Why would someone want to compile a new kernel? It is often not necessary since the default kernel shipped with Ubuntu handles most configurations. However, it is useful to compile a new kernel in order to:
handle special hardware needs, or hardware conflicts with the pre-supplied kernels
handle hardware or options not included in the stock kernel, such as APM or SMP
optimize the kernel by removing useless drivers to speed up boot time
use options of the kernel which are not supported by the default kernel
run an updated or development kernel
impress your friends, try new things
Don't be afraid to try compiling the kernel. It's fun and profitable.
To compile a kernel the Debian way, you need some packages: kernel-package, kernel-source-126.96.36.199 (the most recent version at the time of this writing), fakeroot and a few others which are probably already installed (see /usr/share/doc/kernel-package/README.gz for the complete list).
This method will make a .deb of your kernel source, and, if you have non-standard modules, make a synchronized dependent .deb of those too. It's a better way to manage kernel images; /boot will hold the kernel, the System.map, and a log of the active config file for the build.
Note that you don't have to compile your kernel the ``Debian way''; but we find that using the packaging system to manage your kernel is actually safer and easier. In fact, you can get your kernel sources right from Linus instead of kernel-source-188.8.131.52, yet still use the kernel-package compilation method.
Note that you'll find complete documentation on using kernel-package under /usr/share/doc/kernel-package. This section just contains a brief tutorial.
Hereafter, we'll assume your kernel source will be located in /usr/local/src and that your kernel version is 184.108.40.206. As root, create a directory under /usr/local/src and change the owner of that directory to your normal non-root account. As your normal non-root account, change your directory to where you want to unpack the kernel sources (cd /usr/local/src), extract the kernel sources (tar xjf /usr/src/kernel-source-220.127.116.11.tar.bz2), change your directory to it (cd kernel-source-18.104.22.168/).
Now, you can configure your kernel. Run make xconfig if X11 is installed, configured and being run, make menuconfig otherwise (you'll need ncurses-dev installed). Take the time to read the online help and choose carefully. When in doubt, it is typically better to include the device driver (the software which manages hardware peripherals, such as Ethernet cards, SCSI controllers, and so on) you are unsure about. Be careful: other options, not related to a specific hardware, should be left at the default value if you do not understand them. Do not forget to select ``Kernel module loader'' in ``Loadable module support'' (it is not selected by default). If not included, your Debian installation will experience problems.
Clean the source tree and reset the kernel-package parameters. To do that, do make-kpkg clean.
Now, compile the kernel: fakeroot make-kpkg --revision=custom.1.0 kernel_image. The version number of ``1.0'' can be changed at will; this is just a version number that you will use to track your kernel builds. Likewise, you can put any word you like in place of ``custom'' (e.g., a host name). Kernel compilation may take quite a while, depending on the power of your machine.
If you require PCMCIA support, you'll also need to install the pcmcia-source package. Unpack the gzipped tar file as root in the directory /usr/src (it's important that modules are found where they are expected to be found, namely, /usr/src/modules). Then, as root, do make-kpkg modules_image.
Once the compilation is complete, you can install your custom kernel like any package. As root, do dpkg -i ../kernel-image-22.214.171.124-subarchitecture_custom.1.0_powerpc.deb. The subarchitecture part is an optional sub-architecture, depending on what kernel options you set. dpkg -i kernel-image... will install the kernel, along with some other nice supporting files. For instance, the System.map will be properly installed (helpful for debugging kernel problems), and /boot/config-126.96.36.199 will be installed, containing your current configuration set. Your new kernel-image-188.8.131.52 package is also clever enough to automatically use your platform's boot-loader to run an update on the booting, allowing you to boot without re-running the boot loader. If you have created a modules package, e.g., if you have PCMCIA, you'll need to install that package as well.
It is time to reboot the system: read carefully any warning that the above step may have produced, then shutdown -r now.
For more information on kernel-package, read the fine documentation in /usr/share/doc/kernel-package.