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Configuring Buildroot and Linux Kernel

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Yuriy Kolerov edited this page Aug 1, 2023 · 1 revision

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.

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)

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

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

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

Selecting a Prebuilt Toolchain

ℹ️ Each release of a toolchain may demand a specific set of options which must be selected/deselected manually. It happens, because when a custom toolchain is selected, Buildroot cannot detect toolchain's features automatically. Follow Buildroot's error messages for details.

By default, Buildroot builds a Linux toolchain for ARC from scratch and may use relatively old sources for tools like GCC, Binutils, etc. If you want to use the latest available toolchain for ARC processors, then consider using prebuilt components from official releases page.

In addition to the default values, select/deselect these options for the toolchain for ARC HS3x/4x with glibc library (2023.03 release):

Toolchain -> Toolchain type -> (X) External toolchain
          -> (X) Custom toolchain
          -> Toolchain origin -> (X) Toolchain to be downloaded and installed
          -> Toolchain URL -> https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/releases/download/arc-2023.03-release/arc_gnu_2023.03_prebuilt_glibc_le_archs_linux_install.tar.gz
          -> External toolchain gcc version -> (X) 12.x
          -> External toolchain kernel headers series -> (X) 5.16.x
          -> External toolchain C library -> (X) glibc
          -> [ ] Toolchain has RPC support?
          -> [X] Toolchain has C++ support?
          -> [X] Toolchain has Fortran support?

Additional options for the toolchain for ARC HS3x/4x with uClibc library (2023.03 release):

Toolchain -> Toolchain type -> (X) External toolchain
          -> (X) Custom toolchain
          -> Toolchain origin -> (X) Toolchain to be downloaded and installed
          -> Toolchain URL -> https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/releases/download/arc-2023.03-release/arc_gnu_2023.03_prebuilt_uclibc_le_archs_linux_install.tar.gz
          -> External toolchain gcc version -> (X) 12.x
          -> External toolchain kernel headers series -> (X) 5.16.x
          -> External toolchain C library -> (X) uClibc/uClibc-ng
          -> [X] Toolchain has locale support?
          -> [ ] Toolchain has RPC support?
          -> [X] Toolchain has C++ support?
          -> [X] Toolchain has Fortran support?

Selecting a Preinstalled Toolchain

A procedure is the same as for selecting a prebuilt toolchain except these ones:

Toolchain -> Toolchain origin -> (X) Pre-installed toolchain
          -> Toolchain path -> /tools/toolchains/arc-linux-gnu

Creating a Custom Configuration File

You can use make savedefconfig to save a current configuration to a 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

Using Custom Configuration Files

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

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.

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