udev.xml 15 KB

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  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. <sect1 id="ch-scripts-udev">
  8. <?dbhtml filename="udev.html"?>
  9. <title>Device and Module Handling on an LFS System</title>
  10. <indexterm zone="ch-scripts-udev">
  11. <primary sortas="a-Udev">Udev</primary>
  12. <secondary>usage</secondary>
  13. </indexterm>
  14. <para>In <xref linkend="chapter-building-system"/>, we installed Udev
  15. from the systemd source package. Before we go into the details regarding
  16. how this works, a brief history of previous methods of handling devices
  17. is in order.</para>
  18. <para>Linux systems in general traditionally use a static device creation
  19. method, whereby a great many device nodes are created under <filename
  20. class="directory">/dev</filename> (sometimes literally thousands of nodes),
  21. regardless of whether the corresponding hardware devices actually exist. This
  22. is typically done via a <command>MAKEDEV</command> script, which contains a
  23. number of calls to the <command>mknod</command> program with the relevant
  24. major and minor device numbers for every possible device that might exist in
  25. the world.</para>
  26. <para>Using the Udev method, only those devices which are detected by the
  27. kernel get device nodes created for them. Because these device nodes will be
  28. created each time the system boots, they will be stored on a <systemitem
  29. class="filesystem">devtmpfs</systemitem> file system (a virtual file system
  30. that resides entirely in system memory). Device nodes do not require much
  31. space, so the memory that is used is negligible.</para>
  32. <sect2>
  33. <title>History</title>
  34. <para>In February 2000, a new filesystem called <systemitem
  35. class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
  36. and was made available during the 2.4 series of stable kernels. Although
  37. it was present in the kernel source itself, this method of creating devices
  38. dynamically never received overwhelming support from the core kernel
  39. developers.</para>
  40. <para>The main problem with the approach adopted by <systemitem
  41. class="filesystem">devfs</systemitem> was the way it handled device
  42. detection, creation, and naming. The latter issue, that of device node
  43. naming, was perhaps the most critical. It is generally accepted that if
  44. device names are allowed to be configurable, then the device naming policy
  45. should be up to a system administrator, not imposed on them by any
  46. particular developer(s). The <systemitem
  47. class="filesystem">devfs</systemitem> file system also suffers from race
  48. conditions that are inherent in its design and cannot be fixed without a
  49. substantial revision to the kernel. It was marked as deprecated for a long
  50. period &ndash; due to a lack of maintenance &ndash; and was finally removed
  51. from the kernel in June, 2006.</para>
  52. <para>With the development of the unstable 2.5 kernel tree, later released
  53. as the 2.6 series of stable kernels, a new virtual filesystem called
  54. <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
  55. <systemitem class="filesystem">sysfs</systemitem> is to export a view of
  56. the system's hardware configuration to userspace processes. With this
  57. userspace-visible representation, the possibility of seeing a userspace
  58. replacement for <systemitem class="filesystem">devfs</systemitem> became
  59. much more realistic.</para>
  60. </sect2>
  61. <sect2>
  62. <title>Udev Implementation</title>
  63. <sect3>
  64. <title>Sysfs</title>
  65. <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
  66. was mentioned briefly above. One may wonder how <systemitem
  67. class="filesystem">sysfs</systemitem> knows about the devices present on
  68. a system and what device numbers should be used for them. Drivers that
  69. have been compiled into the kernel directly register their objects with a
  70. <systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally)
  71. as they are detected by the kernel. For drivers compiled as modules, this
  72. registration will happen when the module is loaded. Once the <systemitem
  73. class="filesystem">sysfs</systemitem> filesystem is mounted (on /sys),
  74. data which the drivers register with <systemitem
  75. class="filesystem">sysfs</systemitem> are available to userspace
  76. processes and to udevd for processing (including modifications to device
  77. nodes).</para>
  78. </sect3>
  79. <sect3>
  80. <title>Device Node Creation</title>
  81. <para>Device files are created by the kernel by the <systemitem
  82. class="filesystem">devtmpfs</systemitem> filesystem. Any driver that
  83. wishes to register a device node will go through <systemitem
  84. class="filesystem">devtmpfs</systemitem> (via the driver core) to do it.
  85. When a <systemitem class="filesystem">devtmpfs</systemitem> instance is
  86. mounted on <filename class="directory">/dev</filename>, the device node
  87. will initially be created with a fixed name, permissions, and
  88. owner.</para>
  89. <para>A short time later, the kernel will send a uevent to <command>
  90. udevd</command>. Based on the rules specified in the files within the
  91. <filename class="directory">/etc/udev/rules.d</filename>, <filename
  92. class="directory">/lib/udev/rules.d</filename>, and <filename
  93. class="directory">/run/udev/rules.d</filename> directories, <command>
  94. udevd</command> will create additional symlinks to the device node, or
  95. change its permissions, owner, or group, or modify the internal
  96. <command>udevd</command> database entry (name) for that object.</para>
  97. <para>The rules in these three directories are numbered in a similar
  98. fashion to the LFS-Bootscripts package and all three directories are
  99. merged together. If <command>udevd</command> can't find a rule for the
  100. device it is creating, it will leave the permissions and ownership at
  101. whatever <systemitem class="filesystem">devtmpfs</systemitem> used
  102. initially.</para>
  103. </sect3>
  104. <sect3>
  105. <title>Module Loading</title>
  106. <para>Device drivers compiled as modules may have aliases built into them.
  107. Aliases are visible in the output of the <command>modinfo</command>
  108. program and are usually related to the bus-specific identifiers of devices
  109. supported by a module. For example, the <emphasis>snd-fm801</emphasis>
  110. driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
  111. and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>.
  112. For most devices, the bus driver exports the alias of the driver that
  113. would handle the device via <systemitem
  114. class="filesystem">sysfs</systemitem>. E.g., the
  115. <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
  116. might contain the string
  117. <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>.
  118. The default rules provided with Udev will cause <command>udevd</command>
  119. to call out to <command>/sbin/modprobe</command> with the contents of the
  120. <envar>MODALIAS</envar> uevent environment variable (which should be the
  121. same as the contents of the <filename>modalias</filename> file in sysfs),
  122. thus loading all modules whose aliases match this string after wildcard
  123. expansion.</para>
  124. <para>In this example, this means that, in addition to
  125. <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
  126. <emphasis>forte</emphasis> driver will be loaded if it is
  127. available. See below for ways in which the loading of unwanted drivers can
  128. be prevented.</para>
  129. <para>The kernel itself is also able to load modules for network
  130. protocols, filesystems and NLS support on demand.</para>
  131. </sect3>
  132. <sect3>
  133. <title>Handling Hotpluggable/Dynamic Devices</title>
  134. <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
  135. player, the kernel recognizes that the device is now connected and
  136. generates a uevent. This uevent is then handled by
  137. <command>udevd</command> as described above.</para>
  138. </sect3>
  139. </sect2>
  140. <sect2>
  141. <title>Problems with Loading Modules and Creating Devices</title>
  142. <para>There are a few possible problems when it comes to automatically
  143. creating device nodes.</para>
  144. <sect3>
  145. <title>A kernel module is not loaded automatically</title>
  146. <para>Udev will only load a module if it has a bus-specific alias and the
  147. bus driver properly exports the necessary aliases to <systemitem
  148. class="filesystem">sysfs</systemitem>. In other cases, one should
  149. arrange module loading by other means. With Linux-&linux-version;, Udev is
  150. known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
  151. SERIO, and FireWire devices.</para>
  152. <para>To determine if the device driver you require has the necessary
  153. support for Udev, run <command>modinfo</command> with the module name as
  154. the argument. Now try locating the device directory under
  155. <filename class="directory">/sys/bus</filename> and check whether there is
  156. a <filename>modalias</filename> file there.</para>
  157. <para>If the <filename>modalias</filename> file exists in <systemitem
  158. class="filesystem">sysfs</systemitem>, the driver supports the device and
  159. can talk to it directly, but doesn't have the alias, it is a bug in the
  160. driver. Load the driver without the help from Udev and expect the issue
  161. to be fixed later.</para>
  162. <para>If there is no <filename>modalias</filename> file in the relevant
  163. directory under <filename class="directory">/sys/bus</filename>, this
  164. means that the kernel developers have not yet added modalias support to
  165. this bus type. With Linux-&linux-version;, this is the case with ISA
  166. busses. Expect this issue to be fixed in later kernel versions.</para>
  167. <para>Udev is not intended to load <quote>wrapper</quote> drivers such as
  168. <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
  169. <emphasis>loop</emphasis> at all.</para>
  170. </sect3>
  171. <sect3>
  172. <title>A kernel module is not loaded automatically, and Udev is not
  173. intended to load it</title>
  174. <para>If the <quote>wrapper</quote> module only enhances the functionality
  175. provided by some other module (e.g., <emphasis>snd-pcm-oss</emphasis>
  176. enhances the functionality of <emphasis>snd-pcm</emphasis> by making the
  177. sound cards available to OSS applications), configure
  178. <command>modprobe</command> to load the wrapper after Udev loads the
  179. wrapped module. To do this, add a <quote>softdep</quote> line in any
  180. <filename>/etc/modprobe.d/<replaceable>&lt;filename&gt;</replaceable>.conf</filename>
  181. file. For example:</para>
  182. <screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen>
  183. <para>Note that the <quote>softdep</quote> command also allows
  184. <literal>pre:</literal> dependencies, or a mixture of both
  185. <literal>pre:</literal> and <literal>post:</literal>. See the
  186. <filename>modprobe.d(5)</filename> manual page for more information
  187. on <quote>softdep</quote> syntax and capabilities.</para>
  188. <para>If the module in question is not a wrapper and is useful by itself,
  189. configure the <command>modules</command> bootscript to load this
  190. module on system boot. To do this, add the module name to the
  191. <filename>/etc/sysconfig/modules</filename> file on a separate line.
  192. This works for wrapper modules too, but is suboptimal in that case.</para>
  193. </sect3>
  194. <sect3>
  195. <title>Udev loads some unwanted module</title>
  196. <para>Either don't build the module, or blacklist it in a
  197. <filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the
  198. <emphasis>forte</emphasis> module in the example below:</para>
  199. <screen role="nodump"><literal>blacklist forte</literal></screen>
  200. <para>Blacklisted modules can still be loaded manually with the
  201. explicit <command>modprobe</command> command.</para>
  202. </sect3>
  203. <sect3>
  204. <title>Udev creates a device incorrectly, or makes a wrong symlink</title>
  205. <para>This usually happens if a rule unexpectedly matches a device. For
  206. example, a poorly-written rule can match both a SCSI disk (as desired)
  207. and the corresponding SCSI generic device (incorrectly) by vendor.
  208. Find the offending rule and make it more specific, with the help of the
  209. <command>udevadm info</command> command.</para>
  210. </sect3>
  211. <sect3>
  212. <title>Udev rule works unreliably</title>
  213. <para>This may be another manifestation of the previous problem. If not,
  214. and your rule uses <systemitem class="filesystem">sysfs</systemitem>
  215. attributes, it may be a kernel timing issue, to be fixed in later kernels.
  216. For now, you can work around it by creating a rule that waits for the used
  217. <systemitem class="filesystem">sysfs</systemitem> attribute and appending
  218. it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
  219. file (create this file if it does not exist). Please notify the LFS
  220. Development list if you do so and it helps.</para>
  221. </sect3>
  222. <sect3>
  223. <title>Udev does not create a device</title>
  224. <para>Further text assumes that the driver is built statically into the
  225. kernel or already loaded as a module, and that you have already checked
  226. that Udev doesn't create a misnamed device.</para>
  227. <para>Udev has no information needed to create a device node if a kernel
  228. driver does not export its data to <systemitem
  229. class="filesystem">sysfs</systemitem>.
  230. This is most common with third party drivers from outside the kernel
  231. tree. Create a static device node in
  232. <filename>/lib/udev/devices</filename> with the appropriate major/minor
  233. numbers (see the file <filename>devices.txt</filename> inside the kernel
  234. documentation or the documentation provided by the third party driver
  235. vendor). The static device node will be copied to
  236. <filename class="directory">/dev</filename> by the
  237. <command>udev</command> bootscript.</para>
  238. </sect3>
  239. <sect3>
  240. <title>Device naming order changes randomly after rebooting</title>
  241. <para>This is due to the fact that Udev, by design, handles uevents and
  242. loads modules in parallel, and thus in an unpredictable order. This will
  243. never be <quote>fixed</quote>. You should not rely upon the kernel device
  244. names being stable. Instead, create your own rules that make symlinks with
  245. stable names based on some stable attributes of the device, such as a
  246. serial number or the output of various *_id utilities installed by Udev.
  247. See <xref linkend="ch-scripts-symlinks"/> and
  248. <xref linkend="ch-scripts-network"/> for examples.</para>
  249. </sect3>
  250. </sect2>
  251. <sect2>
  252. <title>Useful Reading</title>
  253. <para>Additional helpful documentation is available at the following
  254. sites:</para>
  255. <itemizedlist>
  256. <listitem>
  257. <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
  258. <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
  259. </listitem>
  260. <listitem>
  261. <para>The <systemitem class="filesystem">sysfs</systemitem> Filesystem
  262. <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
  263. </listitem>
  264. </itemizedlist>
  265. </sect2>
  266. </sect1>