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diff --git a/chapter07/udev.xml b/chapter07/udev.xml index 64b3a43c7..5f98e6139 100644 --- a/chapter07/udev.xml +++ b/chapter07/udev.xml @@ -1,213 +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"?> - -<indexterm zone="ch-scripts-udev"> -<primary sortas="a-Udev">Udev</primary> -<secondary>usage</secondary></indexterm> - -<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> - -<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">tmpfs</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 has also been marked -as deprecated due to a lack of recent maintenance.</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 -hardrware 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> - -<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 <systemitem -class="filesystem">sysfs</systemitem> 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 <filename class="directory">/sys</filename>), 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>The <command>S10udev</command> initscript takes care of creating these -device nodes when Linux is booted. This script starts by registering -<command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events -(discussed below) are not usually 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 names and permissions 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. These are numbered in -a similar fashion to the LFS-Bootscripts package. If <command>udev</command> -can't find a rule for the device it is creating, it will default permissions to -<emphasis>660</emphasis> and ownership to <emphasis>root:root</emphasis>.</para> - -<para>Once the above stage is complete, all devices that were already present -and have compiled-in drivers will be available for use. This leads us to the -devices that have modular drivers.</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 determine the userspace program -that handles the device's connection. The <command>udev</command> bootscript -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>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 distributions 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 starts up. This allows <command>udev</command> to detect the devices -and create the appropriate device nodes.</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> - -<sect2> -<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 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>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>Problems with Creating Devices</title> - -<para>There are a few known problems when it comes to automatically creating -device nodes:</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. -Udev will be unable to automatically create device nodes for such drivers. 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> + <?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 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> + + <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">tmpfs</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 has also + been marked as deprecated due to a lack of recent maintenance.</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 + hardrware 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> + + <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 + <systemitem class="filesystem">sysfs</systemitem> 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 <filename + class="directory">/sys</filename>), 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>The <command>S10udev</command> initscript takes care of creating + these device nodes when Linux is booted. This script starts by registering + <command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events + (discussed below) are not usually 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 names and + permissions 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. These are + numbered in a similar fashion to the LFS-Bootscripts package. If + <command>udev</command> can't find a rule for the device it is creating, + it will default permissions to <emphasis>660</emphasis> and ownership to + <emphasis>root:root</emphasis>.</para> + + <para>Once the above stage is complete, all devices that were already present + and have compiled-in drivers will be available for use. This leads us to the + devices that have modular drivers.</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 determine the userspace + program that handles the device's connection. The <command>udev</command> + bootscript 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>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 distributions 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 starts up. This allows <command>udev</command> to detect the devices + and create the appropriate device nodes.</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> + + <sect2> + <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 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>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>Problems with Creating Devices</title> + + <para>There are a few known problems when it comes to automatically creating + device nodes:</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. Udev will be unable to automatically create device nodes for such + drivers. 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 role="nodump"><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> -<sect2> -<title>Useful Reading</title> + </itemizedlist> -<para>Additional helpful documentation is available at the following -sites:</para> + </sect2> -<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> + <sect2> + <title>Useful Reading</title> -<listitem><para>udev FAQ -<ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para></listitem> + <para>Additional helpful documentation is available at the following + sites:</para> -<listitem><para>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> + <itemizedlist> -</sect1> + <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>udev FAQ + <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para> + </listitem> + + <listitem> + <para>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> + +</sect1> |