Difference between revisions of "How to compile kernel"

Revision as of 17:17, 20 March 2014

It should be noted that the default kernel that ships with Salix OS is "generic" and will work in most cases, and no recompiling is necessary. You should only recompile if:

• Your system's hardware is not supported by the default kernel
• You need (want) features only available in newer kernel
• To get an extra fraction of a percent hardware performance
• For fun

Before going ahead, it may be easier (and will in many cases be sufficient to solve the problem) to install the kernel from Slackware current, which will always be relatively recent. In some cases, you may wish to build the same kernel with different settings.

The procedure is actually quite simple, as long as you remember to reinstall the bootloader before rebooting. However, it can take a long time. Note that all of this should be done with root privileges.

Preparation

First, download the appropriate kernel source from http://www.kernel.org

Unpack source into /usr/src/ and symlink /usr/src/linux to that directory (changing 3.2.25 to whichever kernel version you are compiling)

ln -s /usr/src/linux-3.2.35 /usr/src/linux

Move to that directory, and do

make mrproper

This is a cleaning procedure and will set everything to default. You can work from here, but it is highly recommended to use the working configuration (i.e. the one you've been running on). The default kernel config is found in /proc/config.gz, so copy this file:

zcat /proc/config.gz > .config

Next, you can configure your kernel. The advantage of using the working configuration as a starting-point is you can leave nearly all the options just as they are, but it may be that you need to turn on support for the hardware that doesn't work with the existing kernel; or it may work with the new kernel without changing anything at all. To build the kernel with the working configuration unchanged, do

make oldconfig 

If you do need to change anything, there is a choice of interfaces for the configuration. To use a graphical interface, do

make gconfig

for the gtk-based config, or on a KDE system

make xconfig

for a qt interface.

make menuconfig

will bring up the ncurses based menu-driven config, which will work even if you're not running X.

The number of different options may appear bewildering, but pretty much all of them can (indeed should) be left unchanged from the working configuration. If in doubt, as a rule of thumb, it is probably best to leave it as is.

Compiling

Once you have configured the kernel to your liking, save and exit. Then do:

make

to build the kernel. This will take some hours. After it's done, do:

make modules_install

to install all of the modules. All of the modules will be installed under /lib/modules/version, where 'version' corresponds to the release number of the kernel compiled.

Finalizing

Next, copy the created kernel image and system map to the boot directory.

cp -v arch/x86/boot/bzImage /boot/vmlinuz-version
cp -v System.map /boot/System.map-version

where 'version' corresponds to the release number of the kernel compiled.

Assuming the kernel in your working system is the standard one that came with Salix, the new one you just built will also be a 'huge' kernel that doesn't need an initrd, so you can omit the next step.

However, if you know you do need to create an initrd.gz file:

/usr/share/mkinitrd/mkinitrd_command_generator.sh -l /boot/vmlinux-version

will generate a mkinitrd command that you can then copy and paste. By default, it will output to /boot/initrd.gz. You may want to change this when cutting and pasting so that it will output to /boot/initrd-version.gz. Note that a suggested lilo stanza is also created, but the initrd itself is only made when you run the 'mkinitrd' command itself.

As a final step, edit /etc/lilo.conf so that it will see the newly created kernel, including the 'initrd' line only if you created one:

image = /boot/vmlinuz-version
root = /dev/sda6
initrd = /boot/initrd-version.gz
label = "New Kernel"
read-only

Don't forget to run

lilo

to take effect of the new configuration; or use whichever tool you prefer to install the bootloader.

That's it. When you reboot, you should see the new kernel in the lilo menu, and everything should work. As a check, after the system reboots itself:

uname -a

You should see that the kernel reelase number is indeed the version you just built.

The old kernel is still present, and your bootloader will still include an option to boot with it as a fallback. Once you are certain everything is working properly with the new one, you may wish to remove the old entry from the bootloader and (optionally, with care) delete the old kernel in /boot.