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< Kernel

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

    $ mkdir ~/build/
    $ cd ~/build/

Install the devtools and base-devel package.

You need a clean kernel to start your customization from. Retrieve PKGBUILD source using Git and few other files into your build directory by running:

    $ pkgctl repo clone --protocol=https linux

At this point, the directory tree looks like (there may be a few other files):

    ~/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.:

    PKGBUILD

    pkgbase=linux-custom

Warning: Do not add `linux` to the `provides` array. Your custom kernel will not be compatible with binary modules built against that kernel, so it cannot satisfy that dependency. Similarly, do not add `linux-headers` to the `provides` array of the headers package, for similar reasons.

### Avoid creating the doc

A large portion of the lengthy compiling effort is devoted to creating the documentation. As of 25 August 2022, the following patch to PKGBUILD avoids its creation:

    63c63
    <   make htmldocs all
    ---
    >   make all
    195c195
    < pkgname=("$pkgbase" "$pkgbase-headers" "$pkgbase-docs")
    ---
    > pkgname=("$pkgbase" "$pkgbase-headers")

This patch changes lines #63 and #195. You might have to edit the PKGBUILD file manually if it does not apply cleanly.

### Changing prepare()

In `prepare()` function, you can apply needed kernel patches or change kernel build configuration. As of 20 August 2023, the PKGBUILD automatically applies all `*.patch` files in source.

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

Or you can use a GUI tool to tweak the options. Comment `make olddefconfig` in the prepare() function of the PKGBUILD, and add your favorite tool (run `make help` to list all of the possible configuration targets):

    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
      make prepare
    ...

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 one before running makepkg, install the pacman-contrib package and 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.
- The compilation can take up to several hours to complete depending on the hardware performance. Running compilation jobs simultaneously can reduce compilation time significantly on multi-core systems.
- It can be informative to run the above `makepkg` using the `time` command to know how long your system took to perform the compilation.

## Installing

The compile step will leave two packages in the `~/build/linux` folder, one for the kernel and one for the kernel headers. They might have names like:

    linux-custom-5.8.12-x86_64.pkg.tar.zst
    linux-custom-headers-5.8.12-x86_64.pkg.tar.zst

Best practice is to install both packages together as they might be both needed (e.g. DKMS):

    # pacman -U linux-custom-headers-5.8.12-x86_64.pkg.tar.zst linux-custom-5.8.12-x86_64.pkg.tar.zst

(substitute the actual names of the files you have in the folder)

## Boot loader

If you have modified `pkgbase` in order to have your new kernel installed alongside the default kernel you will need to update your bootloader configuration file and add new entries ('default' and 'fallback') for your custom kernel and the associated initramfs images.

## Updating

Assuming one has an arch kernel source that they want to update, one method to do that is with https://github.com/archlinux/linux. 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 `https://github.com/archlinux/linux.git`. The other one is at `src/archlinux-linux/`, pulling from the bare repository. Possible local patches, and building, are 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

    $ cd ~/build/linux/archlinux-linux/
    $ git log --oneline --max-count 1 HEAD

    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.

    $ git fetch --verbose

One can see it fetched v5.2.7-arch1, which was the newest archlinux tag, because it prints what new tags were obtained. If no new tags were obtained then there is no newer archlinux source available.

Now the source can be updated where the actual build will take place.

    $ cd ~/build/linux/src/archlinux-linux/
    $ git checkout master
    $ git pull
    $ git fetch --tags --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 --max-count=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 `Arch Linux kernel v5.2.7-arch1` and `Linux 5.2.7`.

The up to date PKGBUILD, as well archlinux kernel configuration file, can be pulled in using `git` in the package directory:

    $ cd ~/build/linux/
    $ git pull

Now you should merge files located in `~/build/linux/linux/*` into `~/build/linux/`. Merging can also done manually, or with specific utilities. Review #Changing prepare(), and run manually most, if not all, the shell commands of PKGBUILD::prepare().

At this point, `makepkg --verifysource` should succeed. While #Compiling, make sure to also add `--noextract` option to the `makepkg` command, since it should be able to build the packages as if the source was extracted by `makepkg --nobuild`. And you are back to #Installing.

### Cleanup

One will probably want to remove `~/build/linux/linux/` after merging. In addition, `~/build/linux/src/archlinux` will accumulate branches in the form of `5.2.7-arch1` if more recent updates are done in this fashion. These can be deleted with

    $ cd ~/build/linux/src/archlinux
    $ git branch --delete --force --verbose 5.2.7-arch1

## See also

- https://docs.kernel.org/kbuild/kconfig.html and the parent directory

Category:

- Kernel