Author: manuel Date: 2005-05-30 13:36:39 -0600 (Mon, 30 May 2005) New Revision: 5542
Modified: branches/cross-lfs/BOOK/bootscripts/common/udev.xml Log: Indented bootscripts/common/udev.xml Modified: branches/cross-lfs/BOOK/bootscripts/common/udev.xml =================================================================== --- branches/cross-lfs/BOOK/bootscripts/common/udev.xml 2005-05-30 19:17:49 UTC (rev 5541) +++ branches/cross-lfs/BOOK/bootscripts/common/udev.xml 2005-05-30 19:36:39 UTC (rev 5542) @@ -1,232 +1,237 @@ <?xml version="1.0" encoding="ISO-8859-1"?> -<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"; [ +<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" + "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"; [ <!ENTITY % general-entities SYSTEM "../../general.ent"> %general-entities; ]> + <sect1 id="ch-scripts-udev"> -<title>Device and Module Handling on an LFS System</title> -<?dbhtml filename="udev.html"?> + <?dbhtml filename="udev.html"?> -<indexterm zone="ch-scripts-udev"> -<primary sortas="a-Udev">Udev</primary> -<secondary>usage</secondary></indexterm> + <title>Device and Module Handling on an LFS System</title> -<para>In <xref linkend="chapter-building-system"/>, we installed the Udev -package. Before we go into the details regarding how this works, -a brief history of previous methods of handling devices is in -order.</para> + <indexterm zone="ch-scripts-udev"> + <primary sortas="a-Udev">Udev</primary> + <secondary>usage</secondary></indexterm> -<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. 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">ramfs</systemitem> (a file system that -resides entirely in memory and does not take up any disk space). -Device nodes do not require much disk space, so the memory that is -used is negligible.</para> + <para>In <xref linkend="chapter-building-system"/>, we installed the Udev + package. Before we go into the details regarding how this works, a brief + history of previous methods of handling devices is in order.</para> -<sect2> -<title>History</title> + <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. 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">ramfs</systemitem> (a file + system that resides entirely in memory and does not take up any disk space). + Device nodes do not require much disk space, so the memory that is used is + negligible.</para> -<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> + <sect2> + <title>History</title> -<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 has also been marked -as deprecated due to a lack of recent maintenance.</para> + <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>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 structure 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> + <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 has also been marked + as deprecated due to a lack of recent maintenance.</para> -</sect2> + <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 structure 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> -<title>Udev Implementation</title> + </sect2> -<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. Drivers that -have been compiled into the kernel directly register their objects -with <systemitem class="filesystem">sysfs</systemitem> as they are -detected by the kernel. For drivers compiled as modules, this will -happen when the module is loaded. Once the <systemitem -class="filesystem">sysfs</systemitem> filesystem is mounted (on -<filename class="directory">/sys</filename>), the data which the -built-in drivers registered with <systemitem -class="filesystem">sysfs</systemitem> are available to userspace -processes and to <command>udev</command> for device node creation.</para> + <sect2> + <title>Udev Implementation</title> -<para>The <command>S10udev</command> initscript takes care of creating -these device nodes when Linux is booted. This script starts with -registering <command>/sbin/udev</command> as a hotplug event handler. -Hotplug events (discussed below) should not be generated during this -stage, but <command>udev</command> is registered just in case they do -occur. The <command>udevstart</command> program then walks through -the <systemitem class="filesystem">/sys</systemitem> filesystem and -creates devices under <filename class="directory">/dev</filename> that -match the descriptions. For example, -<filename>/sys/class/tty/vcs/dev</filename> contains the string -<quote>7:0</quote> This string is used by <command>udevstart</command> -to create <filename>/dev/vcs</filename> with major number -<emphasis>7</emphasis> and minor <emphasis>0</emphasis>. The -permissions of each and every device that <command>udevstart</command> -creates are set using files from the <filename -class="directory">/etc/udev.d/permissions.d/</filename> directory. -These are numbered in a similar fashion to the LFS bootscripts. If -<command>udev</command> cannot find a permissions file for the device -it is creating, it will default permissions to -<emphasis>600</emphasis> and ownership to -<emphasis>root:root</emphasis>. The names of the nodes created under -the <filename class="directory">/dev</filename> directory are -configured according to the rules specified in the files within the -<filename class="directory">/etc/udev/rules.d/</filename> -directory.</para> + <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. + Drivers that have been compiled into the kernel directly register + their objects with <systemitem class="filesystem">sysfs</systemitem> + as they are detected by the kernel. For drivers compiled as modules, + this will happen when the module is loaded. Once the <systemitem + class="filesystem">sysfs</systemitem> filesystem is mounted (on + <filename class="directory">/sys</filename>), the data which the + built-in drivers registered with <systemitem + class="filesystem">sysfs</systemitem> are available to userspace + processes and to <command>udev</command> for device node creation.</para> -<para>Once the above stage is complete, all devices that were already -present and have compiled-in drivers will be available for use. What -about those devices that have modular drivers?</para> + <para>The <command>S10udev</command> initscript takes care of creating + these device nodes when Linux is booted. This script starts with + registering <command>/sbin/udev</command> as a hotplug event handler. + Hotplug events (discussed below) should not be generated during this + stage, but <command>udev</command> is registered just in case they do + occur. The <command>udevstart</command> program then walks through the + <systemitem class="filesystem">/sys</systemitem> filesystem and creates + devices under <filename class="directory">/dev</filename> that match the + descriptions. For example, <filename>/sys/class/tty/vcs/dev</filename> + contains the string <quote>7:0</quote> This string is used by + <command>udevstart</command> to create <filename>/dev/vcs</filename> with + major number <emphasis>7</emphasis> and minor <emphasis>0</emphasis>. The + permissions of each and every device that <command>udevstart</command> + creates are set using files from the <filename + class="directory">/etc/udev.d/permissions.d/</filename> directory. These + are numbered in a similar fashion to the LFS bootscripts. If + <command>udev</command> cannot find a permissions file for the device it + is creating, it will default permissions to <emphasis>600</emphasis> and + ownership to <emphasis>root:root</emphasis>. The names of the nodes + created under the <filename class="directory">/dev</filename> directory + are configured according to the rules specified in the files within the + <filename class="directory">/etc/udev/rules.d/</filename> directory.</para> -<para>Earlier, we mentioned the concept of a <quote>hotplug event -handler.</quote> When a new device connection is detected by the -kernel, the kernel will generate a hotplug event and look at the file -<filename>/proc/sys/kernel/hotplug</filename> to find out the -userspace program that handles the device's connection. The -<command>udev</command> initscript registered <command>udevsend</command> -as this handler. When these hotplug events are generated, the kernel -will tell <command>udev</command> to check the <filename -class="directory">/sys</filename> filesystem for the information -pertaining to this new device and create the <filename -class="directory">/dev</filename> entry for it.</para> + <para>Once the above stage is complete, all devices that were already + present and have compiled-in drivers will be available for use. What + about those devices that have modular drivers?</para> -<para>This brings us to one problem that exists with -<command>udev</command>, and likewise with <systemitem -class="filesystem">devfs</systemitem> before it. It is commonly -referred to as the <quote>chicken and egg</quote> problem. Most Linux -distrubtions handle loading modules via entries in -<filename>/etc/modules.conf</filename>. Access to a device node causes -the appropriate kernel module to load. With <command>udev</command>, -this method will not work because the device node does not exist until -the module is loaded. To solve this, the -<command>S05modules</command> bootscript was added to the -lfs-bootscripts package, along with the -<filename>/etc/sysconfig/modules</filename> file. By - adding module -names to the <filename>modules</filename> file, these modules will be -loaded when the computer is starting up. This allows -<command>udev</command> to detect the devices and create the -appropriate device nodes.</para> + <para>Earlier, we mentioned the concept of a <quote>hotplug event + handler.</quote> When a new device connection is detected by the kernel, + the kernel will generate a hotplug event and look at the file + <filename>/proc/sys/kernel/hotplug</filename> to find out the userspace + program that handles the device's connection. The <command>udev</command> + initscript registered <command>udevsend</command> as this handler. When + these hotplug events are generated, the kernel will tell + <command>udev</command> to check the <filename + class="directory">/sys</filename> filesystem for the information + pertaining to this new device and create the <filename + class="directory">/dev</filename> entry for it.</para> -<para>Note that on slower machines or for drivers that create a lot -of device nodes, the process of creating devices may take a few -seconds to complete. This means that some device nodes may not be -immediately accessible.</para> -</sect2> + <para>This brings us to one problem that exists with + <command>udev</command>, and likewise with <systemitem + class="filesystem">devfs</systemitem> before it. It is commonly + referred to as the <quote>chicken and egg</quote> problem. Most + Linux distrubtions handle loading modules via entries in + <filename>/etc/modules.conf</filename>. Access to a device node + causes the appropriate kernel module to load. With + <command>udev</command>, this method will not work because the device + node does not exist until the module is loaded. To solve this, the + <command>S05modules</command> bootscript was added to the lfs-bootscripts + package, along with the <filename>/etc/sysconfig/modules</filename> file. + By adding module names to the <filename>modules</filename> file, these + modules will be loaded when the computer is starting up. This allows + <command>udev</command> to detect the devices and create the appropriate + device nodes.</para> -<sect2> -<title>Handling Hotpluggable/Dynamic Devices</title> + <para>Note that on slower machines or for drivers that create a lot + of device nodes, the process of creating devices may take a few + seconds to complete. This means that some device nodes may not be + immediately accessible.</para> -<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 hotplug -event. If the driver is already loaded (either because it was compiled -into the kernel or because it was loaded via the -<command>S05modules</command> bootscript), <command>udev</command> will -be called upon to create the relevant device node(s) according to the -<systemitem class="filesystem">sysfs</systemitem> data available in -<filename class="directory">/sys</filename>.</para> + </sect2> -<para>If the driver for the just plugged in device is available as a module but -currently unloaded, the Hotplug package will load the appropriate module -and make this device available by creating the device node(s) for it.</para> -</sect2> + <sect2> + <title>Handling Hotpluggable/Dynamic Devices</title> -<sect2> -<title>Problems with Creating 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 hotplug event. If the driver is already loaded (either + because it was compiled into the kernel or because it was loaded via + the <command>S05modules</command> bootscript), <command>udev</command> + will be called upon to create the relevant device node(s) according to + the <systemitem class="filesystem">sysfs</systemitem> data available in + <filename class="directory">/sys</filename>.</para> -<para>There are a few known problems when it comes to automatically creating -devices nodes:</para> + <para>If the driver for the just plugged in device is available as a + module but currently unloaded, the Hotplug package will load the + appropriate module and make this device available by creating the + device node(s) for it.</para> -<para>1) A kernel driver may not export its data to <systemitem -class="filesystem">sysfs</systemitem>.</para> - -<para>This is most common with third party drivers from outside the -kernel tree. These drivers will not end up having their device nodes -created. Use the -<filename>/etc/sysconfig/createfiles</filename> configuration file to -manually create the devices. Consult the -<filename>devices.txt</filename> file inside the kernel documentation -or the documentation for that driver to find the proper major/minor -numbers.</para> + </sect2> + <sect2> + <title>Problems with Creating Devices</title> -<para>2) A non-hardware device is required. This is most common with -the Advanced Linux Sound Architecture (ALSA) project's Open Sound -System (OSS) compatibility module. These types of devices can be -handled in one of two ways:</para> + <para>There are a few known problems when it comes to automatically + creating devices nodes:</para> -<itemizedlist> + <para>1) A kernel driver may not export its data to <systemitem + class="filesystem">sysfs</systemitem>.</para> -<listitem><para>Adding the module names to -<filename>/etc/sysconfig/modules</filename></para></listitem> -<listitem><para>Using an -<quote>install</quote> line in -<filename>/etc/modprobe.conf</filename>. This tells the -<command>modprobe</command> command <quote>when loading this module, -also load this other module, at the same time.</quote> For example:</para> + <para>This is most common with third party drivers from outside the + kernel tree. These drivers will not end up having their device nodes + created. Use the <filename>/etc/sysconfig/createfiles</filename> + configuration file to manually create the devices. Consult the + <filename>devices.txt</filename> file inside the kernel documentation + or the documentation for that driver to find the proper major/minor + numbers.</para> + + <para>2) A non-hardware device is required. This is most common with + the Advanced Linux Sound Architecture (ALSA) project's Open Sound + System (OSS) compatibility module. These types of devices can be + handled in one of two ways:</para> + + <itemizedlist> + <listitem> + <para>Adding the module names to + <filename>/etc/sysconfig/modules</filename></para> + </listitem> + <listitem> + <para>Using an <quote>install</quote> line in + <filename>/etc/modprobe.conf</filename>. This tells the + <command>modprobe</command> command <quote>when loading this + module, also load this other module, at the same time.</quote> + For example:</para> + <screen><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \ snd-pcm-oss ; true</userinput></screen> -<para>This will cause the system to load both the -<emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis> -modules when any request is made to load the driver -<emphasis>snd-pcm</emphasis>.</para></listitem> -</itemizedlist> -</sect2> + <para>This will cause the system to load both the + <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis> + modules when any request is made to load the driver + <emphasis>snd-pcm</emphasis>.</para> + </listitem> + </itemizedlist> -<sect2> -<title>Useful Reading</title> + </sect2> -<para>Additional helpful documentation is available at the following -sites:</para> + <sect2> + <title>Useful Reading</title> -<itemizedlist> -<listitem><para remap="verbatim">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> + <para>Additional helpful documentation is available at the following + sites:</para> -<listitem><para remap="verbatim">udev FAQ -<ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para></listitem> + <itemizedlist> + <listitem> + <para remap="verbatim">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 remap="verbatim">udev FAQ + <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para> + </listitem> + <listitem> + <para remap="verbatim">The Linux Kernel Driver Model + <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para> + </listitem> + </itemizedlist> -<listitem><para remap="verbatim">The Linux Kernel Driver Model -<ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para></listitem> -</itemizedlist> -</sect2> + </sect2> </sect1> - -- http://linuxfromscratch.org/mailman/listinfo/lfs-book FAQ: http://www.linuxfromscratch.org/lfs/faq.html Unsubscribe: See the above information page
