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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 make a symlink to /etc/localtime using ln:

Code listing 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, 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.

Code listing 2: 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 3: 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 right kernel:

Code listing 4: 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 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. 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 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 5: 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 you compile your kernel with the correct processor family and subarchitecture type:

Code listing 6: Selecting correct processor family

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. Do not select the /dev file system.

Code listing 7: 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)

(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 8: 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 9: 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 10: 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 11: 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 run make && make modules_install:

Code listing 12: 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 13: 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 14: Backing up your kernel configuration

# cp .config /boot/config-2.6.11-gentoo-r3

Now continue with Configuring your System.

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 15: 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 16: Copying over the Installation CD kernel config

# zcat /proc/config.gz > /usr/share/genkernel/x86/kernel-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 17: 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 18: 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 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 19: Emerging and enabling coldplug

# emerge coldplug
# rc-update add coldplug boot

If you want your system to react to hotplugging events, you will need to install and setup hotplug as well:

Code listing 20: Emerging and enabling hotplug

# emerge hotplug
# rc-update add hotplug default

Now continue with Configuring your System.

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