summaryrefslogtreecommitdiffstats
path: root/Documentation/DocBook/uio-howto.tmpl
blob: c119484258b8428d59b07b3046a5c5d48def9519 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" []>

<book id="index">
<bookinfo>
<title>The Userspace I/O HOWTO</title>

<author>
      <firstname>Hans-Jürgen</firstname>
      <surname>Koch</surname>
      <authorblurb><para>Linux developer, Linutronix</para></authorblurb>
	<affiliation>
	<orgname>
		<ulink url="http://www.linutronix.de">Linutronix</ulink>
	</orgname>

	<address>
	   <email>hjk@linutronix.de</email>
	</address>
    </affiliation>
</author>

<pubdate>2006-12-11</pubdate>

<abstract>
	<para>This HOWTO describes concept and usage of Linux kernel's
		Userspace I/O system.</para>
</abstract>

<revhistory>
	<revision>
	<revnumber>0.3</revnumber>
	<date>2007-04-29</date>
	<authorinitials>hjk</authorinitials>
	<revremark>Added section about userspace drivers.</revremark>
	</revision>
	<revision>
	<revnumber>0.2</revnumber>
	<date>2007-02-13</date>
	<authorinitials>hjk</authorinitials>
	<revremark>Update after multiple mappings were added.</revremark>
	</revision>
	<revision>
	<revnumber>0.1</revnumber>
	<date>2006-12-11</date>
	<authorinitials>hjk</authorinitials>
	<revremark>First draft.</revremark>
	</revision>
</revhistory>
</bookinfo>

<chapter id="aboutthisdoc">
<?dbhtml filename="about.html"?>
<title>About this document</title>

<sect1 id="copyright">
<?dbhtml filename="copyright.html"?>
<title>Copyright and License</title>
<para>
      Copyright (c) 2006 by Hans-Jürgen Koch.</para>
<para>
This documentation is Free Software licensed under the terms of the
GPL version 2.
</para>
</sect1>

<sect1 id="translations">
<?dbhtml filename="translations.html"?>
<title>Translations</title>

<para>If you know of any translations for this document, or you are
interested in translating it, please email me
<email>hjk@linutronix.de</email>.
</para>
</sect1>

<sect1 id="preface">
<title>Preface</title>
	<para>
	For many types of devices, creating a Linux kernel driver is
	overkill.  All that is really needed is some way to handle an
	interrupt and provide access to the memory space of the
	device.  The logic of controlling the device does not
	necessarily have to be within the kernel, as the device does
	not need to take advantage of any of other resources that the
	kernel provides.  One such common class of devices that are
	like this are for industrial I/O cards.
	</para>
	<para>
	To address this situation, the userspace I/O system (UIO) was
	designed.  For typical industrial I/O cards, only a very small
	kernel module is needed. The main part of the driver will run in
	user space. This simplifies development and reduces the risk of
	serious bugs within a kernel module.
	</para>
</sect1>

<sect1 id="thanks">
<title>Acknowledgments</title>
	<para>I'd like to thank Thomas Gleixner and Benedikt Spranger of
	Linutronix, who have not only written most of the UIO code, but also
	helped greatly writing this HOWTO by giving me all kinds of background
	information.</para>
</sect1>

<sect1 id="feedback">
<title>Feedback</title>
	<para>Find something wrong with this document? (Or perhaps something
	right?) I would love to hear from you. Please email me at
	<email>hjk@linutronix.de</email>.</para>
</sect1>
</chapter>

<chapter id="about">
<?dbhtml filename="about.html"?>
<title>About UIO</title>

<para>If you use UIO for your card's driver, here's what you get:</para>

<itemizedlist>
<listitem>
	<para>only one small kernel module to write and maintain.</para>
</listitem>
<listitem>
	<para>develop the main part of your driver in user space,
	with all the tools and libraries you're used to.</para>
</listitem>
<listitem>
	<para>bugs in your driver won't crash the kernel.</para>
</listitem>
<listitem>
	<para>updates of your driver can take place without recompiling
	the kernel.</para>
</listitem>
</itemizedlist>

<sect1 id="how_uio_works">
<title>How UIO works</title>
	<para>
	Each UIO device is accessed through a device file and several
	sysfs attribute files. The device file will be called
	<filename>/dev/uio0</filename> for the first device, and
	<filename>/dev/uio1</filename>, <filename>/dev/uio2</filename>
	and so on for subsequent devices.
	</para>

	<para><filename>/dev/uioX</filename> is used to access the
	address space of the card. Just use
	<function>mmap()</function> to access registers or RAM
	locations of your card.
	</para>

	<para>
	Interrupts are handled by reading from
	<filename>/dev/uioX</filename>. A blocking
	<function>read()</function> from
	<filename>/dev/uioX</filename> will return as soon as an
	interrupt occurs. You can also use
	<function>select()</function> on
	<filename>/dev/uioX</filename> to wait for an interrupt. The
	integer value read from <filename>/dev/uioX</filename>
	represents the total interrupt count. You can use this number
	to figure out if you missed some interrupts.
	</para>

	<para>
	To handle interrupts properly, your custom kernel module can
	provide its own interrupt handler. It will automatically be
	called by the built-in handler.
	</para>

	<para>
	For cards that don't generate interrupts but need to be
	polled, there is the possibility to set up a timer that
	triggers the interrupt handler at configurable time intervals.
	See <filename>drivers/uio/uio_dummy.c</filename> for an
	example of this technique.
	</para>

	<para>
	Each driver provides attributes that are used to read or write
	variables. These attributes are accessible through sysfs
	files.  A custom kernel driver module can add its own
	attributes to the device owned by the uio driver, but not added
	to the UIO device itself at this time.  This might change in the
	future if it would be found to be useful.
	</para>

	<para>
	The following standard attributes are provided by the UIO
	framework:
	</para>
<itemizedlist>
<listitem>
	<para>
	<filename>name</filename>: The name of your device. It is
	recommended to use the name of your kernel module for this.
	</para>
</listitem>
<listitem>
	<para>
	<filename>version</filename>: A version string defined by your
	driver. This allows the user space part of your driver to deal
	with different versions of the kernel module.
	</para>
</listitem>
<listitem>
	<para>
	<filename>event</filename>: The total number of interrupts
	handled by the driver since the last time the device node was
	read.
	</para>
</listitem>
</itemizedlist>
<para>
	These attributes appear under the
	<filename>/sys/class/uio/uioX</filename> directory.  Please
	note that this directory might be a symlink, and not a real
	directory.  Any userspace code that accesses it must be able
	to handle this.
</para>
<para>
	Each UIO device can make one or more memory regions available for
	memory mapping. This is necessary because some industrial I/O cards
	require access to more than one PCI memory region in a driver.
</para>
<para>
	Each mapping has its own directory in sysfs, the first mapping
	appears as <filename>/sys/class/uio/uioX/maps/map0/</filename>.
	Subsequent mappings create directories <filename>map1/</filename>,
	<filename>map2/</filename>, and so on. These directories will only
	appear if the size of the mapping is not 0.
</para>
<para>
	Each <filename>mapX/</filename> directory contains two read-only files
	that show start address and size of the memory:
</para>
<itemizedlist>
<listitem>
	<para>
	<filename>addr</filename>: The address of memory that can be mapped.
	</para>
</listitem>
<listitem>
	<para>
	<filename>size</filename>: The size, in bytes, of the memory
	pointed to by addr.
	</para>
</listitem>
</itemizedlist>

<para>
	From userspace, the different mappings are distinguished by adjusting
	the <varname>offset</varname> parameter of the
	<function>mmap()</function> call. To map the memory of mapping N, you
	have to use N times the page size as your offset:
</para>
<programlisting format="linespecific">
offset = N * getpagesize();
</programlisting>

</sect1>
</chapter>

<chapter id="using-uio_dummy" xreflabel="Using uio_dummy">
<?dbhtml filename="using-uio_dummy.html"?>
<title>Using uio_dummy</title>
	<para>
	Well, there is no real use for uio_dummy. Its only purpose is
	to test most parts of the UIO system (everything except
	hardware interrupts), and to serve as an example for the
	kernel module that you will have to write yourself.
	</para>

<sect1 id="what_uio_dummy_does">
<title>What uio_dummy does</title>
	<para>
	The kernel module <filename>uio_dummy.ko</filename> creates a
	device that uses a timer to generate periodic interrupts. The
	interrupt handler does nothing but increment a counter. The
	driver adds two custom attributes, <varname>count</varname>
	and <varname>freq</varname>, that appear under
	<filename>/sys/devices/platform/uio_dummy/</filename>.
	</para>

	<para>
	The attribute <varname>count</varname> can be read and
	written.  The associated file
	<filename>/sys/devices/platform/uio_dummy/count</filename>
	appears as a normal text file and contains the total number of
	timer interrupts. If you look at it (e.g. using
	<function>cat</function>), you'll notice it is slowly counting
	up.
	</para>

	<para>
	The attribute <varname>freq</varname> can be read and written.
	The content of
	<filename>/sys/devices/platform/uio_dummy/freq</filename>
	represents the number of system timer ticks between two timer
	interrupts. The default value of <varname>freq</varname> is
	the value of the kernel variable <varname>HZ</varname>, which
	gives you an interval of one second. Lower values will
	increase the frequency. Try the following:
	</para>
<programlisting format="linespecific">
cd /sys/devices/platform/uio_dummy/
echo 100 > freq
</programlisting>
	<para>
	Use <function>cat count</function> to see how the interrupt
	frequency changes.
	</para>
</sect1>
</chapter>

<chapter id="custom_kernel_module" xreflabel="Writing your own kernel module">
<?dbhtml filename="custom_kernel_module.html"?>
<title>Writing your own kernel module</title>
	<para>
	Please have a look at <filename>uio_dummy.c</filename> as an
	example. The following paragraphs explain the different
	sections of this file.
	</para>

<sect1 id="uio_info">
<title>struct uio_info</title>
	<para>
	This structure tells the framework the details of your driver,
	Some of the members are required, others are optional.
	</para>

<itemizedlist>
<listitem><para>
<varname>char *name</varname>: Required. The name of your driver as
it will appear in sysfs. I recommend using the name of your module for this.
</para></listitem>

<listitem><para>
<varname>char *version</varname>: Required. This string appears in
<filename>/sys/class/uio/uioX/version</filename>.
</para></listitem>

<listitem><para>
<varname>struct uio_mem mem[ MAX_UIO_MAPS ]</varname>: Required if you
have memory that can be mapped with <function>mmap()</function>. For each
mapping you need to fill one of the <varname>uio_mem</varname> structures.
See the description below for details.
</para></listitem>

<listitem><para>
<varname>long irq</varname>: Required. If your hardware generates an
interrupt, it's your modules task to determine the irq number during
initialization. If you don't have a hardware generated interrupt but
want to trigger the interrupt handler in some other way, set
<varname>irq</varname> to <varname>UIO_IRQ_CUSTOM</varname>. The
uio_dummy module does this as it triggers the event mechanism in a timer
routine. If you had no interrupt at all, you could set
<varname>irq</varname> to <varname>UIO_IRQ_NONE</varname>, though this
rarely makes sense.
</para></listitem>

<listitem><para>
<varname>unsigned long irq_flags</varname>: Required if you've set
<varname>irq</varname> to a hardware interrupt number. The flags given
here will be used in the call to <function>request_irq()</function>.
</para></listitem>

<listitem><para>
<varname>int (*mmap)(struct uio_info *info, struct vm_area_struct
*vma)</varname>: Optional. If you need a special
<function>mmap()</function> function, you can set it here. If this
pointer is not NULL, your <function>mmap()</function> will be called
instead of the built-in one.
</para></listitem>

<listitem><para>
<varname>int (*open)(struct uio_info *info, struct inode *inode)
</varname>: Optional. You might want to have your own
<function>open()</function>, e.g. to enable interrupts only when your
device is actually used.
</para></listitem>

<listitem><para>
<varname>int (*release)(struct uio_info *info, struct inode *inode)
</varname>: Optional. If you define your own
<function>open()</function>, you will probably also want a custom
<function>release()</function> function.
</para></listitem>
</itemizedlist>

<para>
Usually, your device will have one or more memory regions that can be mapped
to user space. For each region, you have to set up a
<varname>struct uio_mem</varname> in the <varname>mem[]</varname> array.
Here's a description of the fields of <varname>struct uio_mem</varname>:
</para>

<itemizedlist>
<listitem><para>
<varname>int memtype</varname>: Required if the mapping is used. Set this to
<varname>UIO_MEM_PHYS</varname> if you you have physical memory on your
card to be mapped. Use <varname>UIO_MEM_LOGICAL</varname> for logical
memory (e.g. allocated with <function>kmalloc()</function>). There's also
<varname>UIO_MEM_VIRTUAL</varname> for virtual memory.
</para></listitem>

<listitem><para>
<varname>unsigned long addr</varname>: Required if the mapping is used.
Fill in the address of your memory block. This address is the one that
appears in sysfs.
</para></listitem>

<listitem><para>
<varname>unsigned long size</varname>: Fill in the size of the
memory block that <varname>addr</varname> points to. If <varname>size</varname>
is zero, the mapping is considered unused. Note that you
<emphasis>must</emphasis> initialize <varname>size</varname> with zero for
all unused mappings.
</para></listitem>

<listitem><para>
<varname>void *internal_addr</varname>: If you have to access this memory
region from within your kernel module, you will want to map it internally by
using something like <function>ioremap()</function>. Addresses
returned by this function cannot be mapped to user space, so you must not
store it in <varname>addr</varname>. Use <varname>internal_addr</varname>
instead to remember such an address.
</para></listitem>
</itemizedlist>

<para>
Please do not touch the <varname>kobj</varname> element of
<varname>struct uio_mem</varname>! It is used by the UIO framework
to set up sysfs files for this mapping. Simply leave it alone.
</para>
</sect1>

<sect1 id="adding_irq_handler">
<title>Adding an interrupt handler</title>
	<para>
	What you need to do in your interrupt handler depends on your
	hardware and on how you want to	handle it. You should try to
	keep the amount of code in your kernel interrupt handler low.
	If your hardware requires no action that you
	<emphasis>have</emphasis> to perform after each interrupt,
	then your handler can be empty.</para> <para>If, on the other
	hand, your hardware <emphasis>needs</emphasis> some action to
	be performed after each interrupt, then you
	<emphasis>must</emphasis> do it in your kernel module. Note
	that you cannot rely on the userspace part of your driver. Your
	userspace program can terminate at any time, possibly leaving
	your hardware in a state where proper interrupt handling is
	still required.
	</para>

	<para>
	There might also be applications where you want to read data
	from your hardware at each interrupt and buffer it in a piece
	of kernel memory you've allocated for that purpose.  With this
	technique you could avoid loss of data if your userspace
	program misses an interrupt.
	</para>

	<para>
	A note on shared interrupts: Your driver should support
	interrupt sharing whenever this is possible. It is possible if
	and only if your driver can detect whether your hardware has
	triggered the interrupt or not. This is usually done by looking
	at an interrupt status register. If your driver sees that the
	IRQ bit is actually set, it will perform its actions, and the
	handler returns IRQ_HANDLED. If the driver detects that it was
	not your hardware that caused the interrupt, it will do nothing
	and return IRQ_NONE, allowing the kernel to call the next
	possible interrupt handler.
	</para>

	<para>
	If you decide not to support shared interrupts, your card
	won't work in computers with no free interrupts. As this
	frequently happens on the PC platform, you can save yourself a
	lot of trouble by supporting interrupt sharing.
	</para>
</sect1>

</chapter>

<chapter id="userspace_driver" xreflabel="Writing a driver in user space">
<?dbhtml filename="userspace_driver.html"?>
<title>Writing a driver in userspace</title>
	<para>
	Once you have a working kernel module for your hardware, you can
	write the userspace part of your driver. You don't need any special
	libraries, your driver can be written in any reasonable language,
	you can use floating point numbers and so on. In short, you can
	use all the tools and libraries you'd normally use for writing a
	userspace application.
	</para>

<sect1 id="getting_uio_information">
<title>Getting information about your UIO device</title>
	<para>
	Information about all UIO devices is available in sysfs. The
	first thing you should do in your driver is check
	<varname>name</varname> and <varname>version</varname> to
	make sure your talking to the right device and that its kernel
	driver has the version you expect.
	</para>
	<para>
	You should also make sure that the memory mapping you need
	exists and has the size you expect.
	</para>
	<para>
	There is a tool called <varname>lsuio</varname> that lists
	UIO devices and their attributes. It is available here:
	</para>
	<para>
	<ulink url="http://www.osadl.org/projects/downloads/UIO/user/">
		http://www.osadl.org/projects/downloads/UIO/user/</ulink>
	</para>
	<para>
	With <varname>lsuio</varname> you can quickly check if your
	kernel module is loaded and which attributes it exports.
	Have a look at the manpage for details.
	</para>
	<para>
	The source code of <varname>lsuio</varname> can serve as an
	example for getting information about an UIO device.
	The file <filename>uio_helper.c</filename> contains a lot of
	functions you could use in your userspace driver code.
	</para>
</sect1>

<sect1 id="mmap_device_memory">
<title>mmap() device memory</title>
	<para>
	After you made sure you've got the right device with the
	memory mappings you need, all you have to do is to call
	<function>mmap()</function> to map the device's memory
	to userspace.
	</para>
	<para>
	The parameter <varname>offset</varname> of the
	<function>mmap()</function> call has a special meaning
	for UIO devices: It is used to select which mapping of
	your device you want to map. To map the memory of
	mapping N, you have to use N times the page size as
	your offset:
	</para>
<programlisting format="linespecific">
	offset = N * getpagesize();
</programlisting>
	<para>
	N starts from zero, so if you've got only one memory
	range to map, set <varname>offset = 0</varname>.
	A drawback of this technique is that memory is always
	mapped beginning with its start address.
	</para>
</sect1>

<sect1 id="wait_for_interrupts">
<title>Waiting for interrupts</title>
	<para>
	After you successfully mapped your devices memory, you
	can access it like an ordinary array. Usually, you will
	perform some initialization. After that, your hardware
	starts working and will generate an interrupt as soon
	as it's finished, has some data available, or needs your
	attention because an error occured.
	</para>
	<para>
	<filename>/dev/uioX</filename> is a read-only file. A
	<function>read()</function> will always block until an
	interrupt occurs. There is only one legal value for the
	<varname>count</varname> parameter of
	<function>read()</function>, and that is the size of a
	signed 32 bit integer (4). Any other value for
	<varname>count</varname> causes <function>read()</function>
	to fail. The signed 32 bit integer read is the interrupt
	count of your device. If the value is one more than the value
	you read the last time, everything is OK. If the difference
	is greater than one, you missed interrupts.
	</para>
	<para>
	You can also use <function>select()</function> on
	<filename>/dev/uioX</filename>.
	</para>
</sect1>

</chapter>

<appendix id="app1">
<title>Further information</title>
<itemizedlist>
	<listitem><para>
			<ulink url="http://www.osadl.org">
				OSADL homepage.</ulink>
		</para></listitem>
	<listitem><para>
		<ulink url="http://www.linutronix.de">
		 Linutronix homepage.</ulink>
		</para></listitem>
</itemizedlist>
</appendix>

</book>
OpenPOWER on IntegriCloud