem>
385	        <listitem>
386	        <emphasis>s </emphasis>or <emphasis>u</emphasis>, specifies if
387	          signed (2's complement) or unsigned.
388	        </listitem>
389	        <listitem><emphasis>bits</emphasis>, is the number of valid data
390	          bits.
391	        </listitem>
392	        <listitem><emphasis>storagebits</emphasis>, is the number of bits
393	          (after padding) that it occupies in the buffer.
394	        </listitem>
395	        <listitem>
396	        <emphasis>shift</emphasis>, if specified, is the shift that needs
397	          to be applied prior to masking out unused bits.
398	        </listitem>
399	        <listitem>
400	        <emphasis>repeat</emphasis>, specifies the number of bits/storagebits
401	        repetitions. When the repeat element is 0 or 1, then the repeat
402	        value is omitted.
403	        </listitem>
404	        </itemizedlist>
405	      </listitem>
406	      </itemizedlist>
407	      For example, a driver for a 3-axis accelerometer with 12 bit
408	      resolution where data is stored in two 8-bits registers as
409	      follows:
410	      <programlisting>
411	        7   6   5   4   3   2   1   0
412	      +---+---+---+---+---+---+---+---+
413	      |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
414	      +---+---+---+---+---+---+---+---+
415	
416	        7   6   5   4   3   2   1   0
417	      +---+---+---+---+---+---+---+---+
418	      |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
419	      +---+---+---+---+---+---+---+---+
420	      </programlisting>
421	
422	      will have the following scan element type for each axis:
423	      <programlisting>
424	      $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
425	      le:s12/16>>4
426	      </programlisting>
427	      A user space application will interpret data samples read from the
428	      buffer as two byte little endian signed data, that needs a 4 bits
429	      right shift before masking out the 12 valid bits of data.
430	    </para>
431	    <para>
432	      For implementing buffer support a driver should initialize the following
433	      fields in <type>iio_chan_spec</type> definition:
434	      <programlisting>
435	          struct iio_chan_spec {
436	              /* other members */
437	              int scan_index
438	              struct {
439	                  char sign;
440	                  u8 realbits;
441	                  u8 storagebits;
442	                  u8 shift;
443	                  u8 repeat;
444	                  enum iio_endian endianness;
445	              } scan_type;
446	          };
447	      </programlisting>
448	      The driver implementing the accelerometer described above will
449	      have the following channel definition:
450	      <programlisting>
451	      struct struct iio_chan_spec accel_channels[] = {
452	          {
453	            .type = IIO_ACCEL,
454	            .modified = 1,
455	            .channel2 = IIO_MOD_X,
456	            /* other stuff here */
457	            .scan_index = 0,
458	            .scan_type = {
459	              .sign = 's',
460	              .realbits = 12,
461	              .storagebits = 16,
462	              .shift = 4,
463	              .endianness = IIO_LE,
464	            },
465	        }
466	        /* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
467	         * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
468	         */
469	    }
470	    </programlisting>
471	    </para>
472	    <para>
473	    Here <emphasis> scan_index </emphasis> defines the order in which
474	    the enabled channels are placed inside the buffer. Channels with a lower
475	    scan_index will be placed before channels with a higher index. Each
476	    channel needs to have a unique scan_index.
477	    </para>
478	    <para>
479	    Setting scan_index to -1 can be used to indicate that the specific
480	    channel does not support buffered capture. In this case no entries will
481	    be created for the channel in the scan_elements directory.
482	    </para>
483	    </sect2>
484	  </sect1>
485	
486	  <sect1 id="iiotrigger"> <title> Industrial I/O triggers  </title>
487	!Finclude/linux/iio/trigger.h iio_trigger
488	!Edrivers/iio/industrialio-trigger.c
489	    <para>
490	      In many situations it is useful for a driver to be able to
491	      capture data based on some external event (trigger) as opposed
492	      to periodically polling for data. An IIO trigger can be provided
493	      by a device driver that also has an IIO device based on hardware
494	      generated events (e.g. data ready or threshold exceeded) or
495	      provided by a separate driver from an independent interrupt
496	      source (e.g. GPIO line connected to some external system, timer
497	      interrupt or user space writing a specific file in sysfs). A
498	      trigger may initiate data capture for a number of sensors and
499	      also it may be completely unrelated to the sensor itself.
500	    </para>
501	
502	    <sect2 id="iiotrigsysfs"> <title> IIO trigger sysfs interface </title>
503	      There are two locations in sysfs related to triggers:
504	      <itemizedlist>
505	        <listitem><filename>/sys/bus/iio/devices/triggerY</filename>,
506	          this file is created once an IIO trigger is registered with
507	          the IIO core and corresponds to trigger with index Y. Because
508	          triggers can be very different depending on type there are few
509	          standard attributes that we can describe here:
510	          <itemizedlist>
511	            <listitem>
512	              <emphasis>name</emphasis>, trigger name that can be later
513	                used for association with a device.
514	            </listitem>
515	            <listitem>
516	            <emphasis>sampling_frequency</emphasis>, some timer based
517	              triggers use this attribute to specify the frequency for
518	              trigger calls.
519	            </listitem>
520	          </itemizedlist>
521	        </listitem>
522	        <listitem>
523	          <filename>/sys/bus/iio/devices/iio:deviceX/trigger/</filename>, this
524	          directory is created once the device supports a triggered
525	          buffer. We can associate a trigger with our device by writing
526	          the trigger's name in the <filename>current_trigger</filename> file.
527	        </listitem>
528	      </itemizedlist>
529	    </sect2>
530	
531	    <sect2 id="iiotrigattr"> <title> IIO trigger setup</title>
532	
533	    <para>
534	      Let's see a simple example of how to setup a trigger to be used
535	      by a driver.
536	
537	      <programlisting>
538	      struct iio_trigger_ops trigger_ops = {
539	          .set_trigger_state = sample_trigger_state,
540	          .validate_device = sample_validate_device,
541	      }
542	
543	      struct iio_trigger *trig;
544	
545	      /* first, allocate memory for our trigger */
546	      trig = iio_trigger_alloc(dev, "trig-%s-%d", name, idx);
547	
548	      /* setup trigger operations field */
549	      trig->ops = &amp;trigger_ops;
550	
551	      /* now register the trigger with the IIO core */
552	      iio_trigger_register(trig);
553	      </programlisting>
554	    </para>
555	    </sect2>
556	
557	    <sect2 id="iiotrigsetup"> <title> IIO trigger ops</title>
558	!Finclude/linux/iio/trigger.h iio_trigger_ops
559	     <para>
560	        Notice that a trigger has a set of operations attached:
561	        <itemizedlist>
562	        <listitem>
563	          <function>set_trigger_state</function>, switch the trigger on/off
564	          on demand.
565	        </listitem>
566	        <listitem>
567	          <function>validate_device</function>, function to validate the
568	          device when the current trigger gets changed.
569	        </listitem>
570	        </itemizedlist>
571	      </para>
572	    </sect2>
573	  </sect1>
574	  <sect1 id="iiotriggered_buffer">
575	    <title> Industrial I/O triggered buffers </title>
576	    <para>
577	    Now that we know what buffers and triggers are let's see how they
578	    work together.
579	    </para>
580	    <sect2 id="iiotrigbufsetup"> <title> IIO triggered buffer setup</title>
581	!Edrivers/iio/buffer/industrialio-triggered-buffer.c
582	!Finclude/linux/iio/iio.h iio_buffer_setup_ops
583	
584	
585	    <para>
586	    A typical triggered buffer setup looks like this:
587	    <programlisting>
588	    const struct iio_buffer_setup_ops sensor_buffer_setup_ops = {
589	      .preenable    = sensor_buffer_preenable,
590	      .postenable   = sensor_buffer_postenable,
591	      .postdisable  = sensor_buffer_postdisable,
592	      .predisable   = sensor_buffer_predisable,
593	    };
594	
595	    irqreturn_t sensor_iio_pollfunc(int irq, void *p)
596	    {
597	        pf->timestamp = iio_get_time_ns((struct indio_dev *)p);
598	        return IRQ_WAKE_THREAD;
599	    }
600	
601	    irqreturn_t sensor_trigger_handler(int irq, void *p)
602	    {
603	        u16 buf[8];
604	        int i = 0;
605	
606	        /* read data for each active channel */
607	        for_each_set_bit(bit, active_scan_mask, masklength)
608	            buf[i++] = sensor_get_data(bit)
609	
610	        iio_push_to_buffers_with_timestamp(indio_dev, buf, timestamp);
611	
612	        iio_trigger_notify_done(trigger);
613	        return IRQ_HANDLED;
614	    }
615	
616	    /* setup triggered buffer, usually in probe function */
617	    iio_triggered_buffer_setup(indio_dev, sensor_iio_polfunc,
618	                               sensor_trigger_handler,
619	                               sensor_buffer_setup_ops);
620	    </programlisting>
621	    </para>
622	    The important things to notice here are:
623	    <itemizedlist>
624	    <listitem><function> iio_buffer_setup_ops</function>, the buffer setup
625	    functions to be called at predefined points in the buffer configuration
626	    sequence (e.g. before enable, after disable). If not specified, the
627	    IIO core uses the default <type>iio_triggered_buffer_setup_ops</type>.
628	    </listitem>
629	    <listitem><function>sensor_iio_pollfunc</function>, the function that
630	    will be used as top half of poll function. It should do as little
631	    processing as possible, because it runs in interrupt context. The most
632	    common operation is recording of the current timestamp and for this reason
633	    one can use the IIO core defined <function>iio_pollfunc_store_time
634	    </function> function.
635	    </listitem>
636	    <listitem><function>sensor_trigger_handler</function>, the function that
637	    will be used as bottom half of the poll function. This runs in the
638	    context of a kernel thread and all the processing takes place here.
639	    It usually reads data from the device and stores it in the internal
640	    buffer together with the timestamp recorded in the top half.
641	    </listitem>
642	    </itemizedlist>
643	    </sect2>
644	  </sect1>
645	  </chapter>
646	  <chapter id='iioresources'>
647	    <title> Resources </title>
648	      IIO core may change during time so the best documentation to read is the
649	      source code. There are several locations where you should look:
650	      <itemizedlist>
651	        <listitem>
652	          <filename>drivers/iio/</filename>, contains the IIO core plus
653	          and directories for each sensor type (e.g. accel, magnetometer,
654	          etc.)
655	        </listitem>
656	        <listitem>
657	          <filename>include/linux/iio/</filename>, contains the header
658	          files, nice to read for the internal kernel interfaces.
659	        </listitem>
660	        <listitem>
661	        <filename>include/uapi/linux/iio/</filename>, contains files to be
662	          used by user space applications.
663	        </listitem>
664	        <listitem>
665	         <filename>tools/iio/</filename>, contains tools for rapidly
666	          testing buffers, events and device creation.
667	        </listitem>
668	        <listitem>
669	          <filename>drivers/staging/iio/</filename>, contains code for some
670	          drivers or experimental features that are not yet mature enough
671	          to be moved out.
672	        </listitem>
673	      </itemizedlist>
674	    <para>
675	    Besides the code, there are some good online documentation sources:
676	    <itemizedlist>
677	    <listitem>
678	      <ulink url="http://marc.info/?l=linux-iio"> Industrial I/O mailing
679	      list </ulink>
680	    </listitem>
681	    <listitem>
682	      <ulink url="http://wiki.analog.com/software/linux/docs/iio/iio">
683	      Analog Device IIO wiki page </ulink>
684	    </listitem>
685	    <listitem>
686	      <ulink url="https://fosdem.org/2015/schedule/event/iiosdr/">
687	      Using the Linux IIO framework for SDR, Lars-Peter Clausen's
688	      presentation at FOSDEM </ulink>
689	    </listitem>
690	    </itemizedlist>
691	    </para>
692	  </chapter>
693	</book>
694	
695	<!--
696	vim: softtabstop=2:shiftwidth=2:expandtab:textwidth=72
697	-->
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