Buildroot Practical Guide: Building the IMX6ULL Embedded Root Filesystem from Scratch

Introduction

In a previous article, we introduced how to use busybox to build a filesystem:

https://www.guyuehome.com/detail?id=1928835293522231297

However, the filesystem built with busybox requires a lot of cross-compilation and manual addition of components, and some software needs to be ported by yourself, making the porting process quite cumbersome.

There is another very practical method for building a root filesystem—using Buildroot to create the root filesystem. Buildroot not only integrates busybox but also various common third-party libraries and software; you just need to select the software you need.

In a previous article, we also detailed the components and usage of Buildroot:

https://www.guyuehome.com/detail?id=1929555441212260353

Next, we will directly use Buildroot to create the root filesystem.

To create the root filesystem, we need to understand Buildroot based on the following questions:

How to decide whether to compile certain packages:

make menuconfig

Where does the menu content come from?

How to download these packages

Each package should provide its own download address

How to compile these packages

Each package should provide its own compilation method

How to create image files

The selected packages are compiled and placed into a unified directory

Create the rootfs image

This is not enough; we want to create an emmc image file: it contains a partition table, root filesystem partition, and other partitions

1. Buildroot Directory

1. Directory Structure

As detailed in a previous article, we will represent it directly with a diagram here:

2. Directory Usage During Execution

2. Buildroot Execution Process

3. Compiling the Root Filesystem with Default Configuration

1. Using Default Configuration and Script

wget https://buildroot.org/downloads/buildroot-2025.02.tar.xztar xJf buildroot-2025.02.tar.xzcd buildroot-2025.02ls configs/*imx6ull*make freescale_imx6ullevk_defconfigmake menuconfig make -j 4

Download the source package version 2025.02 from the Buildroot website

After extraction, a directory buildroot-2025.02 is generated

Switch to the extracted Buildroot working directory

List all configuration files in the configs/ directory that contain imx6ull (usually ending with _defconfig)

Apply the freescale_imx6ullevk_defconfig configuration file to initialize Buildroot’s compilation options

Open the ncurses-based graphical configuration interface, remove the kernel and uboot

Compile

Remove the kernel and Uboot:

At the end of the compilation, the script board/freescale/common/imx/post-image.sh will be used to generate the image file, resulting in the following error:

Create a new file board/freescale/100ask/genimage.cfg:

image  emmc.img {  hdimage {              // Generates DOS partition images.  }  partition rootfs-1 {    partition-type = 0xC    bootable = "yes"    size = 10M            // Partition size is 10M  }  partition rootfs-2 {    partition-type = 0x83    bootable = "yes"    image = "rootfs.ext4"// This partition directly writes the image file myrootfs.ext4    size = 1000M            // Partition size is 100M  }

Modify board/freescale/common/imx/post-image.sh, the content is as follows:

Note that if files are transferred from the PC to the virtual machine, you need to execute: convert post-image.sh to Unix format

dos2unix board/freescale/common/imx/post-image.sh

Because Windows-style line endings (CRLF, i.e., \r\n) are not recognized by Linux systems, which only recognize Unix-style line endings (LF, i.e., \n).

Add the kernel and device tree:

cp ~/100ask_imx6ull-sdk/Linux-4.9.88/arch/arm/boot/zImage board/freescale/100ask/rootfs-overlay/boot
cp ~/100ask_imx6ull-sdk/Linux-4.9.88/arch/arm/boot/dts/100ask_imx6ull-14x14.dtb board/freescale/100ask/rootfs-overlay/boot

Reconfigure to set BR2_ROOTFS_OVERLAY equal to board/freescale/100ask/rootfs-overlay/

board/freescale/100ask/rootfs-overlay

Re-execute make, and you will get output/images/emmc.img

3. Compiling the Root Filesystem with External Toolchain

wget https://buildroot.org/downloads/buildroot-2025.02.tar.xztar xJf buildroot-2025.02.tar.xzcd buildroot-2025.02ls configs/*imx6ull*make freescale_imx6ullevk_defconfigmake menuconfig # Select external toolchain, remove kernel, ubootmake -j 4

Download the source package version 2025.02 from the Buildroot website

After extraction, a directory buildroot-2025.02 is generated

Switch to the extracted Buildroot working directory

List all configuration files in the configs/ directory that contain imx6ull (usually ending with _defconfig)

Apply the freescale_imx6ullevk_defconfig configuration file to initialize Buildroot’s compilation options

Open the ncurses-based graphical configuration interface, remove the kernel and uboot

Compile

Select external toolchain:

Remove the kernel and Uboot:

4. Adding Open Source Apps and Dependency Libraries

1. Adding Software Already Supported by Buildroot

For example, in the package directory, there are already openocd and libmodbus, which can be added by executing make menuconfig in the buildroot directory and then re-running make to compile.

2. Adding Local Makefile Project to Buildroot

2.1 Project Source Code

hello_make is a simple Makefile project, with the following content:

$ cat hello_make.c#include<stdio.h>
int main(int argc, char **argv){  printf("hello, I am a make project\n");  return 0;}
$ cat Makefilehello_make: hello_make.c  $(CC) -o $@ $^
clean:  rm -f hello_make

Contains hello_make.c and Makefile, compressed as follows:

2.2 Create a New Package Directory in the Package Directory

mkdir package/hello_makecd package/hello_maketouch hello_make.mk Config.in

hello_make.mk content is as follows:

################################################################################## hello_make#################################################################################
HELLO_MAKE_SOURCE = hello_make.tar.gz
HELLO_MAKE_SITE = $(TOPDIR)/dl/hello_make

define HELLO_MAKE_BUILD_CMDS    $(MAKE) CC="$(TARGET_CC)" LD="$(TARGET_LD)" -C $(@D) endef

define HELLO_MAKE_INSTALL_TARGET_CMDS    $(INSTALL) -D -m 0755 $(@D)/hello_make $(TARGET_DIR)/usr/binendef

$(eval $(generic-package))

Config.in content is as follows:

config BR2_PACKAGE_HELLO_MAKE  bool "hello_make"  help    hello_make for test.

2.3 Place Files in the dl Directory

Place the compressed package of the previous project source code:

dl/hello_make/hello_make.tar.gz

2.4 Modify package/Config.in

2.5 Graphical Configuration

Execute make menuconfig in the Buildroot directory:

2.6 Compile

Compile separately:

make hello_make

If you want to modify the code and recompile:

Directly modify output/build/hello_make/hello_make.c, then re-run make hello_make

Compile and create a series of image files:

make -j4

3. Adding Local CMake Project to Buildroot

3.1 Project Source Code

hello_cmake is a simple CMake project, with the following content:

$ cat hello_cmake.c#include<stdio.h>
int main(int argc, char **argv){  printf("hello, I am a cmake project\n");  return 0;}
$ cat CMakeLists.txt# Minimum required CMake version cmake_minimum_required(VERSION 3.10)
# Define project name and version project(hello_cmake VERSION 1.0.0)
# Specify cross-compilation toolchain path #set(CMAKE_C_COMPILER arm-buildroot-linux-gnueabihf-gcc) #set(CMAKE_CXX_COMPILER arm-buildroot-linux-gnueabihf-g++)
# Add executable file add_executable(hello_cmake hello_cmake.c)
# Optional: Install to system path (e.g., /usr/local/bin) install(TARGETS hello_cmake DESTINATION bin) 

Contains hello_make.c and CMakeLists.txt, compressed as follows:

3.2 Create a New Package Directory in the Package Directory

mkdir  package/hello_cmakecd package/hello_cmaketouch hello_cmake.mk Config.in

hello_cmake.mk content is as follows:

################################################################################## hello_cmake#################################################################################
HELLO_CMAKE_SOURCE = hello_cmake.tar.bz2
HELLO_CMAKE_SITE = $(TOPDIR)/dl/hello_cmake

$(eval $(cmake-package))

Config.in content is as follows:

config BR2_PACKAGE_HELLO_CMAKE  bool "hello_cmake"  help    hello_cmake for test.

3.3 Place Files in the dl Directory

Place the compressed package of the previous project source code:

dl/hello_cmake/hello_cmake.tar.bz2

3.4 Modify package/Config.in

3.5 Graphical Configuration

Execute make menuconfig in the Buildroot directory:

3.6 Compile

Compile separately:

make hello_cmake

Compile and create a series of image files:

make