Cross-compile an RT kernel and deploy it to a headless Pi

Building natively on a Pi 4 takes hours. Cross-compiling on a normal workstation takes about forty minutes. And if you’ve never done it, the scary part isn’t the build — it’s that a bad kernel on a headless box means physically extracting the SD card. This page addresses both.

Or skip it

We publish the built artifacts. See Downloads.

1. Toolchain

sudo apt install crossbuild-essential-arm64 bc bison flex libssl-dev make \
                 libc6-dev libncurses5-dev

2. Source, matched to your running kernel

git clone --depth=1 --branch rpi-6.12.y \
  https://github.com/raspberrypi/linux.git
cd linux

Use Raspberry Pi’s tree, not vanilla. RPi’s PREEMPT_RT is already merged in 6.12 and the board’s DT/overlays live there.

3. Configure

export ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-
make bcm2711_defconfig                    # Pi 4
scripts/config --enable PREEMPT_RT
scripts/config --set-str LOCALVERSION "-rt-cuckoo"
make olddefconfig

Now apply the patch — this is the whole reason you’re building a kernel rather than installing one.

Check that MMC and EXT4 are =y

bcm2711_defconfig builds the SD card driver and filesystem into the image, not as modules. That means you don’t need an initramfs — which removes the single most common way a hand-built Pi kernel fails to boot.

grep -E 'CONFIG_(MMC_BCM2835|EXT4_FS)=' .config   # want =y, not =m

4. Build

make -j$(nproc) Image modules dtbs

5. Stage the artifacts

# Note the ABSOLUTE path. `~` does not expand inside a make variable —
# INSTALL_MOD_PATH=~/out silently installs into a literal "~" directory
# and you end up shipping an 80 KB tarball that contains nothing.
make INSTALL_MOD_PATH=/home/you/out modules_install
tar -C /home/you/out -czf rt-modules.tar.gz lib/modules/
gzip -c arch/arm64/boot/Image > kernel-rt.img.gz

6. Deploy without a rescue trip

The rule: never overwrite the kernel that currently boots.

scp kernel-rt.img.gz rt-modules.tar.gz pi@host:/tmp/
ssh pi@host
sudo tar -C / -xzf /tmp/rt-modules.tar.gz
zcat /tmp/kernel-rt.img.gz | sudo tee /boot/firmware/kernel-rt.img > /dev/null

kernel8.img — the stock kernel — is untouched. Then add one line to /boot/firmware/config.txt:

kernel=kernel-rt.img

sudo reboot

The recovery path

If it doesn’t come back: pull the SD card, mount the FAT boot partition on any machine, delete that one line, put it back. The stock kernel boots. That’s the whole rollback — no initramfs to regenerate, no bootloader to repair, and it works from a Windows laptop if that’s all you have.

Test the rollback before you need it.

7. Confirm

$ uname -a
Linux gps-ntp 6.12.x-rt-cuckoo #1 SMP PREEMPT_RT ... aarch64