lfs-book-zh_CN/chapter07/udev.xml

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<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>

<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>

<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>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>