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diff --git a/chapter07/udev.xml b/chapter07/udev.xml index 7b9be92ad..4ec10b504 100644 --- a/chapter07/udev.xml +++ b/chapter07/udev.xml @@ -12,25 +12,23 @@ <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, +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> (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>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> @@ -54,46 +52,45 @@ 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 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 +<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. 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>), data which the +<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> +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 with registering -<command>/sbin/udevsend</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 +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 +<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 +<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 @@ -101,39 +98,34 @@ 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. What -about those devices that have modular drivers?</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 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 +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 +<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 is starting up. This allows -<command>udev</command> to detect the devices and create the -appropriate device nodes.</para> +<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 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 @@ -167,14 +159,12 @@ 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. 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>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 |