source: chapter07/udev.xml@ 50d538e

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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 does its
job, 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 simply contains a number of
calls to the 'mknod' 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. As 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 in negligable.</para>

<sect2>
<title>History</title>

<para>In Febraury of 2000, a new filesystem called <systemitem
class="filesystem">devfs</systemitem> was merged into the 2.3.46
kernel and was made generally 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 that 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 you are to allow device names to be configurable then
the device naming policy should be up to a system administrator, not
imposed upon them by any particular developer(s).  <systemitem
class="filesystem">devfs</systemitem> 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, came a new virtual
filesystem called <systemitem class="filesystem">sysfs</systemitem>.
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>

<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>), the data which the
built-in drivers registered with <systemitem
class="filesystem">sysfs</systemitem> is 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/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>.  <command>udevstart</command> uses this 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>Once the above stage is complete, all devices that were already
present and have compiled-in drivers will be available for use. How
about those devices that have modular drivers?</para>

<para>We mentioned earlier 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 to handle that device's connection.  The
<command>udev</command> initscript registered <command>udev</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
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>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, e.g. a USB MP3 player, the kernel
recognizes that the device is now connected and generates a hotplug
event. The driver will already have been loaded (either because it was
compiled into the kernel, or it was loaded via the
<command>S05modules</command> bootscript) so <command>udev</command>
will simply be called upon to create the relevant device node
according to the <systemitem class="filesystem">sysfs</systemitem>
data available in <filename class="directory">/sys</filename>.</para>
</sect2>

<sect2>
<title>Problems with creating devices</title>

<para>There are a few problems when it comes to automatically creating
devices nodes:</para>

<para>1) A kernel driver may not export its data to sysfs</para>
   
<para>This is most common with 3rd party drivers from outside the
kernel tree.  These drivers will not end up having their device nodes
created.  You can use the
<filename>/etc/sysconfig/createfiles</filename> configuration file to
manually create the devices. Consult the
<filename>devices.txt</filename> file inside your 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
ALSA project's 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>Use of an
<quote>install</quote> line in
<filename>/etc/modprobe.conf</filename>. Basically, this tells the
modprobe 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>

<sect2>
<title>Useful reading</title>

<para>Some additional documentation that will be useful to
read:</para>

<itemizedlist>
<listitem><para>A Userspace Implementation of devfs -- <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>