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@@ -12,25 +12,23 @@
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<secondary>usage</secondary></indexterm>
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<secondary>usage</secondary></indexterm>
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<para>In <xref linkend="chapter-building-system"/>, we installed the Udev
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<para>In <xref linkend="chapter-building-system"/>, we installed the Udev
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-package. Before we go into the details regarding how this works,
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+package. Before we go into the details regarding how this works,
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a brief history of previous methods of handling devices is in
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a brief history of previous methods of handling devices is in
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order.</para>
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order.</para>
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-<para>Linux systems in general traditionally use a static device
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-creation method, whereby a great many device nodes are created under
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-<filename class="directory">/dev</filename> (sometimes literally
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-thousands of nodes), regardless of whether the corresponding hardware
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-devices actually exist. This is typically done via a
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-<command>MAKEDEV</command> script, which contains a number of
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-calls to the <command>mknod</command> program with the relevant major and minor device
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-numbers for every possible device that might exist in the world. Using
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-the udev method, only those devices which are detected by the kernel
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-get device nodes created for them. Because these device nodes will be
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-created each time the system boots, they will be stored on a
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-<systemitem class="filesystem">tmpfs</systemitem> (a file system that
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-resides entirely in memory and does not take up any disk space).
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-Device nodes do not require much disk space, so the memory that is
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-used is negligible.</para>
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+<para>Linux systems in general traditionally use a static device creation
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+method, whereby a great many device nodes are created under <filename
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+class="directory">/dev</filename> (sometimes literally thousands of nodes),
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+regardless of whether the corresponding hardware devices actually exist. This is
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+typically done via a <command>MAKEDEV</command> script, which contains a number
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+of calls to the <command>mknod</command> program with the relevant major and
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+minor device numbers for every possible device that might exist in the world.
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+Using the Udev method, only those devices which are detected by the kernel get
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+device nodes created for them. Because these device nodes will be created each
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+time the system boots, they will be stored on a <systemitem
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+class="filesystem">tmpfs</systemitem> file system (a virtual file system that
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+resides entirely in system memory). Device nodes do not require much space, so
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+the memory that is used is negligible.</para>
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<sect2>
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<sect2>
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<title>History</title>
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<title>History</title>
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@@ -54,46 +52,45 @@ conditions that are inherent in its design and cannot be fixed
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without a substantial revision to the kernel. It has also been marked
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without a substantial revision to the kernel. It has also been marked
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as deprecated due to a lack of recent maintenance.</para>
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as deprecated due to a lack of recent maintenance.</para>
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-<para>With the development of the unstable 2.5 kernel tree, later
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-released as the 2.6 series of stable kernels, a new virtual filesystem
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-called <systemitem class="filesystem">sysfs</systemitem> came to be.
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-The job of <systemitem class="filesystem">sysfs</systemitem> is to
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-export a view of the system's structure to userspace processes. With
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-this userspace visible representation, the possibility of seeing a
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-userspace replacement for <systemitem
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-class="filesystem">devfs</systemitem> became much more
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+<para>With the development of the unstable 2.5 kernel tree, later released as
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+the 2.6 series of stable kernels, a new virtual filesystem called <systemitem
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+class="filesystem">sysfs</systemitem> came to be. The job of <systemitem
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+class="filesystem">sysfs</systemitem> is to export a view of the system's
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+hardrware configuration to userspace processes. With this userspace-visible
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+representation, the possibility of seeing a userspace replacement for
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+<systemitem class="filesystem">devfs</systemitem> became much more
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realistic.</para>
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realistic.</para>
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+
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</sect2>
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</sect2>
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<sect2>
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<sect2>
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<title>Udev Implementation</title>
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<title>Udev Implementation</title>
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-<para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
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-was mentioned briefly above. One may wonder how <systemitem
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-class="filesystem">sysfs</systemitem> knows about the devices present
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-on a system and what device numbers should be used. Drivers that
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-have been compiled into the kernel directly register their objects
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-with <systemitem class="filesystem">sysfs</systemitem> as they are
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-detected by the kernel. For drivers compiled as modules, this will
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-happen when the module is loaded. Once the <systemitem
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-class="filesystem">sysfs</systemitem> filesystem is mounted (on
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-<filename class="directory">/sys</filename>), data which the
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+<para>The <systemitem class="filesystem">sysfs</systemitem> filesystem was
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+mentioned briefly above. One may wonder how <systemitem
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+class="filesystem">sysfs</systemitem> knows about the devices present on a
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+system and what device numbers should be used for them. Drivers that have been
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+compiled into the kernel directly register their objects with <systemitem
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+class="filesystem">sysfs</systemitem> as they are detected by the kernel. For
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+drivers compiled as modules, this registration will happen when the module is
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+loaded. Once the <systemitem class="filesystem">sysfs</systemitem> filesystem is
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+mounted (on <filename class="directory">/sys</filename>), data which the
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built-in drivers registered with <systemitem
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built-in drivers registered with <systemitem
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-class="filesystem">sysfs</systemitem> are available to userspace
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-processes and to <command>udev</command> for device node creation.</para>
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+class="filesystem">sysfs</systemitem> are available to userspace processes and
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+to <command>udev</command> for device node creation.</para>
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<para>The <command>S10udev</command> initscript takes care of creating these
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<para>The <command>S10udev</command> initscript takes care of creating these
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-device nodes when Linux is booted. This script starts with registering
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-<command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events
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-(discussed below) should not be generated during this stage, but
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-<command>udev</command> is registered just in case they do occur. The
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+device nodes when Linux is booted. This script starts by registering
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+<command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events
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+(discussed below) are not usually generated during this stage, but
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+<command>udev</command> is registered just in case they do occur. The
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<command>udevstart</command> program then walks through the <systemitem
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<command>udevstart</command> program then walks through the <systemitem
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class="filesystem">/sys</systemitem> filesystem and creates devices under
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class="filesystem">/sys</systemitem> filesystem and creates devices under
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-<filename class="directory">/dev</filename> that match the descriptions. For
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+<filename class="directory">/dev</filename> that match the descriptions. For
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example, <filename>/sys/class/tty/vcs/dev</filename> contains the string
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example, <filename>/sys/class/tty/vcs/dev</filename> contains the string
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<quote>7:0</quote> This string is used by <command>udevstart</command> to create
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<quote>7:0</quote> This string is used by <command>udevstart</command> to create
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<filename>/dev/vcs</filename> with major number <emphasis>7</emphasis> and minor
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<filename>/dev/vcs</filename> with major number <emphasis>7</emphasis> and minor
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-<emphasis>0</emphasis>. The names and permissions of the nodes created under
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+<emphasis>0</emphasis>. The names and permissions of the nodes created under
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the <filename class="directory">/dev</filename> directory are configured
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the <filename class="directory">/dev</filename> directory are configured
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according to the rules specified in the files within the <filename
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according to the rules specified in the files within the <filename
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class="directory">/etc/udev/rules.d/</filename> directory. These are numbered in
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class="directory">/etc/udev/rules.d/</filename> directory. These are numbered in
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@@ -101,39 +98,34 @@ a similar fashion to the LFS-Bootscripts package. If <command>udev</command>
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can't find a rule for the device it is creating, it will default permissions to
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can't find a rule for the device it is creating, it will default permissions to
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<emphasis>660</emphasis> and ownership to <emphasis>root:root</emphasis>.</para>
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<emphasis>660</emphasis> and ownership to <emphasis>root:root</emphasis>.</para>
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-<para>Once the above stage is complete, all devices that were already
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-present and have compiled-in drivers will be available for use. What
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-about those devices that have modular drivers?</para>
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+<para>Once the above stage is complete, all devices that were already present
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+and have compiled-in drivers will be available for use. This leads us to the
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+devices that have modular drivers.</para>
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<para>Earlier, we mentioned the concept of a <quote>hotplug event
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<para>Earlier, we mentioned the concept of a <quote>hotplug event
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-handler.</quote> When a new device connection is detected by the
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-kernel, the kernel will generate a hotplug event and look at the file
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-<filename>/proc/sys/kernel/hotplug</filename> to find out the
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-userspace program that handles the device's connection. The
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-<command>udev</command> initscript registered <command>udevsend</command>
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-as this handler. When these hotplug events are generated, the kernel
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-will tell <command>udev</command> to check the <filename
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-class="directory">/sys</filename> filesystem for the information
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+handler.</quote> When a new device connection is detected by the kernel, the
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+kernel will generate a hotplug event and look at the file
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+<filename>/proc/sys/kernel/hotplug</filename> to determine the userspace program
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+that handles the device's connection. The <command>udev</command> bootscript
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+registered <command>udevsend</command> as this handler. When these hotplug
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+events are generated, the kernel will tell <command>udev</command> to check the
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+<filename class="directory">/sys</filename> filesystem for the information
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pertaining to this new device and create the <filename
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pertaining to this new device and create the <filename
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class="directory">/dev</filename> entry for it.</para>
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class="directory">/dev</filename> entry for it.</para>
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-<para>This brings us to one problem that exists with
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-<command>udev</command>, and likewise with <systemitem
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-class="filesystem">devfs</systemitem> before it. It is commonly
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-referred to as the <quote>chicken and egg</quote> problem. Most Linux
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-distributions handle loading modules via entries in
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-<filename>/etc/modules.conf</filename>. Access to a device node causes
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-the appropriate kernel module to load. With <command>udev</command>,
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-this method will not work because the device node does not exist until
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-the module is loaded. To solve this, the
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-<command>S05modules</command> bootscript was added to the
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+<para>This brings us to one problem that exists with <command>udev</command>,
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+and likewise with <systemitem class="filesystem">devfs</systemitem> before it.
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+It is commonly referred to as the <quote>chicken and egg</quote> problem. Most
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+Linux distributions handle loading modules via entries in
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+<filename>/etc/modules.conf</filename>. Access to a device node causes the
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+appropriate kernel module to load. With <command>udev</command>, this method
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+will not work because the device node does not exist until the module is loaded.
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+To solve this, the <command>S05modules</command> bootscript was added to the
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LFS-Bootscripts package, along with the
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LFS-Bootscripts package, along with the
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-<filename>/etc/sysconfig/modules</filename> file. By
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-adding module
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-names to the <filename>modules</filename> file, these modules will be
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-loaded when the computer is starting up. This allows
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-<command>udev</command> to detect the devices and create the
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-appropriate device nodes.</para>
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+<filename>/etc/sysconfig/modules</filename> file. By adding module names to the
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+<filename>modules</filename> file, these modules will be loaded when the
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+computer is starts up. This allows <command>udev</command> to detect the devices
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+and create the appropriate device nodes.</para>
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<para>Note that on slower machines or for drivers that create a lot
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<para>Note that on slower machines or for drivers that create a lot
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of device nodes, the process of creating devices may take a few
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of device nodes, the process of creating devices may take a few
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@@ -167,14 +159,12 @@ device nodes:</para>
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<para>1) A kernel driver may not export its data to <systemitem
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<para>1) A kernel driver may not export its data to <systemitem
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class="filesystem">sysfs</systemitem>.</para>
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class="filesystem">sysfs</systemitem>.</para>
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-<para>This is most common with third party drivers from outside the
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-kernel tree. These drivers will not end up having their device nodes
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-created. Use the
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-<filename>/etc/sysconfig/createfiles</filename> configuration file to
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-manually create the devices. Consult the
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-<filename>devices.txt</filename> file inside the kernel documentation
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-or the documentation for that driver to find the proper major/minor
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-numbers.</para>
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+<para>This is most common with third party drivers from outside the kernel tree.
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+Udev will be unable to automatically create device nodes for such drivers. Use
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+the <filename>/etc/sysconfig/createfiles</filename> configuration file to
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+manually create the devices. Consult the <filename>devices.txt</filename> file
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+inside the kernel documentation or the documentation for that driver to find the
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+proper major/minor numbers.</para>
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<para>2) A non-hardware device is required. This is most common with
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<para>2) A non-hardware device is required. This is most common with
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the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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