[7152faa] | 1 | <?xml version="1.0" encoding="UTF-8"?>
|
---|
[b06ca36] | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
---|
| 3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
|
---|
[bce08ef] | 4 | <!ENTITY % general-entities SYSTEM "../general.ent">
|
---|
| 5 | %general-entities;
|
---|
| 6 | ]>
|
---|
[d781ffb] | 7 |
|
---|
[afcfd74] | 8 | <sect1 id="ch-config-udev">
|
---|
[d781ffb] | 9 | <?dbhtml filename="udev.html"?>
|
---|
| 10 |
|
---|
[bf58c1e] | 11 | <title>Overview of Device and Module Handling</title>
|
---|
[d781ffb] | 12 |
|
---|
[afcfd74] | 13 | <indexterm zone="ch-config-udev">
|
---|
[d781ffb] | 14 | <primary sortas="a-Udev">Udev</primary>
|
---|
| 15 | <secondary>usage</secondary>
|
---|
| 16 | </indexterm>
|
---|
| 17 |
|
---|
[a3d0817] | 18 | <para>In <xref linkend="chapter-building-system"/>, we installed the udev
|
---|
[b1774158] | 19 | daemon when <phrase revision="sysv">udev</phrase>
|
---|
[9a0d741] | 20 | <phrase revision="systemd">systemd</phrase> was built. Before we go into the
|
---|
[6ef5053] | 21 | details regarding how udev works, a brief history of previous methods of
|
---|
[9a0d741] | 22 | handling devices is in order.</para>
|
---|
[d781ffb] | 23 |
|
---|
[bf58c1e] | 24 | <para>Linux systems in general traditionally used a static device creation
|
---|
| 25 | method, whereby a great many device nodes were created under <filename
|
---|
[d781ffb] | 26 | class="directory">/dev</filename> (sometimes literally thousands of nodes),
|
---|
[bf58c1e] | 27 | regardless of whether the corresponding hardware devices actually existed. This
|
---|
[6ef5053] | 28 | was typically done via a <command>MAKEDEV</command> script, which contained a
|
---|
[d2c332bc] | 29 | number of calls to the <command>mknod</command> program with the relevant
|
---|
| 30 | major and minor device numbers for every possible device that might exist in
|
---|
| 31 | the world.</para>
|
---|
| 32 |
|
---|
[6ef5053] | 33 | <para>Using the udev method, device nodes are only created for those devices
|
---|
| 34 | which are detected by the kernel. These device nodes are
|
---|
| 35 | created each time the system boots; they are stored in a <systemitem
|
---|
[841b723] | 36 | class="filesystem">devtmpfs</systemitem> file system (a virtual file system
|
---|
| 37 | that resides entirely in system memory). Device nodes do not require much
|
---|
| 38 | space, so the memory that is used is negligible.</para>
|
---|
[d781ffb] | 39 |
|
---|
| 40 | <sect2>
|
---|
| 41 | <title>History</title>
|
---|
| 42 |
|
---|
| 43 | <para>In February 2000, a new filesystem called <systemitem
|
---|
| 44 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
|
---|
| 45 | and was made available during the 2.4 series of stable kernels. Although
|
---|
| 46 | it was present in the kernel source itself, this method of creating devices
|
---|
| 47 | dynamically never received overwhelming support from the core kernel
|
---|
| 48 | developers.</para>
|
---|
| 49 |
|
---|
| 50 | <para>The main problem with the approach adopted by <systemitem
|
---|
| 51 | class="filesystem">devfs</systemitem> was the way it handled device
|
---|
| 52 | detection, creation, and naming. The latter issue, that of device node
|
---|
| 53 | naming, was perhaps the most critical. It is generally accepted that if
|
---|
[6ef5053] | 54 | device names are configurable, the device naming policy
|
---|
| 55 | should be chosen by system administrators, and not imposed on them by the
|
---|
| 56 | developer(s). The <systemitem
|
---|
[bf58c1e] | 57 | class="filesystem">devfs</systemitem> file system also suffered from race
|
---|
[6ef5053] | 58 | conditions that were inherent in its design; these could not be fixed without a
|
---|
| 59 | substantial revision of the kernel. <systemitem class="filesystem">devfs</systemitem>
|
---|
| 60 | was marked as deprecated for a long
|
---|
| 61 | time, and was finally removed
|
---|
[8d2b5a3] | 62 | from the kernel in June, 2006.</para>
|
---|
[d2c332bc] | 63 |
|
---|
| 64 | <para>With the development of the unstable 2.5 kernel tree, later released
|
---|
| 65 | as the 2.6 series of stable kernels, a new virtual filesystem called
|
---|
| 66 | <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
|
---|
[6ef5053] | 67 | <systemitem class="filesystem">sysfs</systemitem> is to provide information about
|
---|
[d2c332bc] | 68 | the system's hardware configuration to userspace processes. With this
|
---|
[6ef5053] | 69 | userspace-visible representation, it became possible to develop a userspace
|
---|
| 70 | replacement for <systemitem class="filesystem">devfs</systemitem>.</para>
|
---|
[d781ffb] | 71 |
|
---|
| 72 | </sect2>
|
---|
| 73 |
|
---|
| 74 | <sect2>
|
---|
| 75 | <title>Udev Implementation</title>
|
---|
| 76 |
|
---|
[d2c332bc] | 77 | <sect3>
|
---|
| 78 | <title>Sysfs</title>
|
---|
| 79 |
|
---|
[5cef314] | 80 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
|
---|
| 81 | was mentioned briefly above. One may wonder how <systemitem
|
---|
[d2c332bc] | 82 | class="filesystem">sysfs</systemitem> knows about the devices present on
|
---|
| 83 | a system and what device numbers should be used for them. Drivers that
|
---|
[6ef5053] | 84 | have been compiled into the kernel register their objects in
|
---|
[5cef314] | 85 | <systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally)
|
---|
[6ef5053] | 86 | as they are detected by the kernel. For drivers compiled as modules,
|
---|
| 87 | registration happens when the module is loaded. Once the <systemitem
|
---|
[6beb8ce] | 88 | class="filesystem">sysfs</systemitem> filesystem is mounted (on
|
---|
| 89 | <filename class="directory">/sys</filename>),
|
---|
[6ef5053] | 90 | data which the drivers have registered with <systemitem
|
---|
[5cef314] | 91 | class="filesystem">sysfs</systemitem> are available to userspace
|
---|
| 92 | processes and to udevd for processing (including modifications to device
|
---|
[2ca8941] | 93 | nodes).</para>
|
---|
[2a1c1e0] | 94 |
|
---|
[d2c332bc] | 95 | </sect3>
|
---|
| 96 |
|
---|
[0058818] | 97 | <sect3 id='ch-config-udev-device-node-creation'>
|
---|
[d2c332bc] | 98 | <title>Device Node Creation</title>
|
---|
| 99 |
|
---|
[6ef5053] | 100 | <para>Device files are created by the kernel in the <systemitem
|
---|
| 101 | class="filesystem">devtmpfs</systemitem> file system. Any driver that
|
---|
| 102 | wishes to register a device node will use the <systemitem
|
---|
[841b723] | 103 | class="filesystem">devtmpfs</systemitem> (via the driver core) to do it.
|
---|
| 104 | When a <systemitem class="filesystem">devtmpfs</systemitem> instance is
|
---|
| 105 | mounted on <filename class="directory">/dev</filename>, the device node
|
---|
[6ef5053] | 106 | will initially be exposed to userspace with a fixed name, permissions, and
|
---|
[5cef314] | 107 | owner.</para>
|
---|
[841b723] | 108 |
|
---|
| 109 | <para>A short time later, the kernel will send a uevent to <command>
|
---|
| 110 | udevd</command>. Based on the rules specified in the files within the
|
---|
| 111 | <filename class="directory">/etc/udev/rules.d</filename>, <filename
|
---|
[d7a9421] | 112 | class="directory">/usr/lib/udev/rules.d</filename>, and <filename
|
---|
[841b723] | 113 | class="directory">/run/udev/rules.d</filename> directories, <command>
|
---|
[5cef314] | 114 | udevd</command> will create additional symlinks to the device node, or
|
---|
| 115 | change its permissions, owner, or group, or modify the internal
|
---|
| 116 | <command>udevd</command> database entry (name) for that object.</para>
|
---|
[841b723] | 117 |
|
---|
[bf58c1e] | 118 | <para>The rules in these three directories are numbered and all three
|
---|
| 119 | directories are merged together. If <command>udevd</command> can't find a
|
---|
| 120 | rule for the device it is creating, it will leave the permissions and
|
---|
| 121 | ownership at whatever <systemitem
|
---|
| 122 | class="filesystem">devtmpfs</systemitem> used initially.</para> </sect3>
|
---|
[c34b4fb] | 123 |
|
---|
[36d2dcc] | 124 | <sect3 id="module-loading">
|
---|
[d2c332bc] | 125 | <title>Module Loading</title>
|
---|
| 126 |
|
---|
| 127 | <para>Device drivers compiled as modules may have aliases built into them.
|
---|
| 128 | Aliases are visible in the output of the <command>modinfo</command>
|
---|
| 129 | program and are usually related to the bus-specific identifiers of devices
|
---|
| 130 | supported by a module. For example, the <emphasis>snd-fm801</emphasis>
|
---|
| 131 | driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
|
---|
[360fdfca] | 132 | and has an alias of <literal>pci:v00001319d00000801sv*sd*bc04sc01i*</literal>.
|
---|
[d2c332bc] | 133 | For most devices, the bus driver exports the alias of the driver that
|
---|
| 134 | would handle the device via <systemitem
|
---|
| 135 | class="filesystem">sysfs</systemitem>. E.g., the
|
---|
| 136 | <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
|
---|
| 137 | might contain the string
|
---|
[360fdfca] | 138 | <literal>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</literal>.
|
---|
[a3d0817] | 139 | The default rules provided with udev will cause <command>udevd</command>
|
---|
[61e63d3] | 140 | to call out to <command>/sbin/modprobe</command> with the contents of the
|
---|
| 141 | <envar>MODALIAS</envar> uevent environment variable (which should be the
|
---|
[d2c332bc] | 142 | same as the contents of the <filename>modalias</filename> file in sysfs),
|
---|
| 143 | thus loading all modules whose aliases match this string after wildcard
|
---|
| 144 | expansion.</para>
|
---|
| 145 |
|
---|
| 146 | <para>In this example, this means that, in addition to
|
---|
| 147 | <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
|
---|
| 148 | <emphasis>forte</emphasis> driver will be loaded if it is
|
---|
| 149 | available. See below for ways in which the loading of unwanted drivers can
|
---|
| 150 | be prevented.</para>
|
---|
| 151 |
|
---|
| 152 | <para>The kernel itself is also able to load modules for network
|
---|
[a3d0817] | 153 | protocols, filesystems, and NLS support on demand.</para>
|
---|
[d2c332bc] | 154 |
|
---|
| 155 | </sect3>
|
---|
| 156 |
|
---|
| 157 | <sect3>
|
---|
| 158 | <title>Handling Hotpluggable/Dynamic Devices</title>
|
---|
| 159 |
|
---|
| 160 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
|
---|
| 161 | player, the kernel recognizes that the device is now connected and
|
---|
[a0d96d25] | 162 | generates a uevent. This uevent is then handled by
|
---|
[d2c332bc] | 163 | <command>udevd</command> as described above.</para>
|
---|
| 164 |
|
---|
| 165 | </sect3>
|
---|
[d781ffb] | 166 |
|
---|
| 167 | </sect2>
|
---|
| 168 |
|
---|
| 169 | <sect2>
|
---|
[d2c332bc] | 170 | <title>Problems with Loading Modules and Creating Devices</title>
|
---|
[d781ffb] | 171 |
|
---|
[d2c332bc] | 172 | <para>There are a few possible problems when it comes to automatically
|
---|
| 173 | creating device nodes.</para>
|
---|
[d781ffb] | 174 |
|
---|
[d2c332bc] | 175 | <sect3>
|
---|
[6ef5053] | 176 | <title>A Kernel Module Is Not Loaded Automatically</title>
|
---|
[d781ffb] | 177 |
|
---|
[d2c332bc] | 178 | <para>Udev will only load a module if it has a bus-specific alias and the
|
---|
| 179 | bus driver properly exports the necessary aliases to <systemitem
|
---|
| 180 | class="filesystem">sysfs</systemitem>. In other cases, one should
|
---|
[a3d0817] | 181 | arrange module loading by other means. With Linux-&linux-version;, udev is
|
---|
[a0d96d25] | 182 | known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
|
---|
[0cda898] | 183 | SERIO, and FireWire devices.</para>
|
---|
[d781ffb] | 184 |
|
---|
[d2c332bc] | 185 | <para>To determine if the device driver you require has the necessary
|
---|
[a3d0817] | 186 | support for udev, run <command>modinfo</command> with the module name as
|
---|
[d2c332bc] | 187 | the argument. Now try locating the device directory under
|
---|
| 188 | <filename class="directory">/sys/bus</filename> and check whether there is
|
---|
| 189 | a <filename>modalias</filename> file there.</para>
|
---|
| 190 |
|
---|
| 191 | <para>If the <filename>modalias</filename> file exists in <systemitem
|
---|
| 192 | class="filesystem">sysfs</systemitem>, the driver supports the device and
|
---|
| 193 | can talk to it directly, but doesn't have the alias, it is a bug in the
|
---|
[a3d0817] | 194 | driver. Load the driver without the help from udev and expect the issue
|
---|
[d2c332bc] | 195 | to be fixed later.</para>
|
---|
[d781ffb] | 196 |
|
---|
[d2c332bc] | 197 | <para>If there is no <filename>modalias</filename> file in the relevant
|
---|
| 198 | directory under <filename class="directory">/sys/bus</filename>, this
|
---|
| 199 | means that the kernel developers have not yet added modalias support to
|
---|
[a0d96d25] | 200 | this bus type. With Linux-&linux-version;, this is the case with ISA
|
---|
| 201 | busses. Expect this issue to be fixed in later kernel versions.</para>
|
---|
[d2c332bc] | 202 |
|
---|
| 203 | <para>Udev is not intended to load <quote>wrapper</quote> drivers such as
|
---|
| 204 | <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
|
---|
| 205 | <emphasis>loop</emphasis> at all.</para>
|
---|
| 206 |
|
---|
| 207 | </sect3>
|
---|
| 208 |
|
---|
| 209 | <sect3>
|
---|
[6ef5053] | 210 | <title>A Kernel Module Is Not Loaded Automatically, and Udev Is Not
|
---|
| 211 | Intended to Load It</title>
|
---|
[d2c332bc] | 212 |
|
---|
[0d84af1] | 213 | <para>If the <quote>wrapper</quote> module only enhances the
|
---|
| 214 | functionality provided by some other module (e.g.,
|
---|
| 215 | <emphasis>snd-pcm-oss</emphasis> enhances the functionality of
|
---|
| 216 | <emphasis>snd-pcm</emphasis> by making the sound cards available to OSS
|
---|
| 217 | applications), configure <command>modprobe</command> to load the wrapper
|
---|
[a3d0817] | 218 | after udev loads the wrapped module. To do this, add a
|
---|
[0d84af1] | 219 | <quote>softdep</quote> line to the corresponding
|
---|
[b67f2d6] | 220 | <filename>/etc/modprobe.d/<replaceable><filename></replaceable>.conf</filename>
|
---|
| 221 | file. For example:</para>
|
---|
[d2c332bc] | 222 |
|
---|
[841b723] | 223 | <screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen>
|
---|
| 224 |
|
---|
| 225 | <para>Note that the <quote>softdep</quote> command also allows
|
---|
| 226 | <literal>pre:</literal> dependencies, or a mixture of both
|
---|
[c34b4fb] | 227 | <literal>pre:</literal> and <literal>post:</literal> dependencies. See
|
---|
[ea93c11] | 228 | the <ulink role='man' url='&man;modprobe.d.5'>modprobe.d(5)</ulink>
|
---|
| 229 | manual page for more information on <quote>softdep</quote> syntax and
|
---|
| 230 | capabilities.</para>
|
---|
[d2c332bc] | 231 |
|
---|
[c34b4fb] | 232 | <para revision="sysv">If the module in question is not a wrapper and is
|
---|
| 233 | useful by itself, configure the <command>modules</command> bootscript to
|
---|
[9e7475a] | 234 | load this module on system boot. To do this, add the module name to the
|
---|
[d2c332bc] | 235 | <filename>/etc/sysconfig/modules</filename> file on a separate line.
|
---|
| 236 | This works for wrapper modules too, but is suboptimal in that case.</para>
|
---|
| 237 |
|
---|
| 238 | </sect3>
|
---|
| 239 |
|
---|
| 240 | <sect3>
|
---|
[6ef5053] | 241 | <title>Udev Loads Some Unwanted Module</title>
|
---|
[d2c332bc] | 242 |
|
---|
[b67f2d6] | 243 | <para>Either don't build the module, or blacklist it in a
|
---|
| 244 | <filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the
|
---|
[d2c332bc] | 245 | <emphasis>forte</emphasis> module in the example below:</para>
|
---|
| 246 |
|
---|
| 247 | <screen role="nodump"><literal>blacklist forte</literal></screen>
|
---|
| 248 |
|
---|
| 249 | <para>Blacklisted modules can still be loaded manually with the
|
---|
| 250 | explicit <command>modprobe</command> command.</para>
|
---|
| 251 |
|
---|
| 252 | </sect3>
|
---|
| 253 |
|
---|
| 254 | <sect3>
|
---|
[6ef5053] | 255 | <title>Udev Creates a Device Incorrectly, or Makes the Wrong Symlink</title>
|
---|
[d2c332bc] | 256 |
|
---|
| 257 | <para>This usually happens if a rule unexpectedly matches a device. For
|
---|
[edbeeb5] | 258 | example, a poorly-written rule can match both a SCSI disk (as desired)
|
---|
[d2c332bc] | 259 | and the corresponding SCSI generic device (incorrectly) by vendor.
|
---|
[61e63d3] | 260 | Find the offending rule and make it more specific, with the help of the
|
---|
| 261 | <command>udevadm info</command> command.</para>
|
---|
[d2c332bc] | 262 |
|
---|
| 263 | </sect3>
|
---|
| 264 |
|
---|
| 265 | <sect3>
|
---|
[6ef5053] | 266 | <title>Udev Rule Works Unreliably</title>
|
---|
[d2c332bc] | 267 |
|
---|
| 268 | <para>This may be another manifestation of the previous problem. If not,
|
---|
| 269 | and your rule uses <systemitem class="filesystem">sysfs</systemitem>
|
---|
| 270 | attributes, it may be a kernel timing issue, to be fixed in later kernels.
|
---|
| 271 | For now, you can work around it by creating a rule that waits for the used
|
---|
| 272 | <systemitem class="filesystem">sysfs</systemitem> attribute and appending
|
---|
| 273 | it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
|
---|
[61e63d3] | 274 | file (create this file if it does not exist). Please notify the LFS
|
---|
| 275 | Development list if you do so and it helps.</para>
|
---|
[d2c332bc] | 276 |
|
---|
| 277 | </sect3>
|
---|
| 278 |
|
---|
[9a0d741] | 279 | <sect3>
|
---|
[6ef5053] | 280 | <title>Udev Does Not Create a Device</title>
|
---|
[9a0d741] | 281 |
|
---|
[6ef5053] | 282 | <para>First, be certain that the driver is built into the
|
---|
| 283 | kernel or already loaded as a module, and that
|
---|
| 284 | udev isn't creating a misnamed device.</para>
|
---|
[9a0d741] | 285 |
|
---|
[6ef5053] | 286 | <para>If a kernel driver does not export its data to
|
---|
| 287 | <systemitem class="filesystem">sysfs</systemitem>, udev lacks the
|
---|
| 288 | information needed to create a device node. This is most likely to happen
|
---|
[9a0d741] | 289 | with third party drivers from outside the kernel tree. Create a static
|
---|
[d7a9421] | 290 | device node in <filename>/usr/lib/udev/devices</filename> with the
|
---|
[9a0d741] | 291 | appropriate major/minor numbers (see the file
|
---|
| 292 | <filename>devices.txt</filename> inside the kernel documentation or the
|
---|
| 293 | documentation provided by the third party driver vendor). The static
|
---|
| 294 | device node will be copied to <filename class="directory">/dev</filename>
|
---|
| 295 | by <command>udev</command>.</para>
|
---|
| 296 |
|
---|
| 297 | </sect3>
|
---|
| 298 |
|
---|
[d2c332bc] | 299 | <sect3>
|
---|
[6ef5053] | 300 | <title>Device Naming Order Changes Randomly After Rebooting</title>
|
---|
[d781ffb] | 301 |
|
---|
[a3d0817] | 302 | <para>This is due to the fact that udev, by design, handles uevents and
|
---|
[d2c332bc] | 303 | loads modules in parallel, and thus in an unpredictable order. This will
|
---|
[360fdfca] | 304 | never be <quote>fixed.</quote> You should not rely upon the kernel device
|
---|
[d2c332bc] | 305 | names being stable. Instead, create your own rules that make symlinks with
|
---|
| 306 | stable names based on some stable attributes of the device, such as a
|
---|
[a3d0817] | 307 | serial number or the output of various *_id utilities installed by udev.
|
---|
[afcfd74] | 308 | See <xref linkend="ch-config-symlinks"/> and
|
---|
| 309 | <xref linkend="ch-config-network"/> for examples.</para>
|
---|
[d2c332bc] | 310 |
|
---|
| 311 | </sect3>
|
---|
[81fd230] | 312 |
|
---|
[d781ffb] | 313 | </sect2>
|
---|
[81fd230] | 314 |
|
---|
[d781ffb] | 315 | <sect2>
|
---|
| 316 | <title>Useful Reading</title>
|
---|
[81fd230] | 317 |
|
---|
[d781ffb] | 318 | <para>Additional helpful documentation is available at the following
|
---|
| 319 | sites:</para>
|
---|
[81fd230] | 320 |
|
---|
[d781ffb] | 321 | <itemizedlist>
|
---|
[bce08ef] | 322 |
|
---|
[d781ffb] | 323 | <listitem>
|
---|
| 324 | <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
|
---|
| 325 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
|
---|
| 326 | </listitem>
|
---|
| 327 |
|
---|
| 328 | <listitem>
|
---|
[e3bc7f5] | 329 | <para>The <systemitem class="filesystem">sysfs</systemitem> Filesystem
|
---|
[fb66c4c] | 330 | <ulink url="https://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
|
---|
[d781ffb] | 331 | </listitem>
|
---|
| 332 |
|
---|
[5cef314] | 333 | <!-- No longer available
|
---|
[d781ffb] | 334 | <listitem>
|
---|
[e3bc7f5] | 335 | <para>Pointers to further reading
|
---|
| 336 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html"/>
|
---|
| 337 | </para>
|
---|
[d781ffb] | 338 | </listitem>
|
---|
[5cef314] | 339 | -->
|
---|
[d781ffb] | 340 | </itemizedlist>
|
---|
| 341 |
|
---|
| 342 | </sect2>
|
---|
| 343 |
|
---|
| 344 | </sect1>
|
---|