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diff --git a/chapter07/udev.xml b/chapter07/udev.xml deleted file mode 100644 index 6060849be..000000000 --- a/chapter07/udev.xml +++ /dev/null @@ -1,342 +0,0 @@ -<?xml version="1.0" encoding="ISO-8859-1"?> -<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" - "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [ - <!ENTITY % general-entities SYSTEM "../general.ent"> - %general-entities; -]> - -<sect1 id="ch-config-udev"> - <?dbhtml filename="udev.html"?> - - <title>Overview of Device and Module Handling</title> - - <indexterm zone="ch-config-udev"> - <primary sortas="a-Udev">Udev</primary> - <secondary>usage</secondary> - </indexterm> - - <para>In <xref linkend="chapter-building-system"/>, we installed the Udev - package when <phrase revision="sysv">eudev</phrase> - <phrase revision="systemd">systemd</phrase> was built. Before we go into the - details regarding how this works, a brief history of previous methods of - handling devices is in order.</para> - - <para>Linux systems in general traditionally used a static device creation - method, whereby a great many device nodes were created under <filename - class="directory">/dev</filename> (sometimes literally thousands of nodes), - regardless of whether the corresponding hardware devices actually existed. This - was typically done via a <command>MAKEDEV</command> script, which contains a - number of calls to the <command>mknod</command> program with the relevant - major and minor device numbers for every possible device that might exist in - the world.</para> - - <para>Using the Udev method, only those devices which are detected by the - kernel get device nodes created for them. Because these device nodes will be - created each time the system boots, they will be stored on a <systemitem - class="filesystem">devtmpfs</systemitem> file system (a virtual file system - that resides entirely in system memory). Device nodes do not require much - space, so the memory that is used is negligible.</para> - - <sect2> - <title>History</title> - - <para>In February 2000, a new filesystem called <systemitem - class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel - and was made available during the 2.4 series of stable kernels. Although - it was present in the kernel source itself, this method of creating devices - dynamically never received overwhelming support from the core kernel - developers.</para> - - <para>The main problem with the approach adopted by <systemitem - class="filesystem">devfs</systemitem> was the way it handled device - detection, creation, and naming. The latter issue, that of device node - naming, was perhaps the most critical. It is generally accepted that if - device names are allowed to be configurable, then the device naming policy - should be up to a system administrator, not imposed on them by any - particular developer(s). The <systemitem - class="filesystem">devfs</systemitem> file system also suffered from race - conditions that were inherent in its design and could not be fixed without a - substantial revision to the kernel. It was marked as deprecated for a long - period – due to a lack of maintenance – and was finally removed - from the kernel in June, 2006.</para> - - <para>With the development of the unstable 2.5 kernel tree, later released - as the 2.6 series of stable kernels, a new virtual filesystem called - <systemitem class="filesystem">sysfs</systemitem> came to be. The job of - <systemitem class="filesystem">sysfs</systemitem> is to export a view of - the system's hardware configuration to userspace processes. With this - userspace-visible representation, the possibility of developing a userspace - replacement for <systemitem class="filesystem">devfs</systemitem> became - much more realistic.</para> - - </sect2> - - <sect2> - <title>Udev Implementation</title> - - <sect3> - <title>Sysfs</title> - - <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem - was mentioned briefly above. One may wonder how <systemitem - class="filesystem">sysfs</systemitem> knows about the devices present on - a system and what device numbers should be used for them. Drivers that - have been compiled into the kernel directly register their objects with a - <systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally) - as they are detected by the kernel. For drivers compiled as modules, this - registration will happen when the module is loaded. Once the <systemitem - class="filesystem">sysfs</systemitem> filesystem is mounted (on /sys), - data which the drivers register with <systemitem - class="filesystem">sysfs</systemitem> are available to userspace - processes and to udevd for processing (including modifications to device - nodes).</para> - - </sect3> - - <sect3> - <title>Device Node Creation</title> - - <para>Device files are created by the kernel by the <systemitem - class="filesystem">devtmpfs</systemitem> filesystem. Any driver that - wishes to register a device node will go through the <systemitem - class="filesystem">devtmpfs</systemitem> (via the driver core) to do it. - When a <systemitem class="filesystem">devtmpfs</systemitem> instance is - mounted on <filename class="directory">/dev</filename>, the device node - will initially be created with a fixed name, permissions, and - owner.</para> - - <para>A short time later, the kernel will send a uevent to <command> - udevd</command>. Based on the rules specified in the files within the - <filename class="directory">/etc/udev/rules.d</filename>, <filename - class="directory">/lib/udev/rules.d</filename>, and <filename - class="directory">/run/udev/rules.d</filename> directories, <command> - udevd</command> will create additional symlinks to the device node, or - change its permissions, owner, or group, or modify the internal - <command>udevd</command> database entry (name) for that object.</para> - - <para>The rules in these three directories are numbered and all three - directories are merged together. If <command>udevd</command> can't find a - rule for the device it is creating, it will leave the permissions and - ownership at whatever <systemitem - class="filesystem">devtmpfs</systemitem> used initially.</para> </sect3> - - <sect3 id="module-loading"> - <title>Module Loading</title> - - <para>Device drivers compiled as modules may have aliases built into them. - Aliases are visible in the output of the <command>modinfo</command> - program and are usually related to the bus-specific identifiers of devices - supported by a module. For example, the <emphasis>snd-fm801</emphasis> - driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801, - and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>. - For most devices, the bus driver exports the alias of the driver that - would handle the device via <systemitem - class="filesystem">sysfs</systemitem>. E.g., the - <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file - might contain the string - <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>. - The default rules provided with Udev will cause <command>udevd</command> - to call out to <command>/sbin/modprobe</command> with the contents of the - <envar>MODALIAS</envar> uevent environment variable (which should be the - same as the contents of the <filename>modalias</filename> file in sysfs), - thus loading all modules whose aliases match this string after wildcard - expansion.</para> - - <para>In this example, this means that, in addition to - <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted) - <emphasis>forte</emphasis> driver will be loaded if it is - available. See below for ways in which the loading of unwanted drivers can - be prevented.</para> - - <para>The kernel itself is also able to load modules for network - protocols, filesystems and NLS support on demand.</para> - - </sect3> - - <sect3> - <title>Handling Hotpluggable/Dynamic Devices</title> - - <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3 - player, the kernel recognizes that the device is now connected and - generates a uevent. This uevent is then handled by - <command>udevd</command> as described above.</para> - - </sect3> - - </sect2> - - <sect2> - <title>Problems with Loading Modules and Creating Devices</title> - - <para>There are a few possible problems when it comes to automatically - creating device nodes.</para> - - <sect3> - <title>A kernel module is not loaded automatically</title> - - <para>Udev will only load a module if it has a bus-specific alias and the - bus driver properly exports the necessary aliases to <systemitem - class="filesystem">sysfs</systemitem>. In other cases, one should - arrange module loading by other means. With Linux-&linux-version;, Udev is - known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI, - SERIO, and FireWire devices.</para> - - <para>To determine if the device driver you require has the necessary - support for Udev, run <command>modinfo</command> with the module name as - the argument. Now try locating the device directory under - <filename class="directory">/sys/bus</filename> and check whether there is - a <filename>modalias</filename> file there.</para> - - <para>If the <filename>modalias</filename> file exists in <systemitem - class="filesystem">sysfs</systemitem>, the driver supports the device and - can talk to it directly, but doesn't have the alias, it is a bug in the - driver. Load the driver without the help from Udev and expect the issue - to be fixed later.</para> - - <para>If there is no <filename>modalias</filename> file in the relevant - directory under <filename class="directory">/sys/bus</filename>, this - means that the kernel developers have not yet added modalias support to - this bus type. With Linux-&linux-version;, this is the case with ISA - busses. Expect this issue to be fixed in later kernel versions.</para> - - <para>Udev is not intended to load <quote>wrapper</quote> drivers such as - <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as - <emphasis>loop</emphasis> at all.</para> - - </sect3> - - <sect3> - <title>A kernel module is not loaded automatically, and Udev is not - intended to load it</title> - - <para>If the <quote>wrapper</quote> module only enhances the - functionality provided by some other module (e.g., - <emphasis>snd-pcm-oss</emphasis> enhances the functionality of - <emphasis>snd-pcm</emphasis> by making the sound cards available to OSS - applications), configure <command>modprobe</command> to load the wrapper - after Udev loads the wrapped module. To do this, add a - <quote>softdep</quote> line to the corresponding - <filename>/etc/modprobe.d/<replaceable><filename></replaceable>.conf</filename> - file. For example:</para> - -<screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen> - - <para>Note that the <quote>softdep</quote> command also allows - <literal>pre:</literal> dependencies, or a mixture of both - <literal>pre:</literal> and <literal>post:</literal>. See the - <filename>modprobe.d(5)</filename> manual page for more information - on <quote>softdep</quote> syntax and capabilities.</para> - - <para>If the module in question is not a wrapper and is useful by itself, - configure the <command>modules</command> bootscript to load this - module on system boot. To do this, add the module name to the - <filename>/etc/sysconfig/modules</filename> file on a separate line. - This works for wrapper modules too, but is suboptimal in that case.</para> - - </sect3> - - <sect3> - <title>Udev loads some unwanted module</title> - - <para>Either don't build the module, or blacklist it in a - <filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the - <emphasis>forte</emphasis> module in the example below:</para> - -<screen role="nodump"><literal>blacklist forte</literal></screen> - - <para>Blacklisted modules can still be loaded manually with the - explicit <command>modprobe</command> command.</para> - - </sect3> - - <sect3> - <title>Udev creates a device incorrectly, or makes a wrong symlink</title> - - <para>This usually happens if a rule unexpectedly matches a device. For - example, a poorly-written rule can match both a SCSI disk (as desired) - and the corresponding SCSI generic device (incorrectly) by vendor. - Find the offending rule and make it more specific, with the help of the - <command>udevadm info</command> command.</para> - - </sect3> - - <sect3> - <title>Udev rule works unreliably</title> - - <para>This may be another manifestation of the previous problem. If not, - and your rule uses <systemitem class="filesystem">sysfs</systemitem> - attributes, it may be a kernel timing issue, to be fixed in later kernels. - For now, you can work around it by creating a rule that waits for the used - <systemitem class="filesystem">sysfs</systemitem> attribute and appending - it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename> - file (create this file if it does not exist). Please notify the LFS - Development list if you do so and it helps.</para> - - </sect3> - - <sect3> - <title>Udev does not create a device</title> - - <para>Further text assumes that the driver is built statically into the - kernel or already loaded as a module, and that you have already checked - that Udev doesn't create a misnamed device.</para> - - <para>Udev has no information needed to create a device node if a kernel - driver does not export its data to - <systemitem class="filesystem">sysfs</systemitem>. This is most common - with third party drivers from outside the kernel tree. Create a static - device node in <filename>/lib/udev/devices</filename> with the - appropriate major/minor numbers (see the file - <filename>devices.txt</filename> inside the kernel documentation or the - documentation provided by the third party driver vendor). The static - device node will be copied to <filename class="directory">/dev</filename> - by <command>udev</command>.</para> - - </sect3> - - <sect3> - <title>Device naming order changes randomly after rebooting</title> - - <para>This is due to the fact that Udev, by design, handles uevents and - loads modules in parallel, and thus in an unpredictable order. This will - never be <quote>fixed</quote>. You should not rely upon the kernel device - names being stable. Instead, create your own rules that make symlinks with - stable names based on some stable attributes of the device, such as a - serial number or the output of various *_id utilities installed by Udev. - See <xref linkend="ch-config-symlinks"/> and - <xref linkend="ch-config-network"/> for examples.</para> - - </sect3> - - </sect2> - - <sect2> - <title>Useful Reading</title> - - <para>Additional helpful documentation is available at the following - sites:</para> - - <itemizedlist> - - <listitem> - <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem> - <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para> - </listitem> - - <listitem> - <para>The <systemitem class="filesystem">sysfs</systemitem> Filesystem - <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para> - </listitem> - -<!-- No longer available - <listitem> - <para>Pointers to further reading - <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html"/> - </para> - </listitem> ---> - </itemizedlist> - - </sect2> - -</sect1> |