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7. Configuring the Kernel


7.a. Installing the Kernel 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 PPC-based systems we have gentoo-sources (kernel source patched for extra features).

Choose your kernel source and install it using emerge.

Code Listing 1.1: 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. In this case, the installed kernel source points to gentoo-sources-3.4.9. Your version may be different, so keep this in mind.

Code Listing 1.2: Viewing the kernel source symlink

# ls -l /usr/src/linux
lrwxrwxrwx    1 root   root    12 Oct 13 11:04 /usr/src/linux -> linux-3.4.9

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.b. 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 few kernels you won'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, it's time to configure your kernel. Start by configuring a kernel that will boot on most 32 Bit PowerPC machines by first running make pmac32_defconfig. After the default configuration has been generated, run make menuconfig to start an ncurses-based configuration menu.

Code Listing 2.1: Invoking menuconfig

# cd /usr/src/linux
# make pmac32_defconfig
# 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). We also have a Gentoo Kernel Configuration Guide on the Gentoo wiki that might help you further.

Activating Required Options

First go to File Systems and select support for the filesystems you use. Don't compile the file system you use for the root filesystem as module, otherwise your Gentoo system will not be able to mount your partition. Also select Virtual memory and /proc file system. Make sure that you also enable support for Amiga partitions if you are using a Pegasos, or Macintosh partitions if you are using an Apple computer.

Code Listing 2.2: Selecting necessary file systems

File systems --->
  Pseudo Filesystems --->
(/proc may already be forced on by your configuration, if so, you'll see --- instead)
    [*] /proc file system support
    [*] Virtual memory file system support (former shm fs)
  Partition Types --->
    [*] Advanced partition support
    [*]   Amiga partition table support
    [*]   Macintosh partition map support

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

Users of NewWorld and OldWorld machines will want HFS support as well. OldWorld users require it for copying compiled kernels to the MacOS partition. NewWorld users require it for configuring the special Apple_Bootstrap partition:

Code Listing 2.3: Activating HFS support

File Systems --->
  Miscellaneous filesystems --->
    <M> Apple Macintosh file system support
    <M> Apple Extended HFS file system 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 2.4: Selecting PPPoE necessary drivers

Device Drivers --->
  Network device 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 always needed. The PPP over Ethernet option might only be used by ppp when configured to perform kernel mode PPPoE.

Don't forget to include support in the kernel for your ethernet card! Most newer Apple computers use the SunGEM ethernet driver. Older iMacs commonly use the BMAC driver.

Code Listing 2.5: Selecting the network driver

Device Drivers --->
  Network device support --->
    Ethernet (10 or 100Mbit) --->
      [*] Ethernet (10 or 100Mbit)
      <*>   Generic Media Independent Interface device support
      <*>   MACE (Power Mac ethernet) support
      <*>   BMAC (G3 ethernet) support
      <*>   Sun GEM support

If you're booting from FireWire, you'll need to enable these options. If you do not want to compile in support, you'll need to include these modules and their dependencies in an initrd.

Code Listing 2.6: Enable support for FireWire devices on boot

Device Drivers --->
  IEEE 1394 (FireWire) support --->
    <*> IEEE 1394 (FireWire) support
    <*>   OHCI-1394 support
    <*>   SBP-2 support (Harddisks etc.)

If you're booting from USB, you'll need to enable these options. If you do not want to compile in support, you'll need to include these modules and their dependencies in an initrd.

Code Listing 2.7: Enable support for USB devices on boot

Device Drivers --->
  USB support --->
    <*> Support for Host-side USB
    <*>   OHCI HCD support
    <*>   USB Mass Storage support

Do not turn off kernel framebuffer support as it is required for a successful boot. If you are using an NVIDIA based chipset, you should use the Open Firmware framebuffer. If you are using an ATI based chipset, you should select the framebuffer driver based upon your chipset (Mach64, Rage128 or Radeon).

Code Listing 2.8: Choosing a Framebuffer Driver

Device Drivers --->
  Graphics support --->
    <*> Support for frame buffer devices
    [*] Open Firmware frame buffer device support
    <*> ATI Radeon display support
    <*> ATI Rage128 display support
    <*> ATI Mach64 display support
    Console display driver support --->
      <*> Framebuffer Console support

Note: If you select more than one framebuffer device, it may default to a less than optimal driver. Either use only one framebuffer device or specify which to use by passing the driver to use to the kernel on boot by appending a video line such as: video=radeonfb.

Next select Maintain a devtmpfs file system to mount at /dev so that critical device files are already available early in the boot process.

Code Listing 2.9: Enabling devtmpfs support

Device Drivers --->
  Generic Driver Options --->
    [*] Maintain a devtmpfs filesystem to mount at /dev
    [ ]   Automount devtmpfs at /dev, after the kernel mounted the rootfs

When you're done configuring your 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 menu and run the following commands:

Code Listing 2.10: Compiling the kernel

# make && make modules_install

When the kernel has finished compiling, copy the kernel image to /boot as shown below. If you have a separate boot partition, as on Pegasos computers, be sure that it is mounted properly. If you are using BootX to boot, we'll copy the kernel later.

Yaboot and BootX expect to use an uncompressed kernel unlike many other bootloaders. The uncompressed kernel is called vmlinux and it is placed in /usr/src/linux after the kernel has finished compiling. If you are using a Pegasos machine, the Pegasos firmware requires a compressed kernel called zImage which can be found in /usr/src/linux/arch/powerpc/boot/images.

Code Listing 2.11: Installing the kernel

# cd /usr/src/linux
Note, your kernel version might be different
# cp vmlinux /boot/kernel-3.4.9-gentoo
# cp arch/powerpc/boot/images/zImage /boot/kernel-3.4.9-gentoo

Now continue with Kernel Modules.

7.c. Alternative: Using genkernel

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 3.1: Emerging genkernel

# emerge genkernel

If you are using FireWire or USB to boot, you'll need to add modules to the initrd. Edit /usr/share/genkernel/arch/ppc/modules_load and change MODULES_FIREWIRE="ieee1394 ohci1394 sbp2" for FireWire support or MODULES_USB="usbcore ohci-hcd ehci-hcd usb-storage" for USB support.

Before compiling your sources, the fstab needs a slight adjustment. The rest of the fstab will be completed during a later step, so don't worry about the details now. If you did not create a separate boot partition (NOT bootstrap, that's different), remove the line referencing /boot from /etc/fstab. This will need to be done on most Apple computers.

Code Listing 3.2: Removing /boot from /etc/fstab on machines without a boot partition

# nano -w /etc/fstab
Remove this line
/dev/BOOT  /boot    ext2    noauto,noatime    0 2

Now, compile your kernel sources by running genkernel --genzimage all. For Pegasos, we will need to use a different config and create a zImage instead of the vmlinux kernel used on Apple machines. Be aware, as genkernel compiles a kernel that supports almost all hardware, this compilation can take quite a while to finish!

Note that, if your partition where the kernel should be located 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 LVM2 will probably want to add --lvm as an argument as well.

Code Listing 3.3: Running genkernel

# genkernel all

Code Listing 3.4: Running genkernel on the Pegasos

# genkernel --genzimage all

Once genkernel completes, a kernel, full set of modules and initial ram disk (initramfs) 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 them 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. Be sure to also copy down the required boot arguments, these are required for a successful boot with genkernel.

Code Listing 3.5: Checking the created kernel image name and initrd

Note, your kernel version might be different
# ls /boot/kernel-genkernel-ppc-3.4.9-gentoo /boot/initramfs-genkernel-ppc-3.4.9-gentoo

Now continue with Kernel Modules.

7.d. Kernel Modules

Configuring the Modules

You should list the modules you want automatically loaded in /etc/conf.d/modules. 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 4.1: Viewing all available modules

# find /lib/modules/<kernel version>/ -type f -iname '*.o' -or -iname '*.ko' | less

For instance, to automatically load the 3c59x.ko module (which is the driver for a specific 3Com network card family), edit the /etc/conf.d/modules file and enter the module name in it.

Code Listing 4.2: Editing /etc/conf.d/modules

# nano -w /etc/conf.d/modules

Continue the installation with Configuring your System.

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Page updated May 11, 2014

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

Sven Vermeulen

Grant Goodyear

Roy Marples

Daniel Robbins

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