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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 1.23 2004/11/28 15:45:41 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="greg_g@gentoo.org">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>0.18</version>
23 <date>2004-11-28</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 laserprinter 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.
193 You do not need to install <c>hotplug</c> unless you want your modules
194 automatically loaded when you plug devices in. <c>hotplug</c> also handles the
195 automated bringup of network devices and firmware downloading.
196 </p>
197
198 <pre caption="Installing optional hotplug scripts">
199 # <i>emerge hotplug</i>
200 </pre>
201
202 <p>
203 If you want modules loaded for devices that have been plugged in before you
204 boot, use the coldplug package:
205 </p>
206
207 <pre caption="Installing the coldplug package">
208 # <i>emerge coldplug</i>
209 </pre>
210
211 <p>
212 Kernelwise, be sure to activate the following options:
213 </p>
214
215 <pre caption="Required kernel options">
216 General setup ---&gt;
217 [*] Support for hot-pluggable devices
218
219 File systems ---&gt;
220 Pseudo filesystems ---&gt;
221 [*] /proc file system support
222 [*] Virtual memory file system support (former shm fs)
223 </pre>
224
225 <p>
226 You can leave the <c>/dev file system support (OBSOLETE)</c> active if you
227 wish but you have to make sure that "Automatically mount at boot" is disabled:
228 </p>
229
230 <pre caption="Don't automatically mount devfsd">
231 File systems ---&gt;
232 Pseudo Filesystems ---&gt;
233 [*] /dev file system support (OBSOLETE)
234 [ ] Automatically mount at boot
235 </pre>
236
237 <p>
238 If you use <c>genkernel</c>, don't forget to run it with the <c>--udev</c>
239 option to enable all the required kernel configuration directives. The default
240 configuration given by this <c>genkernel</c> invocation is sufficient.
241 </p>
242
243 </body>
244 </section>
245 <section>
246 <title>Configuration</title>
247 <body>
248
249 <p>
250 If you want to use the udev-tweaks Gentoo added to make your life
251 comfortable, then read no more. Gentoo will use udev but keep a static
252 <path>/dev</path> so that you will never have any missing device nodes.
253 The Gentoo init scripts won't run the devfsd daemon and will deactivate devfs
254 when you boot up.
255 </p>
256
257 <p>
258 But if you are a die-hard and want to run a udev-only, no-tweaked system as is
259 intended by the udev development (including the difficulties of missing device
260 nodes because udev doesn't support them yet), by all means, read on :)
261 </p>
262
263 <p>
264 We'll deactivate the rules that save the device file nodes: edit the
265 <c>RC_DEVICE_TARBALL</c> variable in <path>/etc/conf.d/rc</path> and set it to
266 <c>no</c>:
267 </p>
268
269 <pre caption="/etc/conf.d/rc">
270 RC_DEVICE_TARBALL="no"
271 </pre>
272
273 <p>
274 If you have included devfs support in your kernel, you can deactivate it in
275 the bootloader configuration: add <c>gentoo=nodevfs</c> as a kernel parameter.
276 If you want to use devfs and deactivate udev, add <c>gentoo=noudev</c> as kernel
277 parameter.
278 </p>
279
280 </body>
281 </section>
282 </chapter>
283
284 <chapter>
285 <title>Known Issues</title>
286 <section>
287 <title>Missing device node files at boot</title>
288 <body>
289
290 <p>
291 If you can't boot successfully because you get an error about
292 <path>/dev/null</path> not found, or because the initial console is missing, the
293 problem is that you lack some device files that must be available <e>before</e>
294 <path>/dev</path> is mounted and handled by udev. This is common on Gentoo
295 machines installed from old media.
296 </p>
297
298 <p>
299 If you run <c>sys-apps/baselayout-1.8.12</c> or later, this problem is
300 alleviated since the boot process should still manage to complete. However, to
301 get rid of those annoying warnings, you should create the missing device nodes
302 as described below.
303 </p>
304
305 <p>
306 To see which devices nodes are present before the <path>/dev</path> filesystem
307 is mounted, run the following commands:
308 </p>
309
310 <pre caption="Listing device nodes available at boot">
311 # <i>mkdir test</i>
312 # <i>mount --bind / test</i>
313 # <i>cd test/dev</i>
314 # <i>ls</i>
315 </pre>
316
317 <p>
318 The devices needed for a successful boot are <path>/dev/null</path> and
319 <path>/dev/console</path>. If they didn't show up in the previous test, you have
320 to create them manually. Issue the following commands in the
321 <path>test/dev/</path> directory:
322 </p>
323
324 <pre caption="Creating necessary device node files">
325 # <i>mknod -m 660 console c 5 1</i>
326 # <i>mknod -m 660 null c 1 3</i>
327 </pre>
328
329 <p>
330 When you're finished, don't forget to unmount the <path>test/</path> directory:
331 </p>
332
333 <pre caption="Unmounting the test/ directory">
334 # <i>cd ../..</i>
335 # <i>umount test</i>
336 # <i>rmdir test</i>
337 </pre>
338
339 </body>
340 </section>
341 <section>
342 <title>udev and nvidia</title>
343 <body>
344
345 <p>
346 If you use the proprietary driver from nVidia and the X server fails to start on
347 a udev-only system, then make sure you have:
348 </p>
349
350 <ul>
351 <li>
352 the <c>nvidia</c> module listed in
353 <path>/etc/modules.autoload.d/kernel-2.6</path>
354 </li>
355 <li>
356 a version of <c>nvidia-kernel</c> equal to or greater than
357 <c>media-video/nvidia-kernel-1.0.5336-r2</c>
358 </li>
359 <li>
360 a version of baselayout equal to or greater than
361 <c>sys-apps/baselayout-1.8.12</c>
362 </li>
363 </ul>
364
365 </body>
366 </section>
367 <section>
368 <title>LVM2 Names Disappear</title>
369 <body>
370
371 <p>
372 When you use <c>udev</c> and LVM2 together, you might notice that your created
373 volume groups and logical volumes have disappeared. Well, they haven't, but they
374 are unfortunately named <path>/dev/dm-#</path> with # being 0, 1, ...
375 </p>
376
377 <p>
378 To fix this, edit <path>/etc/udev/rules.d/50-udev.rules</path> and uncomment the
379 following line:
380 </p>
381
382 <pre caption="Uncomment this line from /etc/udev/rules.d/50-udev.rules">
383 KERNEL="dm-[0-9]*", PROGRAM="/sbin/devmap_name %M %m", NAME="%k", SYMLINK="%c"
384 </pre>
385
386 </body>
387 </section>
388 <section>
389 <title>No Consistent Naming between DevFS and udev</title>
390 <body>
391
392 <p>
393 Even though our intention is to have a consistent naming scheme between both
394 dynamical device management solutions, sometimes naming differences do occur.
395 </p>
396
397 <p>
398 One reported clash is with a HP Smart Array 5i RAID controller (more precisely
399 the <c>cciss</c> kernel module). With udev, the devices are named
400 <path>/dev/cciss/cXdYpZ</path> with X, Y and Z regular numbers. With devfs, the
401 devices are <path>/dev/hostX/targetY/partZ</path> or symlinked from
402 <path>/dev/cciss/cXdY</path>.
403 </p>
404
405 <p>
406 If this is the case, don't forget to update your <path>/etc/fstab</path> and
407 bootloader configuration files accordingly.
408 </p>
409
410 <p>
411 The same happens with all-round symlinks that used to exist in
412 <path>/dev</path>, such as <path>/dev/mouse</path>, which <c>udev</c> doesn't
413 create anymore. Be certain to check your X configuration file and see if the
414 Device rule for your mouse points to an existing device file.
415 </p>
416
417 </body>
418 </section>
419 <section>
420 <title>Other issues</title>
421 <body>
422
423 <p>
424 If device nodes are not created when a module is loaded from
425 <path>/etc/modules.autoload.d/kernel-2.6</path> but they appear when you load
426 the module manually with modprobe then you should try upgrading to
427 <c>sys-apps/baselayout-1.8.12</c> or later.
428 </p>
429
430 <p>
431 Support for the framebuffer devices (<path>/dev/fb/*</path>) comes with the
432 kernel starting from version 2.6.6-rc2.
433 </p>
434
435 <p>
436 For kernels older than 2.6.4 you have to explicitly include support for the
437 <path>/dev/pts</path> filesystem.
438 </p>
439
440 <pre caption="Enabling the /dev/pts filesystem">
441 File systems ---&gt;
442 Pseudo filesystems ---&gt;
443 [*] /dev/pts file system for Unix98 PTYs
444 </pre>
445
446 </body>
447 </section>
448 </chapter>
449
450 <chapter>
451 <title>Resources &amp; Acknowledgements</title>
452 <section>
453 <body>
454
455 <p>
456 The udev talk on the Linux Symposium (Ottawa, Ontario Canada - 2003) given by
457 Greg Kroah-Hartman (IBM Corporation) provided a solid understanding on the udev
458 application.
459 </p>
460
461 <p>
462 <uri link="http://webpages.charter.net/decibelshelp/LinuxHelp_UDEVPrimer.html">Decibel's
463 UDEV Primer</uri> is an in-depth document about udev and Gentoo.
464 </p>
465
466 <p>
467 <uri link="http://www.reactivated.net/udevrules.php">Writing udev rules</uri> by
468 fellow Gentoo developer Daniel Drake is an excellent document to learn how to
469 customize your udev installation.
470 </p>
471
472 </body>
473 </section>
474 </chapter>
475
476 </guide>

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