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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 swift 1.20 <!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/udev-guide.xml,v 1.19 2004/10/18 15:52:08 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     <mail link="g.guidi@sns.it">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 1.20 <version>0.16</version>
23     <date>October 22, 2004</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 1.1 Kernelwise, if you're using the default set by <c>genkernel</c> then you're all
213     set. Otherwise be sure to activate the following options:
214     </p>
215    
216     <pre caption="Required kernel options">
217 swift 1.6 General setup ---&gt;
218 swift 1.1 [*] Support for hot-pluggable devices
219    
220     File systems ---&gt;
221     Pseudo filesystems ---&gt;
222     [*] /proc file system support
223     [*] Virtual memory file system support (former shm fs)
224     </pre>
225    
226     <p>
227     You can leave the <c>/dev file system support (OBSOLETE)</c> active if you
228 swift 1.16 wish but you have to make sure that "Automatically mount at boot" is disabled:
229 swift 1.1 </p>
230    
231 swift 1.16 <pre caption="Don't automatically mount devfsd">
232     File systems ---&gt;
233     Pseudo Filesystems ---&gt;
234     [*] /dev file system support (OBSOLETE)
235     [ ] Automatically mount at boot
236     </pre>
237    
238 swift 1.1 </body>
239     </section>
240     <section>
241     <title>Configuration</title>
242     <body>
243    
244     <p>
245 neysx 1.5 If you want to use the udev-tweaks Gentoo added to make your life
246 swift 1.15 comfortable, then read no more. Gentoo will use udev but keep a static
247     <path>/dev</path> so that you will never have any missing device nodes.
248     The Gentoo init scripts won't run the devfsd daemon and will deactivate devfs
249     when you boot up.
250 swift 1.1 </p>
251    
252     <p>
253     But if you are a die-hard and want to run a udev-only, no-tweaked system as is
254 swift 1.2 intended by the udev development (including the difficulties of missing device
255     nodes because udev doesn't support them yet), by all means, read on :)
256 swift 1.1 </p>
257    
258     <p>
259 bennyc 1.13 We'll deactivate the rules that save the device file nodes: edit the
260 swift 1.2 <c>RC_DEVICE_TARBALL</c> variable in <path>/etc/conf.d/rc</path> and set it to
261     <c>no</c>:
262     </p>
263 swift 1.1
264 swift 1.2 <pre caption="/etc/conf.d/rc">
265     RC_DEVICE_TARBALL="no"
266 swift 1.1 </pre>
267    
268     <p>
269 swift 1.8 If you have included devfs support in your kernel, you can deactivate it in
270 swift 1.14 the bootloader configuration: add <c>gentoo=nodevfs</c> as a kernel parameter.
271     If you want to use devfs and deactivate udev, add <c>gentoo=noudev</c> as kernel
272 swift 1.8 parameter.
273     </p>
274    
275     </body>
276     </section>
277     </chapter>
278    
279     <chapter>
280     <title>Known Issues</title>
281     <section>
282     <title>Missing device node files at boot</title>
283     <body>
284    
285     <p>
286 neysx 1.9 If you can't boot successfully because you get an error about
287 swift 1.8 <path>/dev/null</path> not found, or because the initial console is missing, the
288     problem is that you lack some device files that must be available <e>before</e>
289     <path>/dev</path> is mounted and handled by udev. This is common on Gentoo
290     machines installed from old media.
291     </p>
292    
293     <p>
294     If you run <c>sys-apps/baselayout-1.8.12</c> or later, this problem is
295     alleviated since the boot process should still manage to complete. However, to
296 swift 1.10 get rid of those annoying warnings, you should create the missing device nodes
297 swift 1.8 as described below.
298     </p>
299    
300     <p>
301     To see which devices nodes are present before the <path>/dev</path> filesystem
302     is mounted, run the following commands:
303     </p>
304    
305     <pre caption="Listing device nodes available at boot">
306     # <i>mkdir test</i>
307     # <i>mount --bind / test</i>
308     # <i>cd test/dev</i>
309     # <i>ls</i>
310     </pre>
311    
312     <p>
313 neysx 1.9 The devices needed for a successful boot are <path>/dev/null</path> and
314 swift 1.8 <path>/dev/console</path>. If they didn't show up in the previous test, you have
315 cam 1.12 to create them manually. Issue the following commands in the
316     <path>test/dev/</path> directory:
317 swift 1.1 </p>
318    
319     <pre caption="Creating necessary device node files">
320 cam 1.11 # <i>mknod -m 660 console c 5 1</i>
321     # <i>mknod -m 660 null c 1 3</i>
322 swift 1.1 </pre>
323    
324     <p>
325 swift 1.8 When you're finished, don't forget to unmount the <path>test/</path> directory:
326 swift 1.1 </p>
327    
328 swift 1.8 <pre caption="Unmounting the test/ directory">
329 cam 1.11 # <i>cd ../..</i>
330 swift 1.8 # <i>umount test</i>
331 cam 1.11 # <i>rmdir test</i>
332 swift 1.8 </pre>
333    
334     </body>
335     </section>
336     <section>
337     <title>udev and nvidia</title>
338     <body>
339    
340 swift 1.1 <p>
341 swift 1.8 If you use the proprietary driver from nVidia and the X server fails to start on
342     a udev-only system, then make sure you have:
343 swift 1.1 </p>
344    
345 swift 1.8 <ul>
346     <li>
347     the <c>nvidia</c> module listed in
348     <path>/etc/modules.autoload.d/kernel-2.6</path>
349     </li>
350     <li>
351 swift 1.10 a version of <c>nvidia-kernel</c> equal to or greater than
352 swift 1.8 <c>media-video/nvidia-kernel-1.0.5336-r2</c>
353     </li>
354     <li>
355     a version of baselayout equal to or greater than
356     <c>sys-apps/baselayout-1.8.12</c>
357     </li>
358     </ul>
359    
360 swift 1.1 </body>
361     </section>
362     <section>
363 swift 1.17 <title>LVM2 Names Disappear</title>
364     <body>
365    
366     <p>
367     When you use <c>udev</c> and LVM2 together, you might notice that your created
368     volume groups and logical volumes have disappeared. Well, they haven't, but they
369     are unfortunately named <path>/dev/dm-#</path> with # being 0, 1, ...
370     </p>
371    
372     <p>
373     To fix this, edit <path>/etc/udev/rules.d/50-udev.rules</path> and uncomment the
374     following line:
375     </p>
376    
377     <pre caption="Uncomment this line from /etc/udev/rules.d/50-udev.rules">
378     KERNEL="dm-[0-9]*", PROGRAM="/sbin/devmap_name %M %m", NAME="%k", SYMLINK="%c"
379     </pre>
380    
381     </body>
382     </section>
383     <section>
384 swift 1.18 <title>No Consistent Naming between DevFS and udev</title>
385     <body>
386    
387     <p>
388     Even though our intention is to have a consistent naming scheme between both
389     dynamical device management solutions, sometimes naming differences do occur.
390     One reported clash is with a HP Smart Array 5i RAID controller (more precisely
391     the <c>cciss</c> kernel module). With udev, the devices are named
392     <path>/dev/cciss/cXdYpZ</path> with X, Y and Z regular numbers. With devfs, the
393     devices are <path>/dev/hostX/targetY/partZ</path> or symlinked from
394     <path>/dev/cciss/cXdY</path>.
395     </p>
396    
397     <p>
398     If this is the case, don't forget to update your <path>/etc/fstab</path> and
399     bootloader configuration files accordingly.
400     </p>
401    
402     </body>
403     </section>
404     <section>
405 swift 1.8 <title>Other issues</title>
406 swift 1.1 <body>
407    
408 swift 1.8 <p>
409     If device nodes are not created when a module is loaded from
410     <path>/etc/modules.autoload.d/kernel-2.6</path> but they appear when you load
411     the module manually with modprobe then you should try upgrading to
412     <c>sys-apps/baselayout-1.8.12</c> or later.
413     </p>
414 swift 1.7
415     <p>
416 swift 1.8 Support for the framebuffer devices (<path>/dev/fb/*</path>) comes with the
417     kernel starting from version 2.6.6-rc2.
418 swift 1.7 </p>
419    
420     <p>
421 swift 1.8 For kernels older than 2.6.4 you have to explicitly include support for the
422     <path>/dev/pts</path> filesystem.
423 swift 1.1 </p>
424    
425 swift 1.8 <pre caption="Enabling the /dev/pts filesystem">
426     File systems ---&gt;
427     Pseudo filesystems ---&gt;
428     [*] /dev/pts file system for Unix98 PTYs
429     </pre>
430    
431 swift 1.1 </body>
432     </section>
433     </chapter>
434    
435     <chapter>
436     <title>Resources &amp; Acknowledgements</title>
437     <section>
438     <body>
439    
440     <p>
441     The udev talk on the Linux Symposium (Ottawa, Ontario Canada - 2003) given by
442     Greg Kroah-Hartman (IBM Corporation) provided a solid understanding on the udev
443     application.
444     </p>
445    
446     <p>
447     <uri link="http://webpages.charter.net/decibelshelp/LinuxHelp_UDEVPrimer.html">Decibel's
448     UDEV Primer</uri> is an in-depth document about udev and Gentoo.
449     </p>
450    
451 swift 1.8 <p>
452     <uri link="http://www.reactivated.net/udevrules.php">Writing udev rules</uri> by
453     fellow Gentoo developer Daniel Drake is an excellent document to learn how to
454     customize your udev installation.
455     </p>
456    
457 swift 1.1 </body>
458     </section>
459     </chapter>
460    
461     </guide>

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