Kernel/Configuration

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This article describes the manual configuration and setup of the Linux kernel. For an automatic method see the genkernel article.

Set symlink

The symlink /usr/src/linux should always point to the kernel sources that is currently being used. This can be done in one of three ways:

1. Install the kernel sources with the symlink USE flag active.
This will make the /usr/src/linux point to the newly installed kernel sources. If necessary, it can still be modified later with the following two methods:
2. Setting the symlink with the eselect tool
root # eselect kernel list
Available kernel symlink targets:
  [1] linux-3.3.8-gentoo *
  [2] linux-3.4.9-gentoo
This outputs the available kernel sources. The asterisk indicates the chosen sources. To change the kernel sources, e.g. to the second entry, do:
root #eselect kernel set 2
3. Setting the symlink manually
root #ln -sf /usr/src/linux-3.4.9-gentoo /usr/src/linux
root #ls -l /usr/src/linux
lrwxrwxrwx 1 root root 11 Aug 29 22:10 /usr/src/linux -> /usr/src/linux-3.4.9-gentoo

Configuration tools

The kernel offers several tools to configure itself:

make config
Text based configuration. The options are prompted one after another. All options need to be answered, and out-of-order access to former options is not possible.
make menuconfig
Graphical menu (only text input). Navigate through the menu to modify the desired options.
make nconfig
Graphical menu based on ncurses. Requires sys-libs/ncurses to be installed.
make xconfig
Graphical menu using Qt4. Requires dev-qt/qtgui to be installed.
make gconfig
Graphical menu using GTK+. Requires x11-libs/gtk+, dev-libs/glib and gnome-base/libglade to be installed.
make oldconfig
Review changes between kernel versions and update to create new .config for kernel.


There are also several scripts to create miscellaneous default configurations. See:

root #make help

Configuration

This article describes the configuration using the make menuconfig tool, but the procedure is similar for the other menus.

root #cd /usr/src/linux
root #make menuconfig

Usage

In the shown menu the blue bar indicates the position of the cursor. With the and arrow keys change the position of the cursor. The and arrow keys traverse the menu bar in the bottom and define what happens when the Enter key is pressed. For the menu bar below, Select switches to a sub menu for the menu entries ending with ---> while Exit exits a sub menu. As an alternative the Esc key can be pressed twice to exit the application.

Menu entries, which start with brackets, are drivers or features that can be activated:

[ ], [*]
Options in square brackets can be activated or deactivated. The asterisk marks the menu entry as activated. The value can be changed with the space key. It is also possible to press Y key (Yes) to activate or N key (No) to deactivate the selected entry.
If the option is activated, the selected feature/driver will be built into the kernel and will always be available at boot time.
< >, <M>, <*>
Options in angle brackets can be activated or deactivated, but also activated as module (indicated by a M). The values can be modified by pressing Y/N keys as before or by pressing the M key to activate the feature/driver as a module.
See the Kernel Modules article for differentiation.
{M}, {*}
Options in curly brackets can be activated or activated as module but not be deactivated. This happens because another driver is dependent on this feature.
-M-, -*-
Options between hyphens are activated in the shown way by another driver. There is no choice.


Furthermore some menu entries have a tag at the end:

(NEW)
This driver is new in the kernel and is maybe not stable enough.
(EXPERIMENTAL)
This driver is experimental and most likely not stable enough.
(DEPRECATED)
This driver is deprecated and not needed for most systems.
(OBSOLETE)
This driver is obsolete and should not be activated.


Most options have a description, which see by pressing the H key or choosing Help in the menu bar.

Driver selection

See the hardware detection article and the articles in the Hardware category.

Search modules

Within menuconfig, use the / key to search modules by name.

As shown below, the search result will show numbers in front of the matches. Pressing 1 in the example below would make menuconfig jump straight to the option Bluetooth device drivers in the menu structure.

KERNEL
Symbol: BT_HCIBTUSB [=m]                       
Type  : tristate                               
Prompt: HCI USB driver                         
  Location:                                    
    -> Networking support (NET [=y])           
      -> Bluetooth subsystem support (BT [=y]) 
(1)     -> Bluetooth device drivers            
  Defined at drivers/bluetooth/Kconfig:5       
  Depends on: NET [=y] && BT [=y] && USB [=m]

Enabling Gentoo Linux common settings

There is a kernel configuration option called CONFIG_GENTOO_LINUX only present in sys-kernel/gentoo-sources and other Kernel Project maintained kernels. It does nothing on its own, but sets various required configuration options for typical installations.

This setting automatically selects tmpfs and devtmpfs support, which are needed for handling /dev on Gentoo Linux, but might be expanded in the future to enable other mandatory settings for a Gentoo Linux system. For more information, read the help information available through the kernel configuration system as described earlier in this guide.

Build

After configuration has been accomplished successfully, compile the kernel:

root #make

For processors with multiple cores, make all the cores do the work: add the option -j(NUMBER_CORES +1). For example, a dual core processor contains two cores plus one (2+1):

root #make -j3

Setup

If drivers are activated as modules, they must be installed:

root #make modules_install

The modules will be copied to a sub directory of /lib/modules.

To install the actual kernel:

root #make install

This command executes /sbin/installkernel, which is part of the sys-apps/debianutils package. The new kernel is installed into /boot/vmlinuz-{version}. If a symbolic link /boot/vmlinuz already exists, it is refreshed by making a link from /boot/vmlinuz to the new kernel, and the previously installed kernel is available as /boot/vmlinuz.old. (installkernel man page). The same for config and System.map files. These symlinks are handy, because they always point to the newest kernel without changing the file path (e.g. they can be used in the bootloader configuration).

Bootloader

Change the system's bootloader configuration to pick up at boot the new kernel.

Finally restart the system with the new kernel.