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1 | <?xml version="1.0" encoding="UTF-8"?> |
2 | <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" []> | |
4 | ||
5 | <book id="index"> | |
6 | <bookinfo> | |
7 | <title>The Userspace I/O HOWTO</title> | |
8 | ||
9 | <author> | |
10 | <firstname>Hans-Jürgen</firstname> | |
11 | <surname>Koch</surname> | |
12 | <authorblurb><para>Linux developer, Linutronix</para></authorblurb> | |
13 | <affiliation> | |
14 | <orgname> | |
15 | <ulink url="http://www.linutronix.de">Linutronix</ulink> | |
16 | </orgname> | |
17 | ||
18 | <address> | |
19 | <email>hjk@linutronix.de</email> | |
20 | </address> | |
21 | </affiliation> | |
22 | </author> | |
23 | ||
17149d9f MF |
24 | <copyright> |
25 | <year>2006-2008</year> | |
26 | <holder>Hans-Jürgen Koch.</holder> | |
27 | </copyright> | |
28 | ||
29 | <legalnotice> | |
30 | <para> | |
31 | This documentation is Free Software licensed under the terms of the | |
32 | GPL version 2. | |
33 | </para> | |
34 | </legalnotice> | |
35 | ||
e3e0a28b HK |
36 | <pubdate>2006-12-11</pubdate> |
37 | ||
38 | <abstract> | |
39 | <para>This HOWTO describes concept and usage of Linux kernel's | |
40 | Userspace I/O system.</para> | |
41 | </abstract> | |
42 | ||
43 | <revhistory> | |
328a14e7 HK |
44 | <revision> |
45 | <revnumber>0.5</revnumber> | |
46 | <date>2008-05-22</date> | |
47 | <authorinitials>hjk</authorinitials> | |
48 | <revremark>Added description of write() function.</revremark> | |
49 | </revision> | |
ed423c24 HK |
50 | <revision> |
51 | <revnumber>0.4</revnumber> | |
52 | <date>2007-11-26</date> | |
53 | <authorinitials>hjk</authorinitials> | |
54 | <revremark>Removed section about uio_dummy.</revremark> | |
55 | </revision> | |
e3e0a28b HK |
56 | <revision> |
57 | <revnumber>0.3</revnumber> | |
58 | <date>2007-04-29</date> | |
59 | <authorinitials>hjk</authorinitials> | |
60 | <revremark>Added section about userspace drivers.</revremark> | |
61 | </revision> | |
62 | <revision> | |
63 | <revnumber>0.2</revnumber> | |
64 | <date>2007-02-13</date> | |
65 | <authorinitials>hjk</authorinitials> | |
66 | <revremark>Update after multiple mappings were added.</revremark> | |
67 | </revision> | |
68 | <revision> | |
69 | <revnumber>0.1</revnumber> | |
70 | <date>2006-12-11</date> | |
71 | <authorinitials>hjk</authorinitials> | |
72 | <revremark>First draft.</revremark> | |
73 | </revision> | |
74 | </revhistory> | |
75 | </bookinfo> | |
76 | ||
77 | <chapter id="aboutthisdoc"> | |
4f7e5309 | 78 | <?dbhtml filename="aboutthis.html"?> |
e3e0a28b HK |
79 | <title>About this document</title> |
80 | ||
e3e0a28b HK |
81 | <sect1 id="translations"> |
82 | <?dbhtml filename="translations.html"?> | |
83 | <title>Translations</title> | |
84 | ||
85 | <para>If you know of any translations for this document, or you are | |
86 | interested in translating it, please email me | |
87 | <email>hjk@linutronix.de</email>. | |
88 | </para> | |
89 | </sect1> | |
90 | ||
91 | <sect1 id="preface"> | |
92 | <title>Preface</title> | |
93 | <para> | |
94 | For many types of devices, creating a Linux kernel driver is | |
95 | overkill. All that is really needed is some way to handle an | |
96 | interrupt and provide access to the memory space of the | |
97 | device. The logic of controlling the device does not | |
98 | necessarily have to be within the kernel, as the device does | |
99 | not need to take advantage of any of other resources that the | |
100 | kernel provides. One such common class of devices that are | |
101 | like this are for industrial I/O cards. | |
102 | </para> | |
103 | <para> | |
104 | To address this situation, the userspace I/O system (UIO) was | |
105 | designed. For typical industrial I/O cards, only a very small | |
106 | kernel module is needed. The main part of the driver will run in | |
107 | user space. This simplifies development and reduces the risk of | |
108 | serious bugs within a kernel module. | |
109 | </para> | |
ed423c24 HK |
110 | <para> |
111 | Please note that UIO is not an universal driver interface. Devices | |
112 | that are already handled well by other kernel subsystems (like | |
113 | networking or serial or USB) are no candidates for an UIO driver. | |
114 | Hardware that is ideally suited for an UIO driver fulfills all of | |
115 | the following: | |
116 | </para> | |
117 | <itemizedlist> | |
118 | <listitem> | |
119 | <para>The device has memory that can be mapped. The device can be | |
120 | controlled completely by writing to this memory.</para> | |
121 | </listitem> | |
122 | <listitem> | |
123 | <para>The device usually generates interrupts.</para> | |
124 | </listitem> | |
125 | <listitem> | |
126 | <para>The device does not fit into one of the standard kernel | |
127 | subsystems.</para> | |
128 | </listitem> | |
129 | </itemizedlist> | |
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130 | </sect1> |
131 | ||
132 | <sect1 id="thanks"> | |
133 | <title>Acknowledgments</title> | |
134 | <para>I'd like to thank Thomas Gleixner and Benedikt Spranger of | |
135 | Linutronix, who have not only written most of the UIO code, but also | |
136 | helped greatly writing this HOWTO by giving me all kinds of background | |
137 | information.</para> | |
138 | </sect1> | |
139 | ||
140 | <sect1 id="feedback"> | |
141 | <title>Feedback</title> | |
142 | <para>Find something wrong with this document? (Or perhaps something | |
143 | right?) I would love to hear from you. Please email me at | |
144 | <email>hjk@linutronix.de</email>.</para> | |
145 | </sect1> | |
146 | </chapter> | |
147 | ||
148 | <chapter id="about"> | |
149 | <?dbhtml filename="about.html"?> | |
150 | <title>About UIO</title> | |
151 | ||
152 | <para>If you use UIO for your card's driver, here's what you get:</para> | |
153 | ||
154 | <itemizedlist> | |
155 | <listitem> | |
156 | <para>only one small kernel module to write and maintain.</para> | |
157 | </listitem> | |
158 | <listitem> | |
159 | <para>develop the main part of your driver in user space, | |
160 | with all the tools and libraries you're used to.</para> | |
161 | </listitem> | |
162 | <listitem> | |
163 | <para>bugs in your driver won't crash the kernel.</para> | |
164 | </listitem> | |
165 | <listitem> | |
166 | <para>updates of your driver can take place without recompiling | |
167 | the kernel.</para> | |
168 | </listitem> | |
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169 | </itemizedlist> |
170 | ||
171 | <sect1 id="how_uio_works"> | |
172 | <title>How UIO works</title> | |
173 | <para> | |
174 | Each UIO device is accessed through a device file and several | |
175 | sysfs attribute files. The device file will be called | |
176 | <filename>/dev/uio0</filename> for the first device, and | |
177 | <filename>/dev/uio1</filename>, <filename>/dev/uio2</filename> | |
178 | and so on for subsequent devices. | |
179 | </para> | |
180 | ||
181 | <para><filename>/dev/uioX</filename> is used to access the | |
182 | address space of the card. Just use | |
183 | <function>mmap()</function> to access registers or RAM | |
184 | locations of your card. | |
185 | </para> | |
186 | ||
187 | <para> | |
188 | Interrupts are handled by reading from | |
189 | <filename>/dev/uioX</filename>. A blocking | |
190 | <function>read()</function> from | |
191 | <filename>/dev/uioX</filename> will return as soon as an | |
192 | interrupt occurs. You can also use | |
193 | <function>select()</function> on | |
194 | <filename>/dev/uioX</filename> to wait for an interrupt. The | |
195 | integer value read from <filename>/dev/uioX</filename> | |
196 | represents the total interrupt count. You can use this number | |
197 | to figure out if you missed some interrupts. | |
198 | </para> | |
328a14e7 HK |
199 | <para> |
200 | For some hardware that has more than one interrupt source internally, | |
201 | but not separate IRQ mask and status registers, there might be | |
202 | situations where userspace cannot determine what the interrupt source | |
203 | was if the kernel handler disables them by writing to the chip's IRQ | |
204 | register. In such a case, the kernel has to disable the IRQ completely | |
205 | to leave the chip's register untouched. Now the userspace part can | |
206 | determine the cause of the interrupt, but it cannot re-enable | |
207 | interrupts. Another cornercase is chips where re-enabling interrupts | |
208 | is a read-modify-write operation to a combined IRQ status/acknowledge | |
209 | register. This would be racy if a new interrupt occurred | |
210 | simultaneously. | |
211 | </para> | |
212 | <para> | |
213 | To address these problems, UIO also implements a write() function. It | |
214 | is normally not used and can be ignored for hardware that has only a | |
215 | single interrupt source or has separate IRQ mask and status registers. | |
216 | If you need it, however, a write to <filename>/dev/uioX</filename> | |
217 | will call the <function>irqcontrol()</function> function implemented | |
218 | by the driver. You have to write a 32-bit value that is usually either | |
219 | 0 or 1 to disable or enable interrupts. If a driver does not implement | |
220 | <function>irqcontrol()</function>, <function>write()</function> will | |
221 | return with <varname>-ENOSYS</varname>. | |
222 | </para> | |
e3e0a28b HK |
223 | |
224 | <para> | |
225 | To handle interrupts properly, your custom kernel module can | |
226 | provide its own interrupt handler. It will automatically be | |
227 | called by the built-in handler. | |
228 | </para> | |
229 | ||
230 | <para> | |
231 | For cards that don't generate interrupts but need to be | |
232 | polled, there is the possibility to set up a timer that | |
233 | triggers the interrupt handler at configurable time intervals. | |
ed423c24 HK |
234 | This interrupt simulation is done by calling |
235 | <function>uio_event_notify()</function> | |
236 | from the timer's event handler. | |
e3e0a28b HK |
237 | </para> |
238 | ||
239 | <para> | |
240 | Each driver provides attributes that are used to read or write | |
241 | variables. These attributes are accessible through sysfs | |
242 | files. A custom kernel driver module can add its own | |
243 | attributes to the device owned by the uio driver, but not added | |
244 | to the UIO device itself at this time. This might change in the | |
245 | future if it would be found to be useful. | |
246 | </para> | |
247 | ||
248 | <para> | |
249 | The following standard attributes are provided by the UIO | |
250 | framework: | |
251 | </para> | |
252 | <itemizedlist> | |
253 | <listitem> | |
254 | <para> | |
255 | <filename>name</filename>: The name of your device. It is | |
256 | recommended to use the name of your kernel module for this. | |
257 | </para> | |
258 | </listitem> | |
259 | <listitem> | |
260 | <para> | |
261 | <filename>version</filename>: A version string defined by your | |
262 | driver. This allows the user space part of your driver to deal | |
263 | with different versions of the kernel module. | |
264 | </para> | |
265 | </listitem> | |
266 | <listitem> | |
267 | <para> | |
268 | <filename>event</filename>: The total number of interrupts | |
269 | handled by the driver since the last time the device node was | |
270 | read. | |
271 | </para> | |
272 | </listitem> | |
273 | </itemizedlist> | |
274 | <para> | |
275 | These attributes appear under the | |
276 | <filename>/sys/class/uio/uioX</filename> directory. Please | |
277 | note that this directory might be a symlink, and not a real | |
278 | directory. Any userspace code that accesses it must be able | |
279 | to handle this. | |
280 | </para> | |
281 | <para> | |
282 | Each UIO device can make one or more memory regions available for | |
283 | memory mapping. This is necessary because some industrial I/O cards | |
284 | require access to more than one PCI memory region in a driver. | |
285 | </para> | |
286 | <para> | |
287 | Each mapping has its own directory in sysfs, the first mapping | |
288 | appears as <filename>/sys/class/uio/uioX/maps/map0/</filename>. | |
289 | Subsequent mappings create directories <filename>map1/</filename>, | |
290 | <filename>map2/</filename>, and so on. These directories will only | |
291 | appear if the size of the mapping is not 0. | |
292 | </para> | |
293 | <para> | |
294 | Each <filename>mapX/</filename> directory contains two read-only files | |
295 | that show start address and size of the memory: | |
296 | </para> | |
297 | <itemizedlist> | |
298 | <listitem> | |
299 | <para> | |
300 | <filename>addr</filename>: The address of memory that can be mapped. | |
301 | </para> | |
302 | </listitem> | |
303 | <listitem> | |
304 | <para> | |
305 | <filename>size</filename>: The size, in bytes, of the memory | |
306 | pointed to by addr. | |
307 | </para> | |
308 | </listitem> | |
309 | </itemizedlist> | |
310 | ||
311 | <para> | |
312 | From userspace, the different mappings are distinguished by adjusting | |
313 | the <varname>offset</varname> parameter of the | |
314 | <function>mmap()</function> call. To map the memory of mapping N, you | |
315 | have to use N times the page size as your offset: | |
316 | </para> | |
317 | <programlisting format="linespecific"> | |
318 | offset = N * getpagesize(); | |
319 | </programlisting> | |
320 | ||
321 | </sect1> | |
322 | </chapter> | |
323 | ||
e3e0a28b HK |
324 | <chapter id="custom_kernel_module" xreflabel="Writing your own kernel module"> |
325 | <?dbhtml filename="custom_kernel_module.html"?> | |
326 | <title>Writing your own kernel module</title> | |
327 | <para> | |
ed423c24 | 328 | Please have a look at <filename>uio_cif.c</filename> as an |
e3e0a28b HK |
329 | example. The following paragraphs explain the different |
330 | sections of this file. | |
331 | </para> | |
332 | ||
333 | <sect1 id="uio_info"> | |
334 | <title>struct uio_info</title> | |
335 | <para> | |
336 | This structure tells the framework the details of your driver, | |
337 | Some of the members are required, others are optional. | |
338 | </para> | |
339 | ||
340 | <itemizedlist> | |
341 | <listitem><para> | |
342 | <varname>char *name</varname>: Required. The name of your driver as | |
343 | it will appear in sysfs. I recommend using the name of your module for this. | |
344 | </para></listitem> | |
345 | ||
346 | <listitem><para> | |
347 | <varname>char *version</varname>: Required. This string appears in | |
348 | <filename>/sys/class/uio/uioX/version</filename>. | |
349 | </para></listitem> | |
350 | ||
351 | <listitem><para> | |
352 | <varname>struct uio_mem mem[ MAX_UIO_MAPS ]</varname>: Required if you | |
353 | have memory that can be mapped with <function>mmap()</function>. For each | |
354 | mapping you need to fill one of the <varname>uio_mem</varname> structures. | |
355 | See the description below for details. | |
356 | </para></listitem> | |
357 | ||
358 | <listitem><para> | |
359 | <varname>long irq</varname>: Required. If your hardware generates an | |
360 | interrupt, it's your modules task to determine the irq number during | |
361 | initialization. If you don't have a hardware generated interrupt but | |
362 | want to trigger the interrupt handler in some other way, set | |
ed423c24 HK |
363 | <varname>irq</varname> to <varname>UIO_IRQ_CUSTOM</varname>. |
364 | If you had no interrupt at all, you could set | |
e3e0a28b HK |
365 | <varname>irq</varname> to <varname>UIO_IRQ_NONE</varname>, though this |
366 | rarely makes sense. | |
367 | </para></listitem> | |
368 | ||
369 | <listitem><para> | |
370 | <varname>unsigned long irq_flags</varname>: Required if you've set | |
371 | <varname>irq</varname> to a hardware interrupt number. The flags given | |
372 | here will be used in the call to <function>request_irq()</function>. | |
373 | </para></listitem> | |
374 | ||
375 | <listitem><para> | |
376 | <varname>int (*mmap)(struct uio_info *info, struct vm_area_struct | |
377 | *vma)</varname>: Optional. If you need a special | |
378 | <function>mmap()</function> function, you can set it here. If this | |
379 | pointer is not NULL, your <function>mmap()</function> will be called | |
380 | instead of the built-in one. | |
381 | </para></listitem> | |
382 | ||
383 | <listitem><para> | |
384 | <varname>int (*open)(struct uio_info *info, struct inode *inode) | |
385 | </varname>: Optional. You might want to have your own | |
386 | <function>open()</function>, e.g. to enable interrupts only when your | |
387 | device is actually used. | |
388 | </para></listitem> | |
389 | ||
390 | <listitem><para> | |
391 | <varname>int (*release)(struct uio_info *info, struct inode *inode) | |
392 | </varname>: Optional. If you define your own | |
393 | <function>open()</function>, you will probably also want a custom | |
394 | <function>release()</function> function. | |
395 | </para></listitem> | |
328a14e7 HK |
396 | |
397 | <listitem><para> | |
398 | <varname>int (*irqcontrol)(struct uio_info *info, s32 irq_on) | |
399 | </varname>: Optional. If you need to be able to enable or disable | |
400 | interrupts from userspace by writing to <filename>/dev/uioX</filename>, | |
401 | you can implement this function. The parameter <varname>irq_on</varname> | |
402 | will be 0 to disable interrupts and 1 to enable them. | |
403 | </para></listitem> | |
e3e0a28b HK |
404 | </itemizedlist> |
405 | ||
406 | <para> | |
407 | Usually, your device will have one or more memory regions that can be mapped | |
408 | to user space. For each region, you have to set up a | |
409 | <varname>struct uio_mem</varname> in the <varname>mem[]</varname> array. | |
410 | Here's a description of the fields of <varname>struct uio_mem</varname>: | |
411 | </para> | |
412 | ||
413 | <itemizedlist> | |
414 | <listitem><para> | |
415 | <varname>int memtype</varname>: Required if the mapping is used. Set this to | |
416 | <varname>UIO_MEM_PHYS</varname> if you you have physical memory on your | |
417 | card to be mapped. Use <varname>UIO_MEM_LOGICAL</varname> for logical | |
418 | memory (e.g. allocated with <function>kmalloc()</function>). There's also | |
419 | <varname>UIO_MEM_VIRTUAL</varname> for virtual memory. | |
420 | </para></listitem> | |
421 | ||
422 | <listitem><para> | |
423 | <varname>unsigned long addr</varname>: Required if the mapping is used. | |
424 | Fill in the address of your memory block. This address is the one that | |
425 | appears in sysfs. | |
426 | </para></listitem> | |
427 | ||
428 | <listitem><para> | |
429 | <varname>unsigned long size</varname>: Fill in the size of the | |
430 | memory block that <varname>addr</varname> points to. If <varname>size</varname> | |
431 | is zero, the mapping is considered unused. Note that you | |
432 | <emphasis>must</emphasis> initialize <varname>size</varname> with zero for | |
433 | all unused mappings. | |
434 | </para></listitem> | |
435 | ||
436 | <listitem><para> | |
437 | <varname>void *internal_addr</varname>: If you have to access this memory | |
438 | region from within your kernel module, you will want to map it internally by | |
439 | using something like <function>ioremap()</function>. Addresses | |
440 | returned by this function cannot be mapped to user space, so you must not | |
441 | store it in <varname>addr</varname>. Use <varname>internal_addr</varname> | |
442 | instead to remember such an address. | |
443 | </para></listitem> | |
444 | </itemizedlist> | |
445 | ||
446 | <para> | |
447 | Please do not touch the <varname>kobj</varname> element of | |
448 | <varname>struct uio_mem</varname>! It is used by the UIO framework | |
449 | to set up sysfs files for this mapping. Simply leave it alone. | |
450 | </para> | |
451 | </sect1> | |
452 | ||
453 | <sect1 id="adding_irq_handler"> | |
454 | <title>Adding an interrupt handler</title> | |
455 | <para> | |
456 | What you need to do in your interrupt handler depends on your | |
457 | hardware and on how you want to handle it. You should try to | |
458 | keep the amount of code in your kernel interrupt handler low. | |
459 | If your hardware requires no action that you | |
460 | <emphasis>have</emphasis> to perform after each interrupt, | |
461 | then your handler can be empty.</para> <para>If, on the other | |
462 | hand, your hardware <emphasis>needs</emphasis> some action to | |
463 | be performed after each interrupt, then you | |
464 | <emphasis>must</emphasis> do it in your kernel module. Note | |
465 | that you cannot rely on the userspace part of your driver. Your | |
466 | userspace program can terminate at any time, possibly leaving | |
467 | your hardware in a state where proper interrupt handling is | |
468 | still required. | |
469 | </para> | |
470 | ||
471 | <para> | |
472 | There might also be applications where you want to read data | |
473 | from your hardware at each interrupt and buffer it in a piece | |
474 | of kernel memory you've allocated for that purpose. With this | |
475 | technique you could avoid loss of data if your userspace | |
476 | program misses an interrupt. | |
477 | </para> | |
478 | ||
479 | <para> | |
480 | A note on shared interrupts: Your driver should support | |
481 | interrupt sharing whenever this is possible. It is possible if | |
482 | and only if your driver can detect whether your hardware has | |
483 | triggered the interrupt or not. This is usually done by looking | |
484 | at an interrupt status register. If your driver sees that the | |
485 | IRQ bit is actually set, it will perform its actions, and the | |
486 | handler returns IRQ_HANDLED. If the driver detects that it was | |
487 | not your hardware that caused the interrupt, it will do nothing | |
488 | and return IRQ_NONE, allowing the kernel to call the next | |
489 | possible interrupt handler. | |
490 | </para> | |
491 | ||
492 | <para> | |
493 | If you decide not to support shared interrupts, your card | |
494 | won't work in computers with no free interrupts. As this | |
495 | frequently happens on the PC platform, you can save yourself a | |
496 | lot of trouble by supporting interrupt sharing. | |
497 | </para> | |
498 | </sect1> | |
499 | ||
500 | </chapter> | |
501 | ||
502 | <chapter id="userspace_driver" xreflabel="Writing a driver in user space"> | |
503 | <?dbhtml filename="userspace_driver.html"?> | |
504 | <title>Writing a driver in userspace</title> | |
505 | <para> | |
506 | Once you have a working kernel module for your hardware, you can | |
507 | write the userspace part of your driver. You don't need any special | |
508 | libraries, your driver can be written in any reasonable language, | |
509 | you can use floating point numbers and so on. In short, you can | |
510 | use all the tools and libraries you'd normally use for writing a | |
511 | userspace application. | |
512 | </para> | |
513 | ||
514 | <sect1 id="getting_uio_information"> | |
515 | <title>Getting information about your UIO device</title> | |
516 | <para> | |
517 | Information about all UIO devices is available in sysfs. The | |
518 | first thing you should do in your driver is check | |
519 | <varname>name</varname> and <varname>version</varname> to | |
520 | make sure your talking to the right device and that its kernel | |
521 | driver has the version you expect. | |
522 | </para> | |
523 | <para> | |
524 | You should also make sure that the memory mapping you need | |
525 | exists and has the size you expect. | |
526 | </para> | |
527 | <para> | |
528 | There is a tool called <varname>lsuio</varname> that lists | |
529 | UIO devices and their attributes. It is available here: | |
530 | </para> | |
531 | <para> | |
532 | <ulink url="http://www.osadl.org/projects/downloads/UIO/user/"> | |
533 | http://www.osadl.org/projects/downloads/UIO/user/</ulink> | |
534 | </para> | |
535 | <para> | |
536 | With <varname>lsuio</varname> you can quickly check if your | |
537 | kernel module is loaded and which attributes it exports. | |
538 | Have a look at the manpage for details. | |
539 | </para> | |
540 | <para> | |
541 | The source code of <varname>lsuio</varname> can serve as an | |
542 | example for getting information about an UIO device. | |
543 | The file <filename>uio_helper.c</filename> contains a lot of | |
544 | functions you could use in your userspace driver code. | |
545 | </para> | |
546 | </sect1> | |
547 | ||
548 | <sect1 id="mmap_device_memory"> | |
549 | <title>mmap() device memory</title> | |
550 | <para> | |
551 | After you made sure you've got the right device with the | |
552 | memory mappings you need, all you have to do is to call | |
553 | <function>mmap()</function> to map the device's memory | |
554 | to userspace. | |
555 | </para> | |
556 | <para> | |
557 | The parameter <varname>offset</varname> of the | |
558 | <function>mmap()</function> call has a special meaning | |
559 | for UIO devices: It is used to select which mapping of | |
560 | your device you want to map. To map the memory of | |
561 | mapping N, you have to use N times the page size as | |
562 | your offset: | |
563 | </para> | |
564 | <programlisting format="linespecific"> | |
565 | offset = N * getpagesize(); | |
566 | </programlisting> | |
567 | <para> | |
568 | N starts from zero, so if you've got only one memory | |
569 | range to map, set <varname>offset = 0</varname>. | |
570 | A drawback of this technique is that memory is always | |
571 | mapped beginning with its start address. | |
572 | </para> | |
573 | </sect1> | |
574 | ||
575 | <sect1 id="wait_for_interrupts"> | |
576 | <title>Waiting for interrupts</title> | |
577 | <para> | |
578 | After you successfully mapped your devices memory, you | |
579 | can access it like an ordinary array. Usually, you will | |
580 | perform some initialization. After that, your hardware | |
581 | starts working and will generate an interrupt as soon | |
582 | as it's finished, has some data available, or needs your | |
583 | attention because an error occured. | |
584 | </para> | |
585 | <para> | |
586 | <filename>/dev/uioX</filename> is a read-only file. A | |
587 | <function>read()</function> will always block until an | |
588 | interrupt occurs. There is only one legal value for the | |
589 | <varname>count</varname> parameter of | |
590 | <function>read()</function>, and that is the size of a | |
591 | signed 32 bit integer (4). Any other value for | |
592 | <varname>count</varname> causes <function>read()</function> | |
593 | to fail. The signed 32 bit integer read is the interrupt | |
594 | count of your device. If the value is one more than the value | |
595 | you read the last time, everything is OK. If the difference | |
596 | is greater than one, you missed interrupts. | |
597 | </para> | |
598 | <para> | |
599 | You can also use <function>select()</function> on | |
600 | <filename>/dev/uioX</filename>. | |
601 | </para> | |
602 | </sect1> | |
603 | ||
604 | </chapter> | |
605 | ||
606 | <appendix id="app1"> | |
607 | <title>Further information</title> | |
608 | <itemizedlist> | |
609 | <listitem><para> | |
610 | <ulink url="http://www.osadl.org"> | |
611 | OSADL homepage.</ulink> | |
612 | </para></listitem> | |
613 | <listitem><para> | |
614 | <ulink url="http://www.linutronix.de"> | |
615 | Linutronix homepage.</ulink> | |
616 | </para></listitem> | |
617 | </itemizedlist> | |
618 | </appendix> | |
619 | ||
620 | </book> |