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7. Configuring the Kernel

Content:

7.a. Timezone

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
(Suppose you want to use GMT)
# 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 Guide.

For x86-based systems we have, amongst other kernels, vanilla-sources (the default 2.4 kernel source as developed by the linux-kernel developers), gentoo-sources (2.4 kernel source patched with performance-enhancing features), gentoo-dev-sources (kernel v2.6 source patched with performance-enhancing features), development-sources (vanilla 2.6 kernel source), ...

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-2.4.26-r9:

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-2.4.26-gentoo-r9

If the symlink doesn't point to the kernel source of your choice (note that linux-2.4.26-gentoo-r9 is merely an example) you should change it to the right kernel:

Code Listing 2.3: Changing the kernel source symlink

# rm /usr/src/linux
# cd /usr/src
# ln -s linux-2.4.26-gentoo-r9 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 LiveCD. 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 genkernel instead.

7.c. Default: Manual Configuration

Introduction

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 viewing the contents of /proc/pci (or by using lspci if available). You can also run lsmod to see what kernel modules the LiveCD 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

First of all, activate the use of development and experimental code/drivers. You need this, otherwise some very important code/drivers won't show up:

Code Listing 3.2: Selecting experimental code/drivers

Code maturity level options --->
  [*] Prompt for development and/or incomplete code/drivers

Make sure that you compile your kernel with the correct processor family:

Code Listing 3.3: Selecting correct processor family

Processor type and features --->
  (Change according to your system)
  (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 Virtual memory, /proc file system, /dev file system + Automatically mount at boot:

Code Listing 3.4: Selecting necessary file systems

(With a 2.4.x kernel)
File systems --->
  [*] Virtual memory file system support (former shm fs)
  [*] /proc file system support
  [*] /dev file system support (EXPERIMENTAL)
  [*]   Automatically mount at boot
  [ ] /dev/pts file system for Unix98 PTYs

(With a 2.6.x kernel)
File systems --->
  Pseudo Filesystems --->
    [*] /proc file system support
    [*] /dev file system support (OBSOLETE)
    [*]   Automatically mount at boot
    [*] Virtual memory file system support (former shm fs)

(Select one or more of the following options as needed by your system)
  <*> Reiserfs support
  <*> Ext3 journalling file system support
  <*> JFS filesystem support
  <*> Second extended fs support
  <*> XFS filesystem support

If your BIOS can't handle large harddrives and you jumpered the harddrive to report a limited size you have to enable the following option to gain access to your whole harddrive:

Code Listing 3.5: Selecting autogeometry resizing support

(2.4.x kernel only)
ATA/IDE/MFM/RLL support --->
  IDE, ATA and ATAPI Block devices --->
    <*>   Include IDE/ATA-2 DISK support
    [ ]     Use multi-mode by default
    [*]     Auto-Geometry Resizing support

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 3.6: Selecting PPPoE necessary drivers

(With a 2.4.x kernel)
Network device support --->
  <*> PPP (point-to-point protocol) support
  <*>   PPP support for async serial ports
  <*>   PPP support for sync tty ports

(With a 2.6.x kernel)
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 ethernet card.

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 3.7: 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 3.8: Activating USB Support for Input Devices

USB Support --->
  <*>   USB Human Interface Device (full HID) support

Laptop-users who want PCMCIA support should not use the PCMCIA drivers if they choose to use a 2.4 kernel. More recent drivers are available through the pcmcia-cs package which will be installed later on. 2.6-kernel users however should use the PCMCIA drivers from the kernel.

When you've finished configuring the kernel, continue with Compiling and Installing.

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

(For 2.4 kernel)
# make dep && make bzImage modules modules_install

(For 2.6 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 2.4.26 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-2.4.26-gentoo-r9
# cp System.map /boot/System.map-2.4.26-gentoo-r9

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-2.4.26-gentoo-r9

Now continue with Installing Separate Kernel Modules.

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 LiveCD 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 Live 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

Now, compile your kernel sources by running genkernel 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).

Code Listing 4.2: Running genkernel

# genkernel 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 Live CD) before your "real" system starts up.

Code Listing 4.3: Checking the created kernel image name and initrd

# ls /boot/kernel* /boot/initrd*

Now, let's perform one more step to get our system to be more like the Live CD -- let's emerge hotplug. While the initrd autodetects hardware that is needed to boot your system, hotplug autodetects everything else. To emerge and enable hotplug, type the following:

Code Listing 4.4: Emerging and enabling hotplug

# emerge hotplug
# rc-update add hotplug default

7.e. Installing Separate Kernel Modules

Installing Extra Modules

If appropriate, you should emerge ebuilds for any additional hardware that is on your system. Here is a list of kernel-related ebuilds that you could emerge:

Ebuild Purpose Command
nvidia-kernel Accelerated NVIDIA graphics for xorg-x11 emerge nvidia-kernel
nforce-audio On-board audio on NVIDIA NForce(2) motherboards emerge nforce-audio
e100 Intel e100 Fast Ethernet Adapters emerge e100
e1000 Intel e1000 Gigabit Ethernet Adapters emerge e1000
emu10k1 Creative Sound Blaster Live!/Audigy support (for 2.4 kernels only) emerge emu10k1
ati-drivers Accelerated ATI Radeon 8500+/FireGL graphics for xorg-x11 emerge ati-drivers

Beware though, some of these ebuilds might deal with big dependencies. To verify what packages will be installed by emerging an ebuild, use emerge --pretend. For instance, for the emu10k1 package:

Code Listing 5.1: View full installation package listing

# emerge --pretend emu10k1

If you don't like the packages it wants to install, use emerge --pretend --verbose to see what USE-flags are checked when deciding the dependencies:

Code Listing 5.2: View USE-flag usage

# emerge --pretend --verbose emu10k1
...
[ebuild  N    ] media-sound/aumix-2.8  +gpm +nls +gtk +gnome +alsa -gtk2

In the previous example you can see that one of emu10k1's dependencies (aumix) uses the gtk and gnome USE-flags, making gtk (which depends on xorg-x11) be compiled with it.

If you don't want all this to be compiled, deselect all USE-flags, for instance:

Code Listing 5.3: Emerging emu10k1 with all USE-flags deselected

# USE="-gpm -nls -gtk -gnome -alsa" emerge --pretend emu10k1

When you're happy with the results, remove the --pretend to start installing emu10k1.

Configuring the Modules

You should list the modules you want automatically loaded in /etc/modules.autoload.d/kernel-2.4 (or kernel-2.6). 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 just compiled:

Code Listing 5.4: 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-2.4 or kernel-2.6 file and enter the module name in it.

Code Listing 5.5: Editing /etc/modules.autoload.d/kernel-2.4

(Example for 2.4 kernels)
# nano -w /etc/modules.autoload.d/kernel-2.4

Code Listing 5.6: /etc/modules.autoload.d/kernel-2.4 or kernel-2.6

3c59x

Now run modules-update to commit your changes to the /etc/modules.conf file:

Code Listing 5.7: Running modules-update

# modules-update

Continue the installation with Configuring your System.


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Page updated March 25, 2005

Summary: The Linux kernel is the core of every distribution. This chapter explains how to configure your kernel.

Sven Vermeulen
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Daniel Robbins
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Chris Houser
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Jerry Alexandratos
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Seemant Kulleen
Gentoo x86 Developer

Tavis Ormandy
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