RT-Kernel

How to compile 64-bit RT-kernel for Raspberry Pi 5 for Debian bookworm (i.e., /boot/firmware/)

Prepare the environment

sudo apt install git bc bison flex libssl-dev make
sudo apt install libncurses5-dev
sudo apt install raspberrypi-kernel-headers

Clone the git, in this case kernel 6.9, from from https://github.com/raspberrypi/linux/tree/rpi-6.9.y

cd ~
git clone --depth 1 --branch rpi-6.9.y https://github.com/raspberrypi/linux

Get the latest RT-patch, in this case RT5 for kernel 6.9, from https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/6.9/

cd ~/kernel
wget -c https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/6.9/patch-6.9-rt5.patch.xz
xz -d patch-6.9-rt5.patch.xz

Go back into the cloned linux

cd ~/linux

Undo prior patch, if necessary, in this case the one for 6.9-rc6-rt4

#patch -R -p1 < ~/kernel/patch-6.9-rc6-rt4.patch

Update if necessary while scrapping all your local stuff

git stash
git pull --rebase
#git stash clear

Or simply pull

#git pull

Patch the kernel

patch -p1 < ~/kernel/patch-6.9-rt5.patch

Make for Raspberry Pi 5

make bcm2712_defconfig

Start menuconfig

make menuconfig

Select General Setup/Preemption Model/Fully Preemptible Kernel (Real-Time)

## I've made the following changes specifically for my NTP server to also enable kernel PPS:
-CPU_FREQ_DEFAULT_GOV_POWERSAVE y
-CPU_FREQ_GOV_CONSERVATIVE y
-CPU_FREQ_GOV_ONDEMAND y
-CPU_FREQ_GOV_PERFORMANCE y
-CPU_FREQ_GOV_SCHEDUTIL y
-CPU_FREQ_GOV_USERSPACE y
-LEDS_TRIGGER_CPU y
-PREEMPT y
 LOCALVERSION "-v8-16k" -> "-v8-16k-NTP"
 PPS_CLIENT_GPIO m -> y
+CPU_FREQ_DEFAULT_GOV_PERFORMANCE y
+EFI_DISABLE_RUNTIME n
+HZ_1000 y
+HZ_PERIODIC y
+NTP_PPS y
+PREEMPT_RT y
+RTC_INTF_DEV_UIE_EMUL y
+VIRT_CPU_ACCOUNTING_GEN y

See also https://github.com/by/RT-Kernel/blob/main/bcm2712_defconfig_RT_NTP

Build the kernel using all 4 cores (and try gcc optimization level -O3, if you like)

make prepare
make -j4 Image.gz modules dtbs
make CFLAGS='-O3 -march=native' -j4 Image.gz modules dtbs
sudo make modules_install

Create the required directories once

sudo mkdir /boot/firmware/NTP
sudo mkdir /boot/firmware/NTP/overlays-NTP

Add this to /boot/firmware/config.txt in order to preserve the standard kernel

os_prefix=NTP/
overlay_prefix=overlays-NTP/
kernel=/kernel_2712-NTP.img

Copy the file ino the right directories

sudo cp arch/arm64/boot/dts/broadcom/*.dtb /boot/firmware/NTP/; sudo cp arch/arm64/boot/dts/overlays/*.dtb* /boot/firmware/NTP/overlays-NTP/; sudo cp arch/arm64/boot/dts/overlays/README /boot/firmware/NTP/overlays-NTP/; sudo cp arch/arm64/boot/Image.gz /boot/firmware/kernel_2712-NTP.img

Reboot to activate the kernel

sudo reboot now