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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 copy it to /etc/localtime. Please avoid the /usr/share/zoneinfo/Etc/GMT* timezones as their names do not indicate the expected zones. For instance, GMT-8 is in fact GMT+8.

Code Listing 1.1: Setting the timezone information

# ls /usr/share/zoneinfo
(Suppose you want to use GMT)
# cp /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, 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 vanilla-sources package.

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 Installation CD.

Choose your kernel source and install it using emerge. The USE="-doc" is necessary to avoid installing xorg-x11 or other dependencies at this point. USE="symlink" is not necessary for a new install, but ensures proper creation of the /usr/src/linux symlink.

Code Listing 2.1: Installing a kernel source

# USE="-doc symlink" emerge gentoo-sources

When you take a look in /usr/src you should see a symlink called linux pointing to your kernel source. In this case, the installed kernel source points to gentoo-sources-2.6.12-r10. Your version may be different, so keep this in mind.

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-2.6.12-gentoo-r10

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 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 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)  --->
  (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 /proc file system and Virtual memory.

Code Listing 3.3: Selecting necessary file systems

File systems --->
  Pseudo Filesystems --->
    <*> /proc file system support
    <*> 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

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 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 3.6: 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 configuration).

Code Listing 3.8: Activating PCMCIA support

Bus options (PCI, PCMCIA, EISA, MCA, ISA)  --->
  PCCARD (PCMCIA/CardBus) support  --->
    <*> PCCard (PCMCIA/CardBus) support
    (select 16 bit if you need support for older PCMCIA cards. Most people want this.)
    <*>   16-bit PCMCIA support
    [*]   32-bit CardBus support
    (select the relevant bridges below)
    --- 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. Use whatever name you feel is appropriate for your kernel choice and remember it as you will need it later on when you configure your bootloader. Remember to replace <kernel-version> with the name and version of your kernel.

Code Listing 3.10: Installing the kernel

# cp arch/i386/boot/bzImage /boot/<kernel-version>

Now continue with Configuring 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 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

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). Users of EVMS2 or LVM2 will probably want to add --evms2 or --lvm2 as argument as well.

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 Installation CD) before your "real" system starts up.

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

# ls /boot/kernel* /boot/initramfs*

If you want your system to be more like the Installation CD you should emerge coldplug. While the initrd autodetects hardware that is needed to boot your system, coldplug autodetects everything else.

Code Listing 4.4: Emerging and enabling coldplug

# emerge 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 just compiled:

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

(Example for 2.6 kernels)
# nano -w /etc/modules.autoload.d/kernel-2.6

Code Listing 5.3: /etc/modules.autoload.d/kernel-2.6

3c59x

Now continue with Configuring your System.


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Page updated January 6, 2006

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

Sven Vermeulen
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Roy Marples
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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

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Jason Huebel
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