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diff --git a/chapter07/udevd.xml b/chapter07/udevd.xml new file mode 100644 index 000000000..548a2d2ee --- /dev/null +++ b/chapter07/udevd.xml @@ -0,0 +1,337 @@ +<?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-scripts-udev"> + <?dbhtml filename="udev.html"?> + + <title>Device and Module Handling on an LFS System</title> + + <indexterm zone="ch-scripts-udev"> + <primary sortas="a-Udev">Udev</primary> + <secondary>usage</secondary> + </indexterm> + + <para>In <xref linkend="chapter-building-system"/>, we installed Udev + from the systemd source package. 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 use a static device creation + method, whereby a great many device nodes are created under <filename + class="directory">/dev</filename> (sometimes literally thousands of nodes), + regardless of whether the corresponding hardware devices actually exist. This + is 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 suffers from race + conditions that are inherent in its design and cannot 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 seeing 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 <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 in a similar + fashion to the LFS-Bootscripts package 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> + <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 in any + <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 the + <command>udev</command> bootscript.</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-scripts-symlinks"/> and + <xref linkend="ch-scripts-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> + + </itemizedlist> + + </sect2> + +</sect1> |