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7. Configuring the Kernel
You first need to select your timezone so that your system knows where it is
located. Look for your timezone in /usr/share/zoneinfo, then make a
symlink to /etc/localtime using ln:
Code Listing 1.1: Setting the timezone information
# ls /usr/share/zoneinfo
# ln -sf /usr/share/zoneinfo/GMT /etc/localtime
7.b. Installing the Sources
Choosing a Kernel
The core around which all distributions are built is the Linux kernel. It is the
layer between the user programs and your system hardware. Gentoo provides its
users several possible kernel sources. A full listing with description is
available at the Gentoo Kernel
For x86-based systems, our main supported kernel is named
gentoo-sources. This kernel is based on the official Linux sources, but
has security, stability, compatibility and bug fixes applied on top.
Alternatively, the plain and unpatched Linux sources are supplied through the
Both kernel sources are based on the official 2.6 kernel sources. If you
want to install a 2.4-based kernel, you will need to install Gentoo with a
working Internet connection as we do not supply these sources on our
Choose your kernel source and install it using emerge.
Code Listing 2.1: Installing a kernel source
# emerge gentoo-sources
When you take a look in /usr/src you should see a symlink called
linux pointing to your kernel source. We will assume the kernel
source installed is gentoo-sources-2.6.11-r3:
Code Listing 2.2: Viewing the kernel source symlink
# ls -l /usr/src/linux
lrwxrwxrwx 1 root root 12 Oct 13 11:04 /usr/src/linux -> linux-2.6.11-gentoo-r3
If the symlink doesn't point to the kernel source of your choice (note that
linux-2.6.11-gentoo-r3 is merely an example) you should change it to the
Code Listing 2.3: Changing the kernel source symlink
# rm /usr/src/linux
# cd /usr/src
# ln -s linux-2.6.11-gentoo-r3 linux
Now it is time to configure and compile your kernel source. You
can use genkernel for this, which will build a generic kernel as used
by the Installation CD. We explain the "manual" configuration first though, as
it is the best way to optimize your environment.
If you want to manually configure your kernel, continue now with Default: Manual Configuration. If you want to use
genkernel you should read Alternative: Using
7.c. Default: Manual Configuration
Manually configuring a kernel is often seen as the most difficult procedure a
Linux user ever has to perform. Nothing is less true -- after configuring a
couple of kernels you don't even remember that it was difficult ;)
However, one thing is true: you must know your system when you start
configuring a kernel manually. Most information can be gathered by emerging
pciutils (emerge pciutils) which contains lspci. You will now
be able to use lspci within the chrooted environment. You may safely
ignore any pcilib warnings (like pcilib: cannot open
/sys/bus/pci/devices) that lspci throws out. Alternatively, you can run
lspci from a non-chrooted environment. The results are the same.
You can also run lsmod to see what kernel modules the Installation CD
uses (it might provide you with a nice hint on what to enable).
Now go to your kernel source directory and execute make menuconfig. This
will fire up an ncurses-based configuration menu.
Code Listing 3.1: Invoking menuconfig
# cd /usr/src/linux
# make menuconfig
You will be greeted with several configuration sections. We'll first list some
options you must activate (otherwise Gentoo will not function, or not function
properly without additional tweaks).
Activating Required Options
Make sure that every driver that is vital to the booting of your system (such as
SCSI controller, ...) is compiled in the kernel and not as a module,
otherwise your system will not be able to boot completely.
Now select the correct processor family:
Code Listing 3.2: General Support and processor family
General setup --->
[*] Support for hot-pluggable devices
Processor type and features --->
Subarchitecture Type (PC-compatible) --->
(Athlon/Duron/K7) Processor family
Now go to File Systems and select support for the filesystems you use.
Don't compile them as modules, otherwise your Gentoo system will not be
able to mount your partitions. Also select /proc file system and
Virtual memory. Do not select the /dev file system.
Code Listing 3.3: Selecting necessary file systems
File systems --->
Pseudo Filesystems --->
<*> /proc file system support
< > /dev file system support (OBSOLETE)
<*> Virtual memory file system support (former shm fs)
<*> Reiserfs support
<*> Ext3 journalling file system support
<*> JFS filesystem support
<*> Second extended fs support
<*> XFS filesystem support
Do not forget to enable DMA for your drives:
Code Listing 3.4: Activating DMA
Device Drivers --->
ATA/ATAPI/MFM/RLL support --->
[*] Generic PCI bus-master DMA support
[*] Use PCI DMA by default when available
If you are using PPPoE to connect to the Internet or you are using a dial-up
modem, you will need the following options in the kernel:
Code Listing 3.5: Selecting PPPoE necessary drivers
Device Drivers --->
Networking support --->
<*> PPP (point-to-point protocol) support
<*> PPP support for async serial ports
<*> PPP support for sync tty ports
The two compression options won't harm but are not definitely needed, neither
does the PPP over Ethernet option, that might only be used by
rp-pppoe when configured to do kernel mode PPPoE.
If you require it, don't forget to include support in the kernel for your
If you have an Intel CPU that supports HyperThreading (tm), or you have a
multi-CPU system, you should activate "Symmetric multi-processing support":
Code Listing 3.6: Activating SMP support
Processor type and features --->
<*> Symmetric multi-processing support
If you use USB Input Devices (like Keyboard or Mouse) don't forget to enable
those as well:
Code Listing 3.7: Activating USB Support for Input Devices
Device Drivers --->
USB Support --->
<*> USB Human Interface Device (full HID) support
[*] HID input layer support
If you are a laptop user and require PCMCIA support, remember to compile it
into the kernel. As well as the option below, be sure to enable support for
the PCMCIA card bridge present in your system (found in the same menu of the
Code Listing 3.8: Activating PCMCIA support
Bus options (PCI, PCMCIA, EISA, MCA, ISA) --->
PCCARD (PCMCIA/CardBus) support --->
<*> PCCard (PCMCIA/CardBus) support
<*> 16-bit PCMCIA support
[*] 32-bit CardBus support
--- PC-card bridges
<*> CardBus yenta-compatible bridge support (NEW)
<*> Cirrus PD6729 compatible bridge support (NEW)
<*> i82092 compatible bridge support (NEW)
<*> i82365 compatible bridge support (NEW)
<*> Databook TCIC host bridge support (NEW)
Compiling and Installing
Now that your kernel is configured, it is time to compile and install it. Exit
the configuration and start the compilation process:
Code Listing 3.9: Compiling the kernel
# make && make modules_install
When the kernel has finished compiling, copy the kernel image to
/boot. From here onwards we assume that the kernel you are
installing is the 2.6.11-r3 version of the gentoo-sources. Use whatever
name you feel is appropriate for your choice and remember it as you will need
it later on when you configure your bootloader.
Code Listing 3.10: Installing the kernel
# cp arch/i386/boot/bzImage /boot/kernel-2.6.11-gentoo-r3
It is also wise to copy over your kernel configuration file to
/boot, just in case :)
Code Listing 3.11: Backing up your kernel configuration
# cp .config /boot/config-2.6.11-gentoo-r3
Now continue with Configuring Kernel
7.d. Alternative: Using genkernel
If you are reading this section, you have chosen to use our genkernel
script to configure your kernel for you.
Now that your kernel source tree is installed, it's now time to compile your
kernel by using our genkernel script to automatically build a kernel for
you. genkernel works by configuring a kernel nearly identically to the
way our Installation CD kernel is configured. This means that when you use
genkernel to build your kernel, your system will generally detect all
your hardware at boot-time, just like our Installation CD does. Because genkernel
doesn't require any manual kernel configuration, it is an ideal solution for
those users who may not be comfortable compiling their own kernels.
Now, let's see how to use genkernel. First, emerge the genkernel ebuild:
Code Listing 4.1: Emerging genkernel
# emerge genkernel
Next, copy over the kernel configuration used by the Installation CD to the
location where genkernel looks for the default kernel configuration:
Code Listing 4.2: Copying over the Installation CD kernel config
# zcat /proc/config.gz > /usr/share/genkernel/x86/kernel-config-2.6
Now, compile your kernel sources by running genkernel --udev all.
Be aware though, as genkernel compiles a kernel that supports almost all
hardware, this compilation will take quite a while to finish!
Note that, if your boot partition doesn't use ext2 or ext3 as filesystem you
might need to manually configure your kernel using genkernel --menuconfig
all and add support for your filesystem in the kernel (i.e.
not as a module). Users of EVMS2 or LVM2 will probably want to add
--evms2 or --lvm2 as argument as well.
Code Listing 4.3: Running genkernel
# genkernel --udev all
Once genkernel completes, a kernel, full set of modules and
initial root disk (initrd) will be created. We will use the kernel
and initrd when configuring a boot loader later in this document. Write
down the names of the kernel and initrd as you will need it when writing
the bootloader configuration file. The initrd will be started immediately after
booting to perform hardware autodetection (just like on the Installation CD)
before your "real" system starts up.
Code Listing 4.4: Checking the created kernel image name and initrd
# ls /boot/kernel* /boot/initrd*
If you want your system to be more like the Installation CD you should,
when your Gentoo installation is over, emerge coldplug. While the
initrd autodetects hardware that is needed to boot your system,
coldplug autodetects everything else. coldplug is available as one
of the packages on the Package CD.
Code Listing 4.5: Emerging and enabling coldplug
# emerge -k coldplug
# rc-update add coldplug boot
7.e. Configuring Kernel Modules
Configuring the Modules
You should list the modules you want automatically loaded in
/etc/modules.autoload.d/kernel-2.6 (or kernel-2.4).
You can add extra options to the modules too if you want.
To view all available modules, run the following find command. Don't
forget to substitute "<kernel version>" with the version of the kernel you
Code Listing 5.1: Viewing all available modules
# find /lib/modules/<kernel version>/ -type f -iname '*.o' -or -iname '*.ko'
For instance, to automatically load the 3c59x.o module, edit the
kernel-2.6 (or kernel-2.4) file and enter the module
name in it.
Code Listing 5.2: Editing /etc/modules.autoload.d/kernel-2.6
# nano -w /etc/modules.autoload.d/kernel-2.6
Code Listing 5.3: /etc/modules.autoload.d/kernel-2.6
Now continue with Configuring your System.
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