Configuring Buildroot and Linux Kernel¶

Preface¶

This guide contains a bunch of recipes which may be useful while building Linux kernel for Synopsys ARC processors. Consider reading the Buildroot user manual for details.

Configuration Files¶

Configuration menu allows to manually tune options for a particular build. Initialize Buildroot with a predefined configuration file and enter the menu:

make snps_archs38_haps_defconfig
make menuconfig

After you finishes your configuration session a configuration file is saved as .config.

You can use make savedefconfig to save a current configuration to a separate configuration file. It reduces all configuration options to a minimum set of options.

make savedefconfig DEFCONFIG=my_defconfig

Configuration file my_defconfig is saved in Buildroot's source tree. Here is an example of a configuration file which selects a preinstalled toolchain:

BR2_arcle=y
BR2_archs38=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_PATH="/tools/toolchains/arc-2023.03/arc-linux-gnu"
BR2_TOOLCHAIN_EXTERNAL_HEADERS_5_16=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM_GLIBC=y
# BR2_TOOLCHAIN_EXTERNAL_INET_RPC is not set
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_TOOLCHAIN_EXTERNAL_FORTRAN=y
BR2_TARGET_GENERIC_HOSTNAME="zebu_hs"
BR2_TARGET_GENERIC_ISSUE="Welcome to the HAPS Development Platform"
BR2_LINUX_KERNEL=y
BR2_LINUX_KERNEL_CUSTOM_VERSION=y
BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.16"
BR2_LINUX_KERNEL_DEFCONFIG="haps_hs_smp"
BR2_LINUX_KERNEL_VMLINUX=y
BR2_TARGET_ROOTFS_INITRAMFS=y

You can place a custom configuration file in config directory in Buildroot's source tree. It allows you to use custom configuration files as usual:

make my_defconfig

You can use defconfig command and DEFCONFIG environment variable to pass a path to the configuration file:

# Using an absolute path
$ make defconfig DEFCONFIG=/home/user/buildroot/my_defconfig

# Using a relative path (it's always relative to Buildroot's source tree)
$ make defconfig DEFCONFIG=my_defconfig

Building in a Separate Directory¶

It's a good practice to build Linux images out of the main source tree:

git clone https://git.busybox.net/buildroot
mkdir buildroot/build
cd buildroot/build
make -C .. O=$(pwd) snps_archs38_haps_defconfig

Option -C .. points to Buildroot source tree and O=$(pwd) passes an output directory's path to make. Note that if you use relative paths in Buildroot configuration options, then they are relative to Buildroot's source tree, but not to build directory.

Selection a Big Endian Configuration¶

Select this option in a configuration menu:

Target options -> Target Architecture -> (X) ARC (big endian)

This option corresponds to BR2_arceb=y configuration line.

Selecting a Linux Kernel Version¶

By default, Buildroot selects the latest available version of the upstream Linux kernel. However, if you want to use a development branch with the latest patches and support of ARCv3 processor families, then consider using arc64 branch of the development repository:

Kernel -> Kernel version -> Custom Git repository
       -> URL of custom repository -> https://github.com/foss-for-synopsys-dwc-arc-processors/linux
       -> Custom repository version -> arc64

This set of options corresponds to these configuration lines:

BR2_LINUX_KERNEL_CUSTOM_GIT=y
BR2_LINUX_KERNEL_CUSTOM_REPO_URL="https://github.com/foss-for-synopsys-dwc-arc-processors/linux"
BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="arc64"

Selecting a Linux Kernel Configuration File¶

You can select a Linux kernel configuration file in Buildroot configuration menu. For example:

Kernel -> Defconfig name -> haps_hs

This option corresponds to BR2_LINUX_KERNEL_DEFCONFIG="haps_hs" configuration line.

You can find a complete list of all Linux kernel configuration files in arch/arc/configs directory of Linux kernel source tree:

$ ls -1 <path-to-kernel-source-tree>/arch/arc/configs
axs101_defconfig
axs103_defconfig
axs103_smp_defconfig
haps_hs_defconfig
haps_hs_smp_defconfig
hsdk_defconfig
nsim_700_defconfig
nsimosci_defconfig
nsimosci_hs_defconfig
nsimosci_hs_smp_defconfig
tb10x_defconfig
vdk_hs38_defconfig
vdk_hs38_smp_defconfig

Using Overlays¶

If you want to place addition files and directories in rootfs, then you can use BR2_ROOTFS_OVERLAY=<path> parameter: it contains a path to the a directory with files and directories which will be placed in the final filesystem image.

You can select it in a configuration menu:

System configuration -> Root filesystem overlay directories -> overlay

Also you can achieve the same result by placing all necessary files and directories in output/target (or just target if you build in a separate directory) directory and rebuilding images using make.

Using Custom Sources Paths¶

You can specify custom sources paths for various components for debugging purposes. The easiest way is to create local.mk file in a build directory. Here is an example:

UCLIBC_OVERRIDE_SRCDIR = /sources/uClibc
GCC_OVERRIDE_SRCDIR = /sources/gcc

Refer to Advanced usage section of Buildroot documentation for details.