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1 swift 1.1 <?xml version='1.0' encoding="UTF-8"?>
2     <!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
3 neysx 1.4
4 neysx 1.24 <!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/udev-guide.xml,v 1.23 2004/11/28 15:45:41 swift Exp $ -->
5 neysx 1.4
6 swift 1.1 <guide link="/doc/en/udev-guide.xml">
7     <title>Gentoo udev Guide</title>
8    
9     <author title="Author">
10 swift 1.8 <mail link="swift@gentoo.org">Sven Vermeulen</mail>
11     </author>
12     <author title="Contributor">
13 neysx 1.24 <mail link="greg_g@gentoo.org">Gregorio Guidi</mail>
14 swift 1.1 </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 swift 1.22 <version>0.18</version>
23     <date>2004-11-28</date>
24 swift 1.1
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 swift 1.3 people who believe Linux is some sort of virus or brand of coffee, the use of
34 swift 1.1 "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 swift 1.3 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 swift 1.1 the first one disappears? How will that affect ongoing transactions? Wouldn't it
52 swift 1.3 be fun that a printing job is suddenly moved from your supernew laserprinter to
53 swift 1.1 your almost-dead matrix printer because your mom decided to pull the plug of the
54 swift 1.20 laserprinter which happened to be the first printer?
55 swift 1.1 </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 swift 1.3 <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 swift 1.1 </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 swift 1.3 by LANANA, used by the majority of Linux systems currently and therefore very
90 swift 1.1 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 swift 1.3 a given setting provided by the user, the accompanying name is used.
128 swift 1.1 </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 swift 1.8 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 swift 1.1 </p>
186    
187 swift 1.8 <pre caption="Installing udev">
188     # <i>emerge udev</i>
189 swift 1.1 </pre>
190    
191     <p>
192 swift 1.19 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 swift 1.1 </p>
197    
198 swift 1.8 <pre caption="Installing optional hotplug scripts">
199     # <i>emerge hotplug</i>
200 swift 1.1 </pre>
201    
202     <p>
203 swift 1.19 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 swift 1.21 Kernelwise, be sure to activate the following options:
213 swift 1.1 </p>
214    
215     <pre caption="Required kernel options">
216 swift 1.6 General setup ---&gt;
217 swift 1.1 [*] 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 swift 1.16 wish but you have to make sure that "Automatically mount at boot" is disabled:
228 swift 1.1 </p>
229    
230 swift 1.16 <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 swift 1.21 <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 swift 1.1 </body>
244     </section>
245     <section>
246     <title>Configuration</title>
247     <body>
248    
249     <p>
250 neysx 1.5 If you want to use the udev-tweaks Gentoo added to make your life
251 swift 1.15 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 swift 1.1 </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 swift 1.2 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 swift 1.1 </p>
262    
263     <p>
264 bennyc 1.13 We'll deactivate the rules that save the device file nodes: edit the
265 swift 1.2 <c>RC_DEVICE_TARBALL</c> variable in <path>/etc/conf.d/rc</path> and set it to
266     <c>no</c>:
267     </p>
268 swift 1.1
269 swift 1.2 <pre caption="/etc/conf.d/rc">
270     RC_DEVICE_TARBALL="no"
271 swift 1.1 </pre>
272    
273     <p>
274 swift 1.8 If you have included devfs support in your kernel, you can deactivate it in
275 swift 1.14 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 swift 1.8 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 neysx 1.9 If you can't boot successfully because you get an error about
292 swift 1.8 <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 swift 1.10 get rid of those annoying warnings, you should create the missing device nodes
302 swift 1.8 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 neysx 1.9 The devices needed for a successful boot are <path>/dev/null</path> and
319 swift 1.8 <path>/dev/console</path>. If they didn't show up in the previous test, you have
320 cam 1.12 to create them manually. Issue the following commands in the
321     <path>test/dev/</path> directory:
322 swift 1.1 </p>
323    
324     <pre caption="Creating necessary device node files">
325 cam 1.11 # <i>mknod -m 660 console c 5 1</i>
326     # <i>mknod -m 660 null c 1 3</i>
327 swift 1.1 </pre>
328    
329     <p>
330 swift 1.8 When you're finished, don't forget to unmount the <path>test/</path> directory:
331 swift 1.1 </p>
332    
333 swift 1.8 <pre caption="Unmounting the test/ directory">
334 cam 1.11 # <i>cd ../..</i>
335 swift 1.8 # <i>umount test</i>
336 cam 1.11 # <i>rmdir test</i>
337 swift 1.8 </pre>
338    
339     </body>
340     </section>
341     <section>
342     <title>udev and nvidia</title>
343     <body>
344    
345 swift 1.1 <p>
346 swift 1.8 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 swift 1.1 </p>
349    
350 swift 1.8 <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 swift 1.10 a version of <c>nvidia-kernel</c> equal to or greater than
357 swift 1.8 <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 swift 1.1 </body>
366     </section>
367     <section>
368 swift 1.17 <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 swift 1.18 <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 swift 1.22 </p>
396    
397     <p>
398 swift 1.18 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 swift 1.22 <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 swift 1.23 Device rule for your mouse points to an existing device file.
415 swift 1.22 </p>
416    
417 swift 1.18 </body>
418     </section>
419     <section>
420 swift 1.8 <title>Other issues</title>
421 swift 1.1 <body>
422    
423 swift 1.8 <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 swift 1.7
430     <p>
431 swift 1.8 Support for the framebuffer devices (<path>/dev/fb/*</path>) comes with the
432     kernel starting from version 2.6.6-rc2.
433 swift 1.7 </p>
434    
435     <p>
436 swift 1.8 For kernels older than 2.6.4 you have to explicitly include support for the
437     <path>/dev/pts</path> filesystem.
438 swift 1.1 </p>
439    
440 swift 1.8 <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 swift 1.1 </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 swift 1.8 <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 swift 1.1 </body>
473     </section>
474     </chapter>
475    
476     </guide>

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