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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 1.16 2004/08/29 12:38:38 swift 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>0.13</version>
23 <date>September 22, 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 but you have to make sure that "Automatically mount at boot" is disabled:
220 </p>
221
222 <pre caption="Don't automatically mount devfsd">
223 File systems ---&gt;
224 Pseudo Filesystems ---&gt;
225 [*] /dev file system support (OBSOLETE)
226 [ ] Automatically mount at boot
227 </pre>
228
229 </body>
230 </section>
231 <section>
232 <title>Configuration</title>
233 <body>
234
235 <p>
236 If you want to use the udev-tweaks Gentoo added to make your life
237 comfortable, then read no more. Gentoo will use udev but keep a static
238 <path>/dev</path> so that you will never have any missing device nodes.
239 The Gentoo init scripts won't run the devfsd daemon and will deactivate devfs
240 when you boot up.
241 </p>
242
243 <p>
244 But if you are a die-hard and want to run a udev-only, no-tweaked system as is
245 intended by the udev development (including the difficulties of missing device
246 nodes because udev doesn't support them yet), by all means, read on :)
247 </p>
248
249 <p>
250 We'll deactivate the rules that save the device file nodes: edit the
251 <c>RC_DEVICE_TARBALL</c> variable in <path>/etc/conf.d/rc</path> and set it to
252 <c>no</c>:
253 </p>
254
255 <pre caption="/etc/conf.d/rc">
256 RC_DEVICE_TARBALL="no"
257 </pre>
258
259 <p>
260 If you have included devfs support in your kernel, you can deactivate it in
261 the bootloader configuration: add <c>gentoo=nodevfs</c> as a kernel parameter.
262 If you want to use devfs and deactivate udev, add <c>gentoo=noudev</c> as kernel
263 parameter.
264 </p>
265
266 </body>
267 </section>
268 </chapter>
269
270 <chapter>
271 <title>Known Issues</title>
272 <section>
273 <title>Missing device node files at boot</title>
274 <body>
275
276 <p>
277 If you can't boot successfully because you get an error about
278 <path>/dev/null</path> not found, or because the initial console is missing, the
279 problem is that you lack some device files that must be available <e>before</e>
280 <path>/dev</path> is mounted and handled by udev. This is common on Gentoo
281 machines installed from old media.
282 </p>
283
284 <p>
285 If you run <c>sys-apps/baselayout-1.8.12</c> or later, this problem is
286 alleviated since the boot process should still manage to complete. However, to
287 get rid of those annoying warnings, you should create the missing device nodes
288 as described below.
289 </p>
290
291 <p>
292 To see which devices nodes are present before the <path>/dev</path> filesystem
293 is mounted, run the following commands:
294 </p>
295
296 <pre caption="Listing device nodes available at boot">
297 # <i>mkdir test</i>
298 # <i>mount --bind / test</i>
299 # <i>cd test/dev</i>
300 # <i>ls</i>
301 </pre>
302
303 <p>
304 The devices needed for a successful boot are <path>/dev/null</path> and
305 <path>/dev/console</path>. If they didn't show up in the previous test, you have
306 to create them manually. Issue the following commands in the
307 <path>test/dev/</path> directory:
308 </p>
309
310 <pre caption="Creating necessary device node files">
311 # <i>mknod -m 660 console c 5 1</i>
312 # <i>mknod -m 660 null c 1 3</i>
313 </pre>
314
315 <p>
316 When you're finished, don't forget to unmount the <path>test/</path> directory:
317 </p>
318
319 <pre caption="Unmounting the test/ directory">
320 # <i>cd ../..</i>
321 # <i>umount test</i>
322 # <i>rmdir test</i>
323 </pre>
324
325 </body>
326 </section>
327 <section>
328 <title>udev and nvidia</title>
329 <body>
330
331 <p>
332 If you use the proprietary driver from nVidia and the X server fails to start on
333 a udev-only system, then make sure you have:
334 </p>
335
336 <ul>
337 <li>
338 the <c>nvidia</c> module listed in
339 <path>/etc/modules.autoload.d/kernel-2.6</path>
340 </li>
341 <li>
342 a version of <c>nvidia-kernel</c> equal to or greater than
343 <c>media-video/nvidia-kernel-1.0.5336-r2</c>
344 </li>
345 <li>
346 a version of baselayout equal to or greater than
347 <c>sys-apps/baselayout-1.8.12</c>
348 </li>
349 </ul>
350
351 </body>
352 </section>
353 <section>
354 <title>LVM2 Names Disappear</title>
355 <body>
356
357 <p>
358 When you use <c>udev</c> and LVM2 together, you might notice that your created
359 volume groups and logical volumes have disappeared. Well, they haven't, but they
360 are unfortunately named <path>/dev/dm-#</path> with # being 0, 1, ...
361 </p>
362
363 <p>
364 To fix this, edit <path>/etc/udev/rules.d/50-udev.rules</path> and uncomment the
365 following line:
366 </p>
367
368 <pre caption="Uncomment this line from /etc/udev/rules.d/50-udev.rules">
369 KERNEL="dm-[0-9]*", PROGRAM="/sbin/devmap_name %M %m", NAME="%k", SYMLINK="%c"
370 </pre>
371
372 </body>
373 </section>
374 <section>
375 <title>Other issues</title>
376 <body>
377
378 <p>
379 If device nodes are not created when a module is loaded from
380 <path>/etc/modules.autoload.d/kernel-2.6</path> but they appear when you load
381 the module manually with modprobe then you should try upgrading to
382 <c>sys-apps/baselayout-1.8.12</c> or later.
383 </p>
384
385 <p>
386 Support for the framebuffer devices (<path>/dev/fb/*</path>) comes with the
387 kernel starting from version 2.6.6-rc2.
388 </p>
389
390 <p>
391 For kernels older than 2.6.4 you have to explicitly include support for the
392 <path>/dev/pts</path> filesystem.
393 </p>
394
395 <pre caption="Enabling the /dev/pts filesystem">
396 File systems ---&gt;
397 Pseudo filesystems ---&gt;
398 [*] /dev/pts file system for Unix98 PTYs
399 </pre>
400
401 </body>
402 </section>
403 </chapter>
404
405 <chapter>
406 <title>Resources &amp; Acknowledgements</title>
407 <section>
408 <body>
409
410 <p>
411 The udev talk on the Linux Symposium (Ottawa, Ontario Canada - 2003) given by
412 Greg Kroah-Hartman (IBM Corporation) provided a solid understanding on the udev
413 application.
414 </p>
415
416 <p>
417 <uri link="http://webpages.charter.net/decibelshelp/LinuxHelp_UDEVPrimer.html">Decibel's
418 UDEV Primer</uri> is an in-depth document about udev and Gentoo.
419 </p>
420
421 <p>
422 <uri link="http://www.reactivated.net/udevrules.php">Writing udev rules</uri> by
423 fellow Gentoo developer Daniel Drake is an excellent document to learn how to
424 customize your udev installation.
425 </p>
426
427 </body>
428 </section>
429 </chapter>
430
431 </guide>

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