Kernel/Arch build system

See Kernels for the main article.

The Arch Build System can be used to build a custom kernel based on the official linux package. This compilation method can automate the entire process, and is based on a very well tested package. You can edit the PKGBUILD to use a custom kernel configuration or add additional patches.

Getting the Ingredients

Since you'll be using makepkg, follow the best practices outlined there first. For example, you cannot run makepkg as root/sudo. Therefore, create a build directory in your user home first.

 $ cd ~/
 $ mkdir build
 $ cd build/

Install the asp package and the base-devel package group.

You need a clean kernel to start your customization from. Fetch the latest kernel package files from ABS into your build directory by running:

$ asp update linux
$ asp checkout linux

At this point, the directory tree looks like:

~/build/linux/-+
               +--config
               \__PKGBUILD

Then, get any other file you need (e.g. custom configuration files, patches, etc.) from the respective sources.

Modifying the PKGBUILD

Edit PKGBUILD and look for the pkgbase parameter. Change this to your custom package name, e.g.:

 pkgbase=linux-custom

Changing prepare()

In prepare() function, you can apply needed kernel patches or change kernel build configuration.

If you need to change a few config options you can edit config file in the source.

Or you can use GUI tool to tweak the options. Comment make olddefconfig in the prepare() function of the PKGBUILD, and add your favorite tool:

PKGBUILD

...
  msg2 "Setting config..."
  cp ../config .config
  #make olddefconfig

  make nconfig # new CLI menu for configuration
  #make menuconfig # CLI menu for configuration
  #make xconfig # X-based configuration
  #make oldconfig # using old config from previous kernel version
  # ... or manually edit .config
...

Warning: systemd has a number of kernel configuration requirements for general use, for specific usecases (e.g., UEFI) and for specific systemd functionality (e.g., bootchart). Failure to meet these requirements can result in your system being degraded or unusable. The list of required and recommended kernel CONFIGs can be found in /usr/share/doc/systemd/README. Check them before you compile.These requirements also change over time. Because Arch assumes you are using the official kernel, there will be no announcement of these changes. Before you install a new version of systemd, check the version release notes to make sure your current custom kernel meets any new systemd requirements.

Generate new checksums

#Changing prepare() suggests a possible modification to $_srcname/.config. Since this path is not where downloading the package files ended, its checksum was not checked by makepkg (which actually checked $_srcname/../../config).

If you replaced the downloaded config with another config file before running makepkg, Install the pacman-contrib package. Which will generate new checksums by running:

$ updpkgsums

Compiling

You can now proceed to compile your kernel by the usual command makepkg

If you have chosen an interactive program for configuring the kernel parameters (like menuconfig), you need to be there during the compilation.

 $ makepkg -s

The -s parameter will download any additional dependencies used by recent kernels such as xml and docs.

Note:

• Kernel sources are PGP signed, and makepkg will attempt to verify them. See Makepkg#Signature checking for details.
• Running compilation jobs simultaneously can reduce compilation time significantly on multi-core systems.

Installing

After running makepkg, you can have a look at the linux.install file. You will see that some variables have changed.

Now, you only have to install the packages as usual. Best practice is to install both packages together as they might be both needed (e.g. DKMS.)

# pacman -U kernel-headers_package kernel_package

Boot Loader

Now, the folders and files for your custom kernel have been created, e.g. /boot/vmlinuz-linux-test. To test your kernel, update your bootloader configuration file and add new entries ('default' and 'fallback') for your custom kernel. If you renamed your kernel in the PKGBUILD pkgbase you may have to rename the initramfs.img in your $build/pkg/kernel/etc before installing with pacman. That way, you can have both the stock kernel and the custom one to choose from.

Updating

Assuming one has an arch kernel source that he wants to update, one method to do that is with https://git.archlinux.org/linux.git. Follows a concrete example. In what follows, the top kernel source directory is assumed at ~/build/linux/. In general, arch sets an arch kernel source with two local git repositories. The one at archlinux-linux/ is a local bare git repository pointing to git://git.archlinux.org/linux.git. The other one is at src/archlinux-linux/, pulling from the first repository. Possible local patches, and building, is expected at src/archlinux-linux/.

For this example, the HEAD of the locally installed bare git repository source at archlinux-linux/ was initially pointing to 4010b622f1d2 Merge branch 'dax-fix-5.3-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm, which is somewhere between v5.2.5-arch1 and v5.2.6-arch1.

$ cd ~/build/linux/archlinux-linux/
$ git fetch --verbose

That was fetching v5.2.7-arch1, which was the newest archlinux tag.

$ cd ~/build/linux/src/archlinux-linux/
$ git checkout master
$ git pull
$ git fetch --tag --verbose
$ git branch --verbose 5.2.7-arch1 v5.2.7-arch1
$ git checkout 5.2.7-arch1

You can verify you are on track with something like

$ git log --oneline 5.2.7-arch1 | head --lines 7
13193bfc03d4 Arch Linux kernel v5.2.7-arch1
9475c6772d05 netfilter: nf_tabf676926c7f60les: fix module autoload for redir
498d650048f6 iwlwifi: Add support for SAR South Korea limitation
bb7293abdbc7 iwlwifi: mvm: disable TX-AMSDU on older NICs
f676926c7f60 ZEN: Add CONFIG for unprivileged_userns_clone
5e4e503f4f28 add sysctl to disallow unprivileged CLONE_NEWUSER by default
5697a9d3d55f Linux 5.2.7

This shows few specific archlinux patches between Linux 5.2.7 and Arch Linux kernel v5.2.7-arch1. The important lines here are Linux 5.2.7 and Arch Linux kernel v5.2.7-arch1. Obviously, there might be other patches at other versions, and the commit identifiers, such as f676926c7f60, as well as the kernel version, will be different for other versions.

The up to date PKGBUILD, as well archlinux kernel configuration file, can be pulled in by the asp command:

$ cd ~/build
$ asp update linux
$ asp -f export linux
$ cd ~/build/linux

Then run manually most, if not all, the shell commands of PKGBUILD::prepare().

At this point, makepkg --verifysource should succseed. And makepkg --noextract should be able to build the packages as if the source was extracted by makepkg --nobuild.

See Also

• https://kernel.org/doc/Documentation/kbuild/kconfig.txt and the parent directory