Linux kernel

Compile the Linux kernel in the build/ directory using the workspace container by defining an appropriate ${dir}/linux.cfg file and targeting build/${dir}/linux/install.

Linux Kconfig build configuration

The .config target is the Kbuild configuration containing options set for the kernel build. This file is generated using scripts/kconfig/merge_config.sh from the kernel source, which combines the defconfig and *.kconfig fragments.

Target: build/${dir}/linux/.config
Required dependencies:
- ${dir}/linux.cfg - See Configuration file.
Optional dependencies:
- *.kconfig - Fragments of Kconfig configuration that get merged into the defconfig, if specified as DEFCONFIG in linux.cfg. If no defconfig is specified, the *.kconfig fragments are merged to create the complete .config.
Built dependencies:
- build/${dir}/linux.src - The sentinel file generated when the Linux source code is downloaded to build/${dir}/linux using one of the file fetching rules.

Build Linux kernel and modules

The install target is the destination directory that the kernel and modules will be installed to when they are built. The directory follows the conventional boot/ for kernel image and device tree, and lib/modules/ for kernel modules.

Target: build/${dir}/linux/install
Required dependencies:
- ${dir}/linux.cfg - See Configuration file.
Built dependencies:
- build/${dir}/linux.src - The sentinel file generated when the Linux source code is downloaded to build/${dir}/linux using one of the file fetching rules.
- build/${dir}/linux/.config - The target of the Linux Kconfig build configuration rule.

Configuration file

The following options are valid in the ${dir}/linux.cfg configuration file.

ARCH - Required - The architecture of the target platform's CPU. Equivalent to the ARCH environment variable when building Linux manually (See Kbuild ARCH documentation).
CROSS_COMPILE - Required - The binutils toolchain prefix for selecting the correct cross-compiling gcc/ld/etc binaries for the target platform. Equivalent to the CROSS_COMPILE environment variable when building Linux manually. Note that the toolchains available to the kernel build are those in the workspace container.
DEFCONFIG - Optional - The stem of the platform's defconfig file, e.g. sunxi for the sunxi_defconfig available for the ARM architecture. Equivalent to make ${platform}_defconfig in Linux's build system.
INSTALL_TARGET - Optional - Architecture-specific install target used when building Linux manually, determines the type of the kernel binary copied to the linux/install directory when built. E.g. ARM's zinstall for compressed image or uinstall for u-boot wrapped image. Default: install
DTB - Optional - The name of a device tree binary to be copied to the linux/install directory when built. Specify relative to the platform architecture's dts directory e.g. allwinner/sun8i-h3-orangepi-one.dtb found in arch/arm/boot/dts/.

Note

The interactive workspace shell can used to discover the available toolchains to use for the CROSS_COMPILE prefix. For example:

$ make shell CMD=bash
/workspace$ compgen -c | grep '\-gcc$' | sort | uniq
aarch64-linux-gnu-gcc
arm-linux-gnueabi-gcc
c89-gcc
c99-gcc
x86_64-linux-gnu-gcc
x86_64-linux-gnux32-gcc

Example

This example shows configuration for the kernel built for the Makrocosm Example Project's Alpine Orange Pi One image.

The linux.cfg configuration sets the cross-compilation options for ARM, the kernel configuration for the Orange Pi One's CPU family (sunxi) and device tree for the Orange Pi One platform.

platform/opi1/linux.cfg

ARCH=arm
CROSS_COMPILE=arm-linux-gnueabi-
DEFCONFIG=sunxi
INSTALL_TARGET=zinstall
DTB=allwinner/sun8i-h3-orangepi-one.dtb

The linux.git.cfg configuration defines the Git repository where the kernel source tree can be cloned from, and the version tag to use (See the Clone Git repository Makefile rule).

platform/opi1/linux.git.cfg

URL=git@github.com:torvalds/linux.git
REFNAME=v6.10

The Makefile fragment adds a filesystem.kconfig dependency to the .config target in the Linux build directory, instructing the build to merge extra SquashFS and OverlayFS options into the nominated defconfig to create the .config.

platform/opi1/build.mk

...

build/platform/opi1/linux/.config: common/alpine/linux/filesystems.kconfig

...

common/alpine/linux/filesystems.kconfig

CONFIG_SQUASHFS=y
CONFIG_SQUASHFS_XZ=y
CONFIG_SQUASHFS_ZSTD=y
CONFIG_OVERLAY_FS=m

The .config is a dependency of the linux/install target. Running the command make build/platform/opi1/linux/install selectively builds the kernel installs it to that directory:

build/platform/opi1/linux/install

├── boot
│   ├── sun8i-h3-orangepi-one.dtb
│   └── zImage
└── lib
    └── modules
        └── 6.10.0
            ├── kernel
            │   └── fs
            │       └── overlayfs
            │           └── overlay.ko
            ├── modules.alias
            ├── modules.alias.bin
            ├── modules.builtin
            ├── modules.builtin.alias.bin
            ├── modules.builtin.bin
            ├── modules.builtin.modinfo
            ├── modules.dep
            ├── modules.dep.bin
            ├── modules.devname
            ├── modules.order
            ├── modules.softdep
            ├── modules.symbols
            └── modules.symbols.bin

The linux/install target is used by the Alpine Orange Pi example build to include the kernel and kernel modules in the root filesystem.