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<sect1 id="kernel-baking"><title>Compiling a New Kernel</title>
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<para>
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Why would someone want to compile a new kernel? It is often not
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necessary since the default kernel shipped with Debian handles most
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configurations. However, it is useful to compile a new kernel in order
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to:
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<itemizedlist>
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<listitem><para>
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handle special hardware needs, or hardware conflicts with the pre-supplied
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kernels
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</para></listitem>
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<listitem><para>
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handle hardware or options not included in the stock kernel, such as
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APM or SMP
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</para></listitem>
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<listitem><para>
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optimize the kernel by removing useless drivers to speed up boot
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time
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</para></listitem>
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<listitem><para>
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use options of the kernel which are not supported by the default
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kernel (such as high memory support)
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</para></listitem>
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<listitem><para>
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run an updated or development kernel
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</para></listitem>
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<listitem><para>
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impress your friends, try new things
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</para></listitem>
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</itemizedlist>
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</para>
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<sect2><title>Kernel Image Management</title>
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<para>
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Don't be afraid to try compiling the kernel. It's fun and profitable.
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</para><para>
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To compile a kernel the Debian way, you need some packages:
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<classname>kernel-package</classname>,
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<classname>kernel-source-&kernelversion;</classname> (the most recent version
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at the time of this writing), <classname>fakeroot</classname> and a
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few others which are probably already installed (see
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<filename>/usr/share/doc/kernel-package/README.gz</filename> for the
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complete list).
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</para><para>
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This method will make a .deb of your kernel source, and, if you have
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non-standard modules, make a synchronized dependent .deb of those
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too. It's a better way to manage kernel images;
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<filename>/boot</filename> will hold the kernel, the System.map, and a
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log of the active config file for the build.
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</para><para>
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Note that you don't <emphasis>have</emphasis> to compile your kernel
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the <quote>Debian way</quote>; but we find that using the packaging system
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to manage your kernel is actually safer and easier. In fact, you can get
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your kernel sources right from Linus instead of
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<classname>kernel-source-&kernelversion;</classname>, yet still use the
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<classname>kernel-package</classname> compilation method.
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</para><para>
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Note that you'll find complete documentation on using
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<classname>kernel-package</classname> under
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<filename>/usr/share/doc/kernel-package</filename>. This section just
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contains a brief tutorial.
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</para><para>
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Hereafter, we'll assume you have free rein over your machine and will
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extract your kernel source to somewhere in your home directory<footnote>
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<para>
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There are other locations where you can extract kernel sources and build
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your custom kernel, but this is easiest as it does not require special
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permissions.
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</para>
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</footnote>. We'll also assume that your kernel version is
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&kernelversion;. Make sure you are in the directory to where you want to
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unpack the kernel sources, extract them using <userinput>tar xjf
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/usr/src/kernel-source-&kernelversion;.tar.bz2</userinput> and change
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to the directory <filename>kernel-source-&kernelversion;</filename>
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that will have been created.
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</para><para>
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Now, you can configure your kernel. Run <userinput>make
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xconfig</userinput> if X11 is installed, configured and being run; run
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<userinput>make menuconfig</userinput> otherwise (you'll need
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<classname>libncurses5-dev</classname> installed). Take the time to read
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the online help and choose carefully. When in doubt, it is typically
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better to include the device driver (the software which manages
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hardware peripherals, such as Ethernet cards, SCSI controllers, and so
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on) you are unsure about. Be careful: other options, not related to a
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specific hardware, should be left at the default value if you do not
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understand them. Do not forget to select <quote>Kernel module loader</quote>
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in <quote>Loadable module support</quote> (it is not selected by default).
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If not included, your Debian installation will experience problems.
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</para><para>
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Clean the source tree and reset the <classname>kernel-package</classname>
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parameters. To do that, do <userinput>make-kpkg clean</userinput>.
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</para><para>
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Now, compile the kernel:
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<userinput>fakeroot make-kpkg --revision=custom.1.0 kernel_image</userinput>.
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The version number of <quote>1.0</quote> can be changed at will; this is just
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a version number that you will use to track your kernel builds.
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Likewise, you can put any word you like in place of <quote>custom</quote>
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(e.g., a host name). Kernel compilation may take quite a while, depending on
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the power of your machine.
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</para><para condition="supports-pcmcia">
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If you require PCMCIA support, you'll also need to install the
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<classname>pcmcia-source</classname> package. Unpack the gzipped tar file
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as root in the directory <filename>/usr/src</filename> (it's important that
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modules are found where they are expected to be found, namely,
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<filename>/usr/src/modules</filename>). Then, as root, do <userinput>make-kpkg
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modules_image</userinput>.
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</para><para>
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Once the compilation is complete, you can install your custom kernel
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like any package. As root, do <userinput>dpkg -i
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../kernel-image-&kernelversion;-<replaceable>subarchitecture</replaceable>_custom.1.0_&architecture;.deb</userinput>.
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The <replaceable>subarchitecture</replaceable> part is an optional
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sub-architecture,
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<phrase arch="i386"> such as <quote>i586</quote>, </phrase>
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depending on what kernel options you set.
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<userinput>dpkg -i kernel-image...</userinput> will install the
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kernel, along with some other nice supporting files. For instance,
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the <filename>System.map</filename> will be properly installed
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(helpful for debugging kernel problems), and
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<filename>/boot/config-&kernelversion;</filename> will be installed,
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containing your current configuration set. Your new
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<classname>kernel-image-&kernelversion;</classname> package is also clever
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enough to automatically use your platform's boot-loader to run an
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update on the booting, allowing you to boot without re-running the
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boot loader. If you have created a modules package, e.g., if you have
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PCMCIA, you'll need to install that package as well.
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</para><para>
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It is time to reboot the system: read carefully any warning that the
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above step may have produced, then <userinput>shutdown -r now</userinput>.
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</para><para>
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For more information on <classname>kernel-package</classname>, read
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the fine documentation in <filename>/usr/share/doc/kernel-package</filename>.
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</para>
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</sect2>
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</sect1>
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