Build and boot a kernel from git using TFTP

This guide will explain how you can build and boot a kernel from git on a single-board computer (SBC) using the TFTP support in u-boot.

Reasoning

The reason for doing this is because as a kernel developer, you want to:

1. Work in a separate git tree (most likely a git worktree). In other words, we don't want to deal with patches or a kernel directory which is not a git tree (which is default behavior in buildroot).
2. Quickly be able to boot and test your changes. In other words, we do not want to write the kernel to a SD-card each and every time we want to test a change.

We will start off with a buildroot defconfig, which will generate a sdcard.img which contains the bootloader (u-boot), the root file system, and a kernel.

We will write sdcard.img to the SD-card, but instead of using the kernel from the SD-card, we will load a kernel that we have built separately (outside buildroot) using TFTP.

What you will need

• Devboard with Ethernet.
• Ethernet support for that devboard in u-boot.
• Ethernet cable from the devboard to your network.
• SD-card that will contain the root file system.
• Philips Hue bridge.
• Philips Hue smart plug.
• DHCP server on your network.
• TFTP server (will be installed on your development machine).

This guide will use a Radxa Rock5b devboard, and a development machine running Fedora Linux.

TFTP server

Installing a TFTP server on your development machine

sudo dnf install tftpd-server tftp
sudo chmod 777 /var/lib/tftpboot/
sudo systemctl start tftp.socket
sudo systemctl enable tftp.socket
sudo firewall-cmd --add-service=tftp --permanent
sudo firewall-cmd --reload

Verify that the TFTP server is working

dd if=/dev/zero of=/var/lib/tftpboot/test bs=1M count=1
tftp localhost
tftp> verbose
tftp> get test
tftp> quit
rm /var/lib/tftpboot/test

The get command should yield something like:

getting from localhost:test to test [netascii] Received 1048576 bytes in 0.0 seconds [186637662 bit/s]

Buildroot

Clone buildroot

mkdir ~/src
cd ~/src
git clone git://git.buildroot.net/buildroot buildroot-rock5b
git checkout 2023.11
cd buildroot-rock5b

Update u-boot version

Edit configs/rock5b_defconfig

From:

BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2023.07"

To:

BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2024.01"

This is because Ethernet support for Rock5b was first added in u-boot 2024.01.

Make u-boot load kernel via TFTP

Edit board/radxa/rock5b/boot.cmd

From:

setenv bootargs root=/dev/mmcblk0p2 rw rootfstype=ext4 clkin_hz=(25000000) earlycon clk_ignore_unused earlyprintk console=ttyS2,1500000n8 rootwait
fatload mmc 1:1 ${loadaddr} image.itb
bootm ${loadaddr}

To:

pci enum
setenv autoload no
dhcp
setenv serverip 192.168.1.10
tftp ${loadaddr} rock5b/image.itb
setenv bootargs root=PARTLABEL=rootfs clkin_hz=(25000000) earlycon clk_ignore_unused earlyprintk console=ttyS2,1500000n8 rootwait
bootm ${loadaddr}

Replace 192.168.1.10 with the IP address of your development machine.

The prefix to image.itb is important, as it will determine the board type that you will use.

(Later, you will see that there are also other board types defined, e.g. "rock5b-ep".)

Build buildroot and write to SD-card

make rock5b_defconfig
make
sudo dd if=output/images/sdcard.img of=/dev/sdX
( echo w ) | sudo fdisk /dev/sdX && sync && sudo eject /dev/sdX

Replace /dev/sdX with your SD-card.

After writing, plug in the SD-card to your devboard.

build_boot script

The build_boot script is meant to be executed while standing in a kernel git tree or a git worktree.

The build_boot script will:

1. Build the kernel
2. Generate a flattened image tree blob (image.itb)
3. Copy the image.itb to your TFTP directory (inside a "board type" subdirectory)
4. Toggle the power on your Philips Hue smart plug, such that u-boot will load the recently built kernel

Install kernel prerequisites

sudo dnf install gcc-aarch64-linux-gnu
sudo dnf install uboot-tools

This is needed because we are building the kernel outside buildroot.

Clone build_boot script

cd ~/src
git clone https://github.com/floatious/boot-scripts.git
cd boot-scripts

Set Philips Hue specific variables

Edit build_boot

Set variables huehostname, hueapikey and huelightid according to your setup.

If you cloned buildroot to a path different from the path used in the instructions above, make sure that you also update the its variables.

Install build_boot script

sudo cp build_boot /usr/bin/

Kernel

Clone kernel

cd ~/src
git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
cd linux

Create a kernel config

ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig

This only needs to be done once.

Build and boot kernel

cd ~/src/linux
build_boot rock5b

Run the build_boot script while standing in a kernel git tree or a git worktree.

The script will automatically build the kernel, build image.itb, copy image.itb to a subdirectory in your TFTP directory, and the toggle power on your Philps Hue smart plug, such that u-boot will load the recently built kernel via TFTP.

Lean back and enjoy the feeling of not needing to plug and unplug your SD-card anymore.