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1 <?xml version='1.0' encoding="UTF-8"?>
2 <!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
3
4 <!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/udev-guide.xml,v 1.12 2004/05/27 16:28:54 cam Exp $ -->
5
6 <guide link="/doc/en/udev-guide.xml">
7 <title>Gentoo udev Guide</title>
8
9 <author title="Author">
10 <mail link="swift@gentoo.org">Sven Vermeulen</mail>
11 </author>
12 <author title="Contributor">
13 <mail link="g.guidi@sns.it">Gregorio Guidi</mail>
14 </author>
15
16 <abstract>
17 This document explains what udev is and how you can use udev to fit your needs.
18 </abstract>
19
20 <license/>
21
22 <version>0.9</version>
23 <date>May 27, 2004</date>
24
25 <chapter>
26 <title>What is udev?</title>
27 <section>
28 <title>The /dev Directory</title>
29 <body>
30
31 <p>
32 When Linux-users talk about the hardware on their system in the vicinity of
33 people who believe Linux is some sort of virus or brand of coffee, the use of
34 "slash dev slash foo" will return a strange look for sure. But for the fortunate
35 user (and that includes you) using <path>/dev/hda1</path> is just a fast way of
36 explaining that we are talking about the primary master IDE, first partition. Or
37 aren't we?
38 </p>
39
40 <p>
41 We all know what a device file is. Some even know why device files have special
42 numbers when we take a closer look at them when we issue <c>ls -l</c> in
43 <path>/dev</path>. But what we always take for granted is that the primary
44 master IDE disk is referred to as <path>/dev/hda</path>. You might not see it
45 this way, but this is a flaw by design.
46 </p>
47
48 <p>
49 Think about hotpluggable devices like USB, IEEE1394, hot-swappable PCI, ... What
50 is the first device? And for how long? What will the other devices be named when
51 the first one disappears? How will that affect ongoing transactions? Wouldn't it
52 be fun that a printing job is suddenly moved from your supernew laserprinter to
53 your almost-dead matrix printer because your mom decided to pull the plug of the
54 inkjet which happened to be the first printer?
55 </p>
56
57 <p>
58 Enter <e>udev</e>. The goals of the udev project are both interesting and
59 needed:
60 </p>
61
62 <ul>
63 <li>Runs in userspace</li>
64 <li>Dynamically creates/removes device files</li>
65 <li>Provides consistent naming</li>
66 <li>Provides a user-space API</li>
67 </ul>
68
69 <p>
70 To provide these features, udev is developed in three separate projects:
71 <e>namedev</e>, <e>libsysfs</e> and, of course, <e>udev</e>.
72 </p>
73
74 </body>
75 </section>
76 <section>
77 <title>namedev</title>
78 <body>
79
80 <p>
81 Namedev allows you to define the device naming separately from the udev program.
82 This allows for flexible naming policies and naming schemes developed by
83 separate entities. This device naming subsystem provides a standard interface
84 that udev can use.
85 </p>
86
87 <p>
88 Currently only a single naming scheme is provided by namedev; the one provided
89 by LANANA, used by the majority of Linux systems currently and therefore very
90 suitable for the majority of Linux users.
91 </p>
92
93 <p>
94 Namedev uses a 5-step procedure to find out the name of a given device. If the
95 device name is found in one of the given steps, that name is used. The steps
96 are:
97 </p>
98
99 <ul>
100 <li>label or serial number</li>
101 <li>bus device number</li>
102 <li>bus topology</li>
103 <li>statically given name</li>
104 <li>kernel provided name</li>
105 </ul>
106
107 <p>
108 The <e>label or serial number</e> step checks if the device has a unique
109 identifier. For instance USB devices have a unique USB serial number; SCSI
110 devices have a unique UUID. If namedev finds a match between this unique number
111 and a given configuration file, the name provided in the configuration file is
112 used.
113 </p>
114
115 <p>
116 The <e>bus device number</e> step checks the device bus number. For
117 non-hot-swappable environments this procedure is sufficient to
118 identify a hardware device. For instance PCI bus numbers rarely change in the
119 lifetime of a system. Again, if namedev finds a match between this position and
120 a given configuration file, the name provided in that configuration file is
121 used.
122 </p>
123
124 <p>
125 Likewise the <e>bus topology</e> is a rather static way of defining devices as
126 long as the user doesn't switch devices. When the position of the device matches
127 a given setting provided by the user, the accompanying name is used.
128 </p>
129
130 <p>
131 The fourth step, <e>statically given name</e>, is a simple string replacement.
132 When the kernel name (the default name) matches a given replacement string, the
133 substitute name will be used.
134 </p>
135
136 <p>
137 The final step (<e>kernel provided name</e>) is a catch-all: this one takes
138 the default name provided by the kernel. In the majority of cases this is
139 sufficient as it matches the device naming used on current Linux systems.
140 </p>
141
142 </body>
143 </section>
144 <section>
145 <title>libsysfs</title>
146 <body>
147
148 <p>
149 udev interacts with the kernel through the sysfs pseudo filesystem. The libsysfs
150 project provides a common API to access the information given by the sysfs
151 filesystem in a generic way. This allows for querying all kinds of hardware
152 without having to make assumptions on the kind of hardware.
153 </p>
154
155 </body>
156 </section>
157 <section>
158 <title>udev</title>
159 <body>
160
161 <p>
162 Every time the kernel notices an update in the device structure, it calls the
163 <path>/sbin/hotplug</path> program. Hotplug runs the applications linked in the
164 <path>/etc/hotplug.d/default</path> directory where you will also find a symlink
165 to the udev application. Hotplug directs the information given by the kernel to
166 the udev application which performs the necessary actions on the
167 <path>/dev</path> structure (creating or deleting device files).
168 </p>
169
170 </body>
171 </section>
172 </chapter>
173
174 <chapter>
175 <title>Using udev on Gentoo</title>
176 <section>
177 <title>Requirements</title>
178 <body>
179
180 <p>
181 udev is meant to be used in combination with a 2.6 kernel (like
182 <c>development-sources</c> or <c>gentoo-dev-sources</c>). If you're using such a
183 kernel then you just have to make sure that you have a recent
184 <c>sys-apps/baselayout</c> version. That's all you need.
185 </p>
186
187 <pre caption="Installing udev">
188 # <i>emerge udev</i>
189 </pre>
190
191 <p>
192 udev will install <c>hotplug-base</c> as one of it's dependencies. If you
193 intend to use hotplug to execute specific actions when you plug in your
194 favorite USB or IEEE1394 device then you should also emerge the whole bunch
195 of hotplug scripts.
196 </p>
197
198 <pre caption="Installing optional hotplug scripts">
199 # <i>emerge hotplug</i>
200 </pre>
201
202 <p>
203 Kernelwise, if you're using the default set by <c>genkernel</c> then you're all
204 set. Otherwise be sure to activate the following options:
205 </p>
206
207 <pre caption="Required kernel options">
208 General setup ---&gt;
209 [*] Support for hot-pluggable devices
210
211 File systems ---&gt;
212 Pseudo filesystems ---&gt;
213 [*] /proc file system support
214 [*] Virtual memory file system support (former shm fs)
215 </pre>
216
217 <p>
218 You can leave the <c>/dev file system support (OBSOLETE)</c> active if you
219 wish.
220 </p>
221
222 </body>
223 </section>
224 <section>
225 <title>Configuration</title>
226 <body>
227
228 <p>
229 If you want to use the udev-tweaks Gentoo added to make your life
230 comfortable, then read no more. You're all set. The Gentoo init scripts won't
231 run the devfsd daemon and will deactivate devfs when you boot up.
232 </p>
233
234 <p>
235 But if you are a die-hard and want to run a udev-only, no-tweaked system as is
236 intended by the udev development (including the difficulties of missing device
237 nodes because udev doesn't support them yet), by all means, read on :)
238 </p>
239
240 <warn>
241 Do <e>not</e> complain if something goes wrong. You're going to remove the hard
242 work of many Gentoo developers that hacked our init scripts to get udev playing
243 nicely with Gentoo!
244 </warn>
245
246 <p>
247 We'll deactivate the rules that save the device file nodes: edit the
248 <c>RC_DEVICE_TARBALL</c> variable in <path>/etc/conf.d/rc</path> and set it to
249 <c>no</c>:
250 </p>
251
252 <pre caption="/etc/conf.d/rc">
253 RC_DEVICE_TARBALL="no"
254 </pre>
255
256 <p>
257 If you have included devfs support in your kernel, you can deactivate it in
258 the bootloader configuration: add <c>devfs=nomount</c> as a kernel parameter. If
259 you want to use devfs and deactivate udev, add <c>gentoo=noudev</c> as kernel
260 parameter.
261 </p>
262
263 </body>
264 </section>
265 </chapter>
266
267 <chapter>
268 <title>Known Issues</title>
269 <section>
270 <title>Missing device node files at boot</title>
271 <body>
272
273 <p>
274 If you can't boot successfully because you get an error about
275 <path>/dev/null</path> not found, or because the initial console is missing, the
276 problem is that you lack some device files that must be available <e>before</e>
277 <path>/dev</path> is mounted and handled by udev. This is common on Gentoo
278 machines installed from old media.
279 </p>
280
281 <p>
282 If you run <c>sys-apps/baselayout-1.8.12</c> or later, this problem is
283 alleviated since the boot process should still manage to complete. However, to
284 get rid of those annoying warnings, you should create the missing device nodes
285 as described below.
286 </p>
287
288 <p>
289 To see which devices nodes are present before the <path>/dev</path> filesystem
290 is mounted, run the following commands:
291 </p>
292
293 <pre caption="Listing device nodes available at boot">
294 # <i>mkdir test</i>
295 # <i>mount --bind / test</i>
296 # <i>cd test/dev</i>
297 # <i>ls</i>
298 </pre>
299
300 <p>
301 The devices needed for a successful boot are <path>/dev/null</path> and
302 <path>/dev/console</path>. If they didn't show up in the previous test, you have
303 to create them manually. Issue the following commands in the
304 <path>test/dev/</path> directory:
305 </p>
306
307 <pre caption="Creating necessary device node files">
308 # <i>mknod -m 660 console c 5 1</i>
309 # <i>mknod -m 660 null c 1 3</i>
310 </pre>
311
312 <p>
313 When you're finished, don't forget to unmount the <path>test/</path> directory:
314 </p>
315
316 <pre caption="Unmounting the test/ directory">
317 # <i>cd ../..</i>
318 # <i>umount test</i>
319 # <i>rmdir test</i>
320 </pre>
321
322 </body>
323 </section>
324 <section>
325 <title>udev and nvidia</title>
326 <body>
327
328 <p>
329 If you use the proprietary driver from nVidia and the X server fails to start on
330 a udev-only system, then make sure you have:
331 </p>
332
333 <ul>
334 <li>
335 the <c>nvidia</c> module listed in
336 <path>/etc/modules.autoload.d/kernel-2.6</path>
337 </li>
338 <li>
339 a version of <c>nvidia-kernel</c> equal to or greater than
340 <c>media-video/nvidia-kernel-1.0.5336-r2</c>
341 </li>
342 <li>
343 a version of baselayout equal to or greater than
344 <c>sys-apps/baselayout-1.8.12</c>
345 </li>
346 </ul>
347
348 </body>
349 </section>
350 <section>
351 <title>Other issues</title>
352 <body>
353
354 <p>
355 If device nodes are not created when a module is loaded from
356 <path>/etc/modules.autoload.d/kernel-2.6</path> but they appear when you load
357 the module manually with modprobe then you should try upgrading to
358 <c>sys-apps/baselayout-1.8.12</c> or later.
359 </p>
360
361 <p>
362 Support for the framebuffer devices (<path>/dev/fb/*</path>) comes with the
363 kernel starting from version 2.6.6-rc2.
364 </p>
365
366 <p>
367 For kernels older than 2.6.4 you have to explicitly include support for the
368 <path>/dev/pts</path> filesystem.
369 </p>
370
371 <pre caption="Enabling the /dev/pts filesystem">
372 File systems ---&gt;
373 Pseudo filesystems ---&gt;
374 [*] /dev/pts file system for Unix98 PTYs
375 </pre>
376
377 </body>
378 </section>
379 </chapter>
380
381 <chapter>
382 <title>Resources &amp; Acknowledgements</title>
383 <section>
384 <body>
385
386 <p>
387 The udev talk on the Linux Symposium (Ottawa, Ontario Canada - 2003) given by
388 Greg Kroah-Hartman (IBM Corporation) provided a solid understanding on the udev
389 application.
390 </p>
391
392 <p>
393 <uri link="http://webpages.charter.net/decibelshelp/LinuxHelp_UDEVPrimer.html">Decibel's
394 UDEV Primer</uri> is an in-depth document about udev and Gentoo.
395 </p>
396
397 <p>
398 <uri link="http://www.reactivated.net/udevrules.php">Writing udev rules</uri> by
399 fellow Gentoo developer Daniel Drake is an excellent document to learn how to
400 customize your udev installation.
401 </p>
402
403 </body>
404 </section>
405 </chapter>
406
407 </guide>

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