RockPi 4

wget "https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.15.49.tar.xz"
tar -xvJf linux-5.15.49.tar.xz
cd linux-*
wget "https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/5.15/patch-5.15.49-rt47.patch.xz"
unxz patch-5.15.49-rt47.patch.xz
cat patch-5.15.49-rt47.patch | patch -p1 

make ARCH=arm64 menuconfig
make -j9 ARCH=arm64 LOCALVERSION=-rockchip64 Image modules dtbs
make ARCH=arm64 modules_install 

In /boot/armbianEnv.txt

extraargs=.... isolcpus=4,5

apt-get install rtirq-init