Set-up a cross-compile environment for buildroot Linux system running on Orange PI Zero 3

Assuming host system Ubuntu 22.04. Install the pre-requisite packages needed for this lab:

sudo apt install build-essential git autoconf bison flex texinfo help2man gawk libtool-bin libncurses5-dev unzip

Setting up the TFTP server#

Let’s install a TFTP server on your development workstation:

sudo apt install tftpd-hpa

Then edit the file /etc/default/tftpd-hpa to change this setting:

TFTP_DIRECTORY="/srv/tftp"

Now restart the service:

systemctl restart tftpd-hpa

Grab and compile buildroot#

git clone https://git.buildroot.net/buildroot
cd buildroot

Load the default config for Orange PI Zero 3. And build it. Note that we shouldn't change the kernel version as it's been assigned to a custom version compatible with Orange PI.

make orangepi_zero3_defconfig
make

Once the build process is finished you will have an image called "sdcard.img" in the output/images/ directory. Flash this image to your SD card and boot.

On login prompt enter root and no password. TA-DA - and you're in your very own custom linux system!

For more details see Using a build system, example with Buildroot chapter in the slides bootlin embedded linux qemu slides.

Compiling a kernel module#

Now we'll setup the cross-compile variables.

Include the following in your PATH:

export PATH="$PATH:~/buildroot/output/host/bin"

And set the following variables:

export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-

Make a new directory somewhere in your home folder called hello. Inside it create a C file hello.c:

#include <linux/module.h>    /* Needed by all modules */
#include <linux/kernel.h>    /* Needed for KERN_INFO */

int init_module(void){
    printk(KERN_INFO "Hello World!\n");

    return 0;
}

void cleanup_module(void){
    printk(KERN_INFO "Goodbye World.\n");
}

MODULE_LICENSE("GPL");

Create Makefile:

CC = $(CROSS_COMPILE)gcc

obj-m := hello.o
KDIR := ~/buildroot/output/build/linux-custom

all:
        $(MAKE) -C $(KDIR) M=$(PWD)

Noting to adjust the KDIR according to where you have buildroot. Assuming Homefolder here.

Now run make. You'll get a bunch of files as result. We are interested in hello.ko file.

Serial console setup#

The board has 3 pins at the top exposing the serial UART. This will act as the serial console to interact with it over the terminal. The following image from the OrangePI website indicates these pins (TX/RX).

You may link this to a PC using a ftdi chip (available extensively on AliExpress), or like what I do use a Pico-probe which is equipped with a serial-to-usb port! On the host you can use Screen or Minicom to connect.

Transfer the compiled module#

We need to transfer this file to our board. We'll use the tftp service we setup earlier.

First copy the hello.ko file to /srv/tftp directory. Then in the board's prompt:

cd /root
tftp -gr hello.ko <server-ip>

Once successfully copied, clear the system log, and load the module:

sudo dmesg -c
insmod hello.ko

Now check for the message, running dmesg again. To remove this module, simply rmmod hello.ko. Check the output again and you'll see Goodbye world.

This tests that we have a successful cross-compiler environment working to compile and study device driver development on Orange PI Zero 3 running Linux!