1 | <?xml version="1.0" encoding="ISO-8859-1"?> |
---|
2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
---|
3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [ |
---|
4 | <!ENTITY % general-entities SYSTEM "../../general.ent"> |
---|
5 | %general-entities; |
---|
6 | ]> |
---|
7 | |
---|
8 | <sect1 id="ch-scripts-udev"> |
---|
9 | <?dbhtml filename="udev.html"?> |
---|
10 | |
---|
11 | <title>Device and Module Handling on a CLFS System</title> |
---|
12 | |
---|
13 | <indexterm zone="ch-scripts-udev"> |
---|
14 | <primary sortas="a-Udev">Udev</primary> |
---|
15 | <secondary>usage</secondary> |
---|
16 | </indexterm> |
---|
17 | |
---|
18 | <para>In <xref linkend="chapter-building-system"/>, we installed the Udev |
---|
19 | package. Before we go into the details regarding how this works, |
---|
20 | a brief history of previous methods of handling devices is in |
---|
21 | order.</para> |
---|
22 | |
---|
23 | <para>Linux systems in general traditionally use a static device creation |
---|
24 | method, whereby a great many device nodes are created under <filename |
---|
25 | class="directory">/dev</filename> (sometimes literally thousands of nodes), |
---|
26 | regardless of whether the corresponding hardware devices actually exist. This |
---|
27 | is typically done via a <command>MAKEDEV</command> script, which contains a |
---|
28 | number of calls to the <command>mknod</command> program with the relevant |
---|
29 | major and minor device numbers for every possible device that might exist in |
---|
30 | the world.</para> |
---|
31 | |
---|
32 | <para>Using the Udev method, only those devices which are detected by the |
---|
33 | kernel get device nodes created for them. Because these device nodes will be |
---|
34 | created each time the system boots, they will be stored on a <systemitem |
---|
35 | class="filesystem">tmpfs</systemitem> file system (a virtual file system that |
---|
36 | resides entirely in system memory). Device nodes do not require much space, so |
---|
37 | the memory that is used is negligible.</para> |
---|
38 | |
---|
39 | <sect2> |
---|
40 | <title>History</title> |
---|
41 | |
---|
42 | <para>In February 2000, a new filesystem called <systemitem |
---|
43 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel |
---|
44 | and was made available during the 2.4 series of stable kernels. Although |
---|
45 | it was present in the kernel source itself, this method of creating devices |
---|
46 | dynamically never received overwhelming support from the core kernel |
---|
47 | developers.</para> |
---|
48 | |
---|
49 | <para>The main problem with the approach adopted by <systemitem |
---|
50 | class="filesystem">devfs</systemitem> was the way it handled device |
---|
51 | detection, creation, and naming. The latter issue, that of device node |
---|
52 | naming, was perhaps the most critical. It is generally accepted that if |
---|
53 | device names are allowed to be configurable, then the device naming policy |
---|
54 | should be up to a system administrator, not imposed on them by any |
---|
55 | particular developer(s). The <systemitem |
---|
56 | class="filesystem">devfs</systemitem> file system also suffers from race |
---|
57 | conditions that are inherent in its design and cannot be fixed without a |
---|
58 | substantial revision to the kernel. It has also been marked as deprecated |
---|
59 | due to a lack of recent maintenance.</para> |
---|
60 | |
---|
61 | <para>With the development of the unstable 2.5 kernel tree, later released |
---|
62 | as the 2.6 series of stable kernels, a new virtual filesystem called |
---|
63 | <systemitem class="filesystem">sysfs</systemitem> came to be. The job of |
---|
64 | <systemitem class="filesystem">sysfs</systemitem> is to export a view of |
---|
65 | the system's hardware configuration to userspace processes. With this |
---|
66 | userspace-visible representation, the possibility of seeing a userspace |
---|
67 | replacement for <systemitem class="filesystem">devfs</systemitem> became |
---|
68 | much more realistic.</para> |
---|
69 | |
---|
70 | </sect2> |
---|
71 | |
---|
72 | <sect2> |
---|
73 | <title>Udev Implementation</title> |
---|
74 | |
---|
75 | <sect3> |
---|
76 | <title>Sysfs</title> |
---|
77 | |
---|
78 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem was |
---|
79 | mentioned briefly above. One may wonder how <systemitem |
---|
80 | class="filesystem">sysfs</systemitem> knows about the devices present on |
---|
81 | a system and what device numbers should be used for them. Drivers that |
---|
82 | have been compiled into the kernel directly register their objects with |
---|
83 | <systemitem class="filesystem">sysfs</systemitem> as they are detected by |
---|
84 | the kernel. For drivers compiled as modules, this registration will happen |
---|
85 | when the module is loaded. Once the <systemitem |
---|
86 | class="filesystem">sysfs</systemitem> filesystem is mounted (on <filename |
---|
87 | class="directory">/sys</filename>), data which the built-in drivers |
---|
88 | registered with <systemitem class="filesystem">sysfs</systemitem> are |
---|
89 | available to userspace processes and to <command>udevd</command> for device |
---|
90 | node creation.</para> |
---|
91 | |
---|
92 | </sect3> |
---|
93 | |
---|
94 | <sect3> |
---|
95 | <title>Udev Bootscript</title> |
---|
96 | |
---|
97 | <para>The <command>S10udev</command> initscript takes care of creating |
---|
98 | device nodes when Linux is booted. The script unsets the uevent handler |
---|
99 | from the default of <command>/sbin/hotplug</command>. This is done |
---|
100 | because the kernel no longer needs to call out to an external binary. |
---|
101 | Instead <command>udevd</command> will listen on a netlink socket for |
---|
102 | uevents that the kernel raises. Next, the bootscript copies any static |
---|
103 | device nodes that exist in <filename |
---|
104 | class="directory">/lib/udev/devices</filename> to <filename |
---|
105 | class="directory">/dev</filename>. This is necessary because some devices, |
---|
106 | directories, and symlinks are needed before the dynamic device handling |
---|
107 | processes are available during the early stages of booting a system. |
---|
108 | Creating static device nodes in <filename |
---|
109 | class="directory">/lib/udev/devices</filename> also provides an easy |
---|
110 | workaround for devices that are not supported by the dynamic device |
---|
111 | handling infrastructure. The bootscript then starts the Udev daemon, |
---|
112 | <command>udevd</command>, which will act on any uevents it receives. |
---|
113 | Finally, the bootscript forces the kernel to replay uevents for any |
---|
114 | devices that have already been registered and then waits for |
---|
115 | <command>udevd</command> to handle them.</para> |
---|
116 | |
---|
117 | </sect3> |
---|
118 | |
---|
119 | <sect3> |
---|
120 | <title>Device Node Creation</title> |
---|
121 | |
---|
122 | <para>To obtain the right major and minor number for a device, Udev relies |
---|
123 | on the information provided by <systemitem |
---|
124 | class="filesystem">sysfs</systemitem> in <filename |
---|
125 | class="directory">/sys</filename>. For example, |
---|
126 | <filename>/sys/class/tty/vcs/dev</filename> contains the string |
---|
127 | <quote>7:0</quote>. This string is used by <command>udevd</command> |
---|
128 | to create a device node with major number <emphasis>7</emphasis> and minor |
---|
129 | <emphasis>0</emphasis>. The names and permissions of the nodes created |
---|
130 | under the <filename class="directory">/dev</filename> directory are |
---|
131 | determined by rules specified in the files within the <filename |
---|
132 | class="directory">/etc/udev/rules.d/</filename> directory. These are |
---|
133 | numbered in a similar fashion to the CLFS-Bootscripts package. If |
---|
134 | <command>udevd</command> can't find a rule for the device it is creating, |
---|
135 | it will default permissions to <emphasis>660</emphasis> and ownership to |
---|
136 | <emphasis>root:root</emphasis>. Documentation on the syntax of the Udev |
---|
137 | rules configuration files are available in |
---|
138 | <filename>/usr/share/doc/udev-&udev-version;/index.html</filename></para> |
---|
139 | |
---|
140 | </sect3> |
---|
141 | |
---|
142 | <sect3> |
---|
143 | <title>Module Loading</title> |
---|
144 | |
---|
145 | <para>Device drivers compiled as modules may have aliases built into them. |
---|
146 | Aliases are visible in the output of the <command>modinfo</command> |
---|
147 | program and are usually related to the bus-specific identifiers of devices |
---|
148 | supported by a module. For example, the <emphasis>snd-fm801</emphasis> |
---|
149 | driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801, |
---|
150 | and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>. |
---|
151 | For most devices, the bus driver exports the alias of the driver that |
---|
152 | would handle the device via <systemitem |
---|
153 | class="filesystem">sysfs</systemitem>. E.g., the |
---|
154 | <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file |
---|
155 | might contain the string |
---|
156 | <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>. |
---|
157 | The rules that CLFS installs will cause <command>udevd</command> to call |
---|
158 | out to <command>/sbin/modprobe</command> with the contents of the |
---|
159 | <envar>MODALIAS</envar> uevent environment variable (that should be the |
---|
160 | same as the contents of the <filename>modalias</filename> file in sysfs), |
---|
161 | thus loading all modules whose aliases match this string after wildcard |
---|
162 | expansion.</para> |
---|
163 | |
---|
164 | <para>In this example, this means that, in addition to |
---|
165 | <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted) |
---|
166 | <emphasis>forte</emphasis> driver will be loaded if it is |
---|
167 | available. See below for ways in which the loading of unwanted drivers can |
---|
168 | be prevented.</para> |
---|
169 | |
---|
170 | <para>The kernel itself is also able to load modules for network |
---|
171 | protocols, filesystems and NLS support on demand.</para> |
---|
172 | |
---|
173 | </sect3> |
---|
174 | |
---|
175 | <sect3> |
---|
176 | <title>Handling Hotpluggable/Dynamic Devices</title> |
---|
177 | |
---|
178 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3 |
---|
179 | player, the kernel recognizes that the device is now connected and |
---|
180 | generates a uevent. This uevent is then handled by |
---|
181 | <command>udevd</command> as described above.</para> |
---|
182 | |
---|
183 | </sect3> |
---|
184 | |
---|
185 | </sect2> |
---|
186 | |
---|
187 | <sect2> |
---|
188 | <title>Problems with Loading Modules and Creating Devices</title> |
---|
189 | |
---|
190 | <para>There are a few possible problems when it comes to automatically |
---|
191 | creating device nodes.</para> |
---|
192 | |
---|
193 | <sect3> |
---|
194 | <title>A kernel module is not loaded automatically</title> |
---|
195 | |
---|
196 | <para>Udev will only load a module if it has a bus-specific alias and the |
---|
197 | bus driver properly exports the necessary aliases to <systemitem |
---|
198 | class="filesystem">sysfs</systemitem>. In other cases, one should |
---|
199 | arrange module loading by other means. With Linux-&linux-version;, Udev is |
---|
200 | known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI, |
---|
201 | SERIO and FireWire devices.</para> |
---|
202 | |
---|
203 | <para>To determine if the device driver you require has the necessary |
---|
204 | support for Udev, run <command>modinfo</command> with the module name as |
---|
205 | the argument. Now try locating the device directory under |
---|
206 | <filename class="directory">/sys/bus</filename> and check whether there is |
---|
207 | a <filename>modalias</filename> file there.</para> |
---|
208 | |
---|
209 | <para>If the <filename>modalias</filename> file exists in <systemitem |
---|
210 | class="filesystem">sysfs</systemitem>, the driver supports the device and |
---|
211 | can talk to it directly, but doesn't have the alias, it is a bug in the |
---|
212 | driver. Load the driver without the help from Udev and expect the issue |
---|
213 | to be fixed later.</para> |
---|
214 | |
---|
215 | <para>If there is no <filename>modalias</filename> file in the relevant |
---|
216 | directory under <filename class="directory">/sys/bus</filename>, this |
---|
217 | means that the kernel developers have not yet added modalias support to |
---|
218 | this bus type. With Linux-&linux-version;, this is the case with ISA |
---|
219 | busses. Expect this issue to be fixed in later kernel versions.</para> |
---|
220 | |
---|
221 | <para>Udev is not intended to load <quote>wrapper</quote> drivers such as |
---|
222 | <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as |
---|
223 | <emphasis>loop</emphasis> at all.</para> |
---|
224 | |
---|
225 | </sect3> |
---|
226 | |
---|
227 | <sect3> |
---|
228 | <title>A kernel module is not loaded automatically, and Udev is not |
---|
229 | intended to load it</title> |
---|
230 | |
---|
231 | <para>If the <quote>wrapper</quote> module only enhances the functionality |
---|
232 | provided by some other module (e.g., <emphasis>snd-pcm-oss</emphasis> |
---|
233 | enhances the functionality of <emphasis>snd-pcm</emphasis> by making the |
---|
234 | sound cards available to OSS applications), configure |
---|
235 | <command>modprobe</command> to load the wrapper after Udev loads the |
---|
236 | wrapped module. To do this, add an <quote>install</quote> line in |
---|
237 | <filename>/etc/modprobe.conf</filename>. For example:</para> |
---|
238 | |
---|
239 | <screen role="nodump"><literal>install snd-pcm /sbin/modprobe -i snd-pcm ; \ |
---|
240 | /sbin/modprobe snd-pcm-oss ; true</literal></screen> |
---|
241 | |
---|
242 | <para>If the module in question is not a wrapper and is useful by itself, |
---|
243 | configure the <command>S05modules</command> bootscript to load this |
---|
244 | module on system boot. To do this, add the module name to the |
---|
245 | <filename>/etc/sysconfig/modules</filename> file on a separate line. |
---|
246 | This works for wrapper modules too, but is suboptimal in that case.</para> |
---|
247 | |
---|
248 | </sect3> |
---|
249 | |
---|
250 | <sect3> |
---|
251 | <title>Udev loads some unwanted module</title> |
---|
252 | |
---|
253 | <para>Either don't build the module, or blacklist it in |
---|
254 | <filename>/etc/modprobe.conf</filename> file as done with the |
---|
255 | <emphasis>forte</emphasis> module in the example below:</para> |
---|
256 | |
---|
257 | <screen role="nodump"><literal>blacklist forte</literal></screen> |
---|
258 | |
---|
259 | <para>Blacklisted modules can still be loaded manually with the |
---|
260 | explicit <command>modprobe</command> command.</para> |
---|
261 | |
---|
262 | </sect3> |
---|
263 | |
---|
264 | <sect3> |
---|
265 | <title>Udev creates a device incorrectly, or makes a wrong symlink</title> |
---|
266 | |
---|
267 | <para>This usually happens if a rule unexpectedly matches a device. For |
---|
268 | example, a poorly-writen rule can match both a SCSI disk (as desired) |
---|
269 | and the corresponding SCSI generic device (incorrectly) by vendor. |
---|
270 | Find the offending rule and make it more specific.</para> |
---|
271 | |
---|
272 | </sect3> |
---|
273 | |
---|
274 | <sect3> |
---|
275 | <title>Udev rule works unreliably</title> |
---|
276 | |
---|
277 | <para>This may be another manifestation of the previous problem. If not, |
---|
278 | and your rule uses <systemitem class="filesystem">sysfs</systemitem> |
---|
279 | attributes, it may be a kernel timing issue, to be fixed in later kernels. |
---|
280 | For now, you can work around it by creating a rule that waits for the used |
---|
281 | <systemitem class="filesystem">sysfs</systemitem> attribute and appending |
---|
282 | it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename> |
---|
283 | file. Please notify the CLFS Development list if you do so and it |
---|
284 | helps.</para> |
---|
285 | |
---|
286 | </sect3> |
---|
287 | |
---|
288 | <sect3> |
---|
289 | <title>Udev does not create a device</title> |
---|
290 | |
---|
291 | <para>Further text assumes that the driver is built statically into the |
---|
292 | kernel or already loaded as a module, and that you have already checked |
---|
293 | that Udev doesn't create a misnamed device.</para> |
---|
294 | |
---|
295 | <para>Udev has no information needed to create a device node if a kernel |
---|
296 | driver does not export its data to <systemitem |
---|
297 | class="filesystem">sysfs</systemitem>. |
---|
298 | This is most common with third party drivers from outside the kernel |
---|
299 | tree. Create a static device node in |
---|
300 | <filename>/lib/udev/devices</filename> with the appropriate major/minor |
---|
301 | numbers (see the file <filename>devices.txt</filename> inside the kernel |
---|
302 | documentation or the documentation provided by the third party driver |
---|
303 | vendor). The static device node will be copied to |
---|
304 | <filename class="directory">/dev</filename> by the |
---|
305 | <command>S10udev</command> bootscript.</para> |
---|
306 | |
---|
307 | </sect3> |
---|
308 | |
---|
309 | <sect3> |
---|
310 | <title>Device naming order changes randomly after rebooting</title> |
---|
311 | |
---|
312 | <para>This is due to the fact that Udev, by design, handles uevents and |
---|
313 | loads modules in parallel, and thus in an unpredictable order. This will |
---|
314 | never be <quote>fixed</quote>. You should not rely upon the kernel device |
---|
315 | names being stable. Instead, create your own rules that make symlinks with |
---|
316 | stable names based on some stable attributes of the device, such as a |
---|
317 | serial number or the output of various *_id utilities installed by Udev. |
---|
318 | See <xref linkend="ch-scripts-symlinks"/> and |
---|
319 | <xref linkend="ch-scripts-network"/> for examples.</para> |
---|
320 | |
---|
321 | </sect3> |
---|
322 | |
---|
323 | </sect2> |
---|
324 | |
---|
325 | <sect2> |
---|
326 | <title>Useful Reading</title> |
---|
327 | |
---|
328 | <para>Additional helpful documentation is available at the following |
---|
329 | sites:</para> |
---|
330 | |
---|
331 | <itemizedlist> |
---|
332 | |
---|
333 | <listitem> |
---|
334 | <para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem> |
---|
335 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para> |
---|
336 | </listitem> |
---|
337 | |
---|
338 | <listitem> |
---|
339 | <para remap="verbatim">The <systemitem class="filesystem">sysfs</systemitem> Filesystem |
---|
340 | <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para> |
---|
341 | </listitem> |
---|
342 | |
---|
343 | </itemizedlist> |
---|
344 | |
---|
345 | </sect2> |
---|
346 | |
---|
347 | </sect1> |
---|