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7. Configuring the Kernel


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 alpha-based systems we have vanilla-sources (the default 2.6 kernel source).

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.

In the next example we install the vanilla-sources. Of course substitute with your choice of sources, this is merely an example:

Code Listing 2.1: Installing a kernel source

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

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


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. You can 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

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, General setup

Code maturity level options --->
  [*] Prompt for development and/or incomplete code/drivers
General setup --->
  [*] Support for hot-pluggable devices

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 only for 2.4 kernel):

Code Listing 3.3: 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
    [*] 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 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 3.4: 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.

The following options are recommended as well:

Code Listing 3.5: Recommended Alpha options

General setup --->
  <*> SRM environment through procfs
  <*> Configure uac policy via sysctl

Plug and Play configuration --->
  <*> Plug and Play support
  <M>   ISA Plug and Play support

SCSI support --->
  SCSI low-level drivers --->
    <*> SYM53C8XX Version 2 SCSI support (NEW)
    <*> Qlogic ISP SCSI support

Network device support --->
  Ethernet (10 or 100 Mbit) --->
    <M> DECchip Tulip (dc21x4x) PCI support
    <M> Generic DECchip & DIGITAL EtherWORKS PCI/EISA
    <M> EtherExpressPro/100 support (eepro100)
    <M> EtherExpressPro/100 support (e100)
  Ethernet (1000 Mbit) --->
    <M> Alteon AceNIC
      [*] Omit support for old Tigon I
    <M> Broadcom Tigon3
  [*] FDDI driver support
  <M> Digital DEFEA and DEFPA
  <*> PPP support
    <*> PPP Deflate compression

Character devices --->
  [*] Support for console on serial port
  [*] Direct Rendering Manager

File systems --->
  <*> Kernel automounter version 4 support
  Network File Systems --->
    <*> NFS
      [*] NFSv3 client
      <*> NFS server
      [*] NFSv3 server
  Partition Types --->
    [*] Advanced partition selection
    [*] Alpha OSF partition support
  Native Language Support
    <*> NLS ISO 8859-1

Sound --->
  <M> Sound card support
    <M> OSS sound modules
      [*] Verbose initialisation
      [*] Persistent DMA buffers
      <M> 100% Sound Blaster compatibles

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 3.6: Compiling the kernel

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

(For 2.6 kernel)
# make && make modules_install

(For all kernels)
# make boot

When the kernel has finished compiling, copy the kernel image to /boot. Recent kernels might create vmlinux instead of vmlinux.gz. Keep this in mind when you copy your kernel image.

Code Listing 3.7: Installing the kernel

# cp arch/alpha/boot/vmlinux.gz /boot/

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 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 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
GenKernel v3.0.1_beta10
* ARCH: Alpha 
* kernel: configuring source
* kernel: running mrproper
(Output removed to increase readability)
* Kernel compiled successfully!
* Required Kernel Params:
*   : root=/dev/ram0 init=/linuxrc real_root=/dev/$ROOT
*     where $ROOT is the devicenode for your root partition as
*     you should have specified in /etc/fstab
* You MUST tell your bootloader to use the generated initrd
* Recommended Kernel Params:
*   : vga=0x317 splash=verbose
* Do NOT report kernel bugs (configs included) as genkernel bugs.
* Make sure you have the latest genkernel before reporting bugs
* For more info see /usr/share/genkernel/README

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*

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

Code Listing 4.4: Emerging and enabling coldplug

# emerge coldplug
# rc-update add coldplug boot

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
x11-drm Accelerated graphics for ATI Radeon up to 9200, Rage128, Matrox, Voodoo and other cards for xorg-x11. Please check the IUSE_VIDEO_CARDS variable in the /usr/portage/x11-base/x11-drm ebuilds to see what you need to fill in as yourcard. VIDEO_CARDS="yourcard" emerge x11-drm

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 x11-drm package:

Code Listing 5.1: View full installation package listing

# emerge --pretend x11-drm

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 5.2: 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 5.3: Editing /etc/modules.autoload.d/kernel-2.4

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

Code Listing 5.4: /etc/modules.autoload.d/kernel-2.4 or kernel-2.6


Continue the installation 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

Roy Marples

Daniel Robbins

Chris Houser

Jerry Alexandratos

Seemant Kulleen
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