udev.html

CLFS(Cross-Compiled Linux From Scratch)的文档 Version CLFS-SVN-20060417

          "filename">/dev</tt> entry for it.
        </p>
        <p>
          This brings us to one problem that exists with <span><strong class=
          "command">udev</strong></span>, and likewise with <tt class=
          "systemitem">devfs</tt> before it. It is commonly referred to as
          the &ldquo;<span class="quote">chicken and egg</span>&rdquo;
          problem. Most Linux distributions handle loading modules via
          entries in <tt class="filename">/etc/modules.conf</tt>. Access to a
          device node causes the appropriate kernel module to load. With
          <span><strong class="command">udev</strong></span>, this method
          will not work because the device node does not exist until the
          module is loaded. To solve this, the <span><strong class=
          "command">S05modules</strong></span> bootscript was added to the
          LFS-Bootscripts package, along with the <tt class=
          "filename">/etc/sysconfig/modules</tt> file. By adding module names
          to the <tt class="filename">modules</tt> file, these modules will
          be loaded when the computer starts up. This allows
          <span><strong class="command">udev</strong></span> to detect the
          devices and create the appropriate device nodes.
        </p>
        <p>
          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.
        </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <h2 class="sect2">
            11.5.3. Handling Hotpluggable/Dynamic Devices
          </h2>
        </div>
        <p>
          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 <span><strong class="command">S05modules</strong></span>
          bootscript), <span><strong class="command">udev</strong></span>
          will be called upon to create the relevant device node(s) according
          to the <tt class="systemitem">sysfs</tt> data available in
          <tt class="filename">/sys</tt>.
        </p>
        <p>
          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.
        </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <h2 class="sect2">
            11.5.4. Problems with Creating Devices
          </h2>
        </div>
        <p>
          There are a few known problems when it comes to automatically
          creating device nodes:
        </p>
        <p>
          1) A kernel driver may not export its data to <tt class=
          "systemitem">sysfs</tt>.
        </p>
        <p>
          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 <tt class=
          "filename">/etc/sysconfig/createfiles</tt> configuration file to
          manually create the devices. Consult the <tt class=
          "filename">devices.txt</tt> file inside the kernel documentation or
          the documentation for that driver to find the proper major/minor
          numbers.
        </p>
        <p>
          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:
        </p>
        <div class="itemizedlist">
          <ul>
            <li>
              <p>
                Adding the module names to <tt class=
                "filename">/etc/sysconfig/modules</tt>
              </p>
            </li>
            <li>
              <p>
                Using an &ldquo;<span class="quote">install</span>&rdquo;
                line in <tt class="filename">/etc/modprobe.conf</tt>. This
                tells the <span><strong class=
                "command">modprobe</strong></span> command
                &ldquo;<span class="quote">when loading this module, also
                load this other module, at the same time.</span>&rdquo; For
                example:
              </p>
              <pre class="userinput">
<kbd class="command">install snd-pcm modprobe -i snd-pcm ; modprobe \
    snd-pcm-oss ; true</kbd>
</pre>
              <p>
                This will cause the system to load both the <span class=
                "emphasis"><em>snd-pcm</em></span> and <span class=
                "emphasis"><em>snd-pcm-oss</em></span> modules when any
                request is made to load the driver <span class=
                "emphasis"><em>snd-pcm</em></span>.
              </p>
            </li>
          </ul>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <h2 class="sect2">
            11.5.5. Useful Reading
          </h2>
        </div>
        <p>
          Additional helpful documentation is available at the following
          sites:
        </p>
        <div class="itemizedlist">
          <ul>
            <li>
              <p>
                A Userspace Implementation of <tt class=
                "systemitem">devfs</tt> <a href=
                "http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf">
                <i>http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf</i></a>
              </p>
            </li>
            <li>
              <p>
                udev FAQ <a href=
                "http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ">
                <i>http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ</i></a>
              </p>
            </li>
            <li>
              <p>
                The Linux Kernel Driver Model <a href=
                "http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf">
                <i>http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf</i></a>
              </p>
            </li>
          </ul>
        </div>
      </div>
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