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-<?xml version="1.0" encoding="ISO-8859-1"?>
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-<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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- "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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- <!ENTITY % general-entities SYSTEM "../general.ent">
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- %general-entities;
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-]>
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-
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-<sect1 id="ch-scripts-udev" revision="systemd">
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- <?dbhtml filename="udev.html"?>
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-
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- <title>Device and Module Handling on an LFS System</title>
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-
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- <indexterm zone="ch-scripts-udev">
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- <primary sortas="a-Udev">Udev</primary>
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- <secondary>usage</secondary>
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- </indexterm>
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-
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- <para>In <xref linkend="chapter-building-system"/>, we installed Udev
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- from the systemd source package. Before we go into the details regarding
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- how this works, a brief history of previous methods of handling devices
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- is in order.</para>
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-
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- <para>Linux systems in general traditionally use a static device creation
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- method, whereby a great many device nodes are created under <filename
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- class="directory">/dev</filename> (sometimes literally thousands of nodes),
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- regardless of whether the corresponding hardware devices actually exist. This
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- is typically done via a <command>MAKEDEV</command> script, which contains a
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- number of calls to the <command>mknod</command> program with the relevant
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- major and minor device numbers for every possible device that might exist in
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- the world.</para>
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-
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- <para>Using the Udev method, only those devices which are detected by the
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- kernel get device nodes created for them. Because these device nodes will be
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- created each time the system boots, they will be stored on a <systemitem
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- class="filesystem">devtmpfs</systemitem> file system (a virtual file system
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- that resides entirely in system memory). Device nodes do not require much
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- space, so the memory that is used is negligible.</para>
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-
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- <sect2>
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- <title>History</title>
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-
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- <para>In February 2000, a new filesystem called <systemitem
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- class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
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- and was made available during the 2.4 series of stable kernels. Although
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- it was present in the kernel source itself, this method of creating devices
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- dynamically never received overwhelming support from the core kernel
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- developers.</para>
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-
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- <para>The main problem with the approach adopted by <systemitem
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- class="filesystem">devfs</systemitem> was the way it handled device
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- detection, creation, and naming. The latter issue, that of device node
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- naming, was perhaps the most critical. It is generally accepted that if
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- device names are allowed to be configurable, then the device naming policy
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- should be up to a system administrator, not imposed on them by any
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- particular developer(s). The <systemitem
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- class="filesystem">devfs</systemitem> file system also suffers from race
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- conditions that are inherent in its design and cannot be fixed without a
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- substantial revision to the kernel. It was marked as deprecated for a long
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- period – due to a lack of maintenance – and was finally removed
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- from the kernel in June, 2006.</para>
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-
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- <para>With the development of the unstable 2.5 kernel tree, later released
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- as the 2.6 series of stable kernels, a new virtual filesystem called
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- <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
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- <systemitem class="filesystem">sysfs</systemitem> is to export a view of
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- the system's hardware configuration to userspace processes. With this
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- userspace-visible representation, the possibility of seeing a userspace
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- replacement for <systemitem class="filesystem">devfs</systemitem> became
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- much more realistic.</para>
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-
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- </sect2>
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-
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- <sect2>
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- <title>Udev Implementation</title>
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-
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- <sect3>
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- <title>Sysfs</title>
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-
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- <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
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- was mentioned briefly above. One may wonder how <systemitem
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- class="filesystem">sysfs</systemitem> knows about the devices present on
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- a system and what device numbers should be used for them. Drivers that
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- have been compiled into the kernel directly register their objects with a
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- <systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally)
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- as they are detected by the kernel. For drivers compiled as modules, this
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- registration will happen when the module is loaded. Once the <systemitem
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- class="filesystem">sysfs</systemitem> filesystem is mounted (on /sys),
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- data which the drivers register with <systemitem
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- class="filesystem">sysfs</systemitem> are available to userspace
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- processes and to udevd for processing (including modifications to device
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- nodes).</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Device Node Creation</title>
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-
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- <para>Device files are created by the kernel by the <systemitem
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- class="filesystem">devtmpfs</systemitem> filesystem. Any driver that
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- wishes to register a device node will go through <systemitem
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- class="filesystem">devtmpfs</systemitem> (via the driver core) to do it.
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- When a <systemitem class="filesystem">devtmpfs</systemitem> instance is
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- mounted on <filename class="directory">/dev</filename>, the device node
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- will initially be created with a fixed name, permissions, and
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- owner.</para>
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-
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- <para>A short time later, the kernel will send a uevent to <command>
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- udevd</command>. Based on the rules specified in the files within the
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- <filename class="directory">/etc/udev/rules.d</filename>, <filename
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- class="directory">/lib/udev/rules.d</filename>, and <filename
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- class="directory">/run/udev/rules.d</filename> directories, <command>
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- udevd</command> will create additional symlinks to the device node, or
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- change its permissions, owner, or group, or modify the internal
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- <command>udevd</command> database entry (name) for that object.</para>
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-
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- <para>The rules in these three directories are numbered in a similar
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- fashion to the LFS-Bootscripts package and all three directories are
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- merged together. If <command>udevd</command> can't find a rule for the
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- device it is creating, it will leave the permissions and ownership at
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- whatever <systemitem class="filesystem">devtmpfs</systemitem> used
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- initially.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Module Loading</title>
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-
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- <para>Device drivers compiled as modules may have aliases built into them.
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- Aliases are visible in the output of the <command>modinfo</command>
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- program and are usually related to the bus-specific identifiers of devices
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- supported by a module. For example, the <emphasis>snd-fm801</emphasis>
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- driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
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- and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>.
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- For most devices, the bus driver exports the alias of the driver that
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- would handle the device via <systemitem
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- class="filesystem">sysfs</systemitem>. E.g., the
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- <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
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- might contain the string
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- <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>.
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- The default rules provided with Udev will cause <command>udevd</command>
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- to call out to <command>/sbin/modprobe</command> with the contents of the
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- <envar>MODALIAS</envar> uevent environment variable (which should be the
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- same as the contents of the <filename>modalias</filename> file in sysfs),
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- thus loading all modules whose aliases match this string after wildcard
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- expansion.</para>
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-
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- <para>In this example, this means that, in addition to
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- <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
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- <emphasis>forte</emphasis> driver will be loaded if it is
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- available. See below for ways in which the loading of unwanted drivers can
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- be prevented.</para>
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-
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- <para>The kernel itself is also able to load modules for network
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- protocols, filesystems and NLS support on demand.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Handling Hotpluggable/Dynamic Devices</title>
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-
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- <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
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- player, the kernel recognizes that the device is now connected and
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- generates a uevent. This uevent is then handled by
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- <command>udevd</command> as described above.</para>
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-
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- </sect3>
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-
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- </sect2>
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-
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- <sect2>
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- <title>Problems with Loading Modules and Creating Devices</title>
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-
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- <para>There are a few possible problems when it comes to automatically
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- creating device nodes.</para>
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-
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- <sect3>
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- <title>A kernel module is not loaded automatically</title>
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-
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- <para>Udev will only load a module if it has a bus-specific alias and the
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- bus driver properly exports the necessary aliases to <systemitem
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- class="filesystem">sysfs</systemitem>. In other cases, one should
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- arrange module loading by other means. With Linux-&linux-version;, Udev is
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- known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
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- SERIO, and FireWire devices.</para>
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-
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- <para>To determine if the device driver you require has the necessary
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- support for Udev, run <command>modinfo</command> with the module name as
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- the argument. Now try locating the device directory under
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- <filename class="directory">/sys/bus</filename> and check whether there is
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- a <filename>modalias</filename> file there.</para>
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-
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- <para>If the <filename>modalias</filename> file exists in <systemitem
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- class="filesystem">sysfs</systemitem>, the driver supports the device and
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- can talk to it directly, but doesn't have the alias, it is a bug in the
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- driver. Load the driver without the help from Udev and expect the issue
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- to be fixed later.</para>
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-
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- <para>If there is no <filename>modalias</filename> file in the relevant
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- directory under <filename class="directory">/sys/bus</filename>, this
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- means that the kernel developers have not yet added modalias support to
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- this bus type. With Linux-&linux-version;, this is the case with ISA
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- busses. Expect this issue to be fixed in later kernel versions.</para>
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-
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- <para>Udev is not intended to load <quote>wrapper</quote> drivers such as
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- <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
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- <emphasis>loop</emphasis> at all.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>A kernel module is not loaded automatically, and Udev is not
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- intended to load it</title>
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-
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- <para>If the <quote>wrapper</quote> module only enhances the functionality
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- provided by some other module (e.g., <emphasis>snd-pcm-oss</emphasis>
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- enhances the functionality of <emphasis>snd-pcm</emphasis> by making the
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- sound cards available to OSS applications), configure
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- <command>modprobe</command> to load the wrapper after Udev loads the
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- wrapped module. To do this, add a <quote>softdep</quote> line in any
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- <filename>/etc/modprobe.d/<replaceable><filename></replaceable>.conf</filename>
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- file. For example:</para>
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-
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-<screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen>
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-
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- <para>Note that the <quote>softdep</quote> command also allows
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- <literal>pre:</literal> dependencies, or a mixture of both
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- <literal>pre:</literal> and <literal>post:</literal>. See the
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- <filename>modprobe.d(5)</filename> manual page for more information
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- on <quote>softdep</quote> syntax and capabilities.</para>
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-
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- <para>If the module in question is not a wrapper and is useful by itself,
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- configure the <command>modules</command> bootscript to load this
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- module on system boot. To do this, add the module name to the
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- <filename>/etc/sysconfig/modules</filename> file on a separate line.
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- This works for wrapper modules too, but is suboptimal in that case.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Udev loads some unwanted module</title>
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-
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- <para>Either don't build the module, or blacklist it in a
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- <filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the
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- <emphasis>forte</emphasis> module in the example below:</para>
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-
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-<screen role="nodump"><literal>blacklist forte</literal></screen>
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-
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- <para>Blacklisted modules can still be loaded manually with the
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- explicit <command>modprobe</command> command.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Udev creates a device incorrectly, or makes a wrong symlink</title>
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-
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- <para>This usually happens if a rule unexpectedly matches a device. For
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- example, a poorly-written rule can match both a SCSI disk (as desired)
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- and the corresponding SCSI generic device (incorrectly) by vendor.
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- Find the offending rule and make it more specific, with the help of the
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- <command>udevadm info</command> command.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Udev rule works unreliably</title>
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-
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- <para>This may be another manifestation of the previous problem. If not,
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- and your rule uses <systemitem class="filesystem">sysfs</systemitem>
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- attributes, it may be a kernel timing issue, to be fixed in later kernels.
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- For now, you can work around it by creating a rule that waits for the used
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- <systemitem class="filesystem">sysfs</systemitem> attribute and appending
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- it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
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- file (create this file if it does not exist). Please notify the LFS
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- Development list if you do so and it helps.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Udev does not create a device</title>
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-
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- <para>Further text assumes that the driver is built statically into the
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- kernel or already loaded as a module, and that you have already checked
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- that Udev doesn't create a misnamed device.</para>
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-
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- <para>Udev has no information needed to create a device node if a kernel
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- driver does not export its data to <systemitem
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- class="filesystem">sysfs</systemitem>.
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- This is most common with third party drivers from outside the kernel
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- tree. Create a static device node in
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- <filename>/lib/udev/devices</filename> with the appropriate major/minor
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- numbers (see the file <filename>devices.txt</filename> inside the kernel
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- documentation or the documentation provided by the third party driver
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- vendor). The static device node will be copied to
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- <filename class="directory">/dev</filename> by the
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- <command>udev</command> bootscript.</para>
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-
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- </sect3>
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-
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- <sect3>
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- <title>Device naming order changes randomly after rebooting</title>
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-
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- <para>This is due to the fact that Udev, by design, handles uevents and
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- loads modules in parallel, and thus in an unpredictable order. This will
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- never be <quote>fixed</quote>. You should not rely upon the kernel device
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- names being stable. Instead, create your own rules that make symlinks with
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- stable names based on some stable attributes of the device, such as a
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- serial number or the output of various *_id utilities installed by Udev.
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- See <xref linkend="ch-scripts-symlinks"/> and
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- <xref linkend="ch-scripts-network"/> for examples.</para>
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-
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- </sect3>
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-
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- </sect2>
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-
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- <sect2>
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- <title>Useful Reading</title>
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-
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- <para>Additional helpful documentation is available at the following
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- sites:</para>
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-
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- <itemizedlist>
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-
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- <listitem>
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- <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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- <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
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- </listitem>
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-
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- <listitem>
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- <para>The <systemitem class="filesystem">sysfs</systemitem> Filesystem
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- <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
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- </listitem>
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-
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- </itemizedlist>
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-
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- </sect2>
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-
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-</sect1>
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DJ Lucas