PREEMPT_RT patch the Raspberry Pi 3B kernel

Useful links for RT patching the kernel:

• https://wiki.linuxfoundation.org/realtime/documentation/start
• https://www.raspberrypi.com/documentation/computers/linux_kernel.html

Tested on kernel 6.1.77 and Raspberry OS Bookworm (link to exact version).

The linux/ contains a submodule with the kernel code already patched (even if it's quite easy to do yourself). You can pull the contents by cloning with --recurse-submodules --shallow-submodules (second option is there to avoid pulling the full kernel repo, which is big).

Kernel compilation

IMPORTANT: Configure full real-time capabilities with menu_config before compilation.

Cross compile the kernel following the indications in the second link.

Installation via ssh

After having built the kernel:

• Run the packing script from this directory: ./pack.sh.
• Copy the installation archive via ssh: scp .\rt-install.tar.gz <dest>:.
• Ssh into your Raspberry Pi: ssh <dest>
• Backup your current kernel image: sudo cp /boot/firmware/kernel8.img /boot/firmware/kernel8-$(date +"%Y%m%d_%H%M%S").img
• Unpack the files tar -xvf rt-install.tar.gz
• Copy the nel kernel files: sudo cp -r rt-install/* /
• Append dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0 to /boot/firmware/cmdline.txt (see https://wiki.linuxfoundation.org/realtime/documentation/known_limitations)
• Reboot: sudo reboot

All in one script to run on ste RPi (DANGER!):

sudo cp /boot/firmware/kernel8.img /boot/firmware/kernel8-$(date +"%Y%m%d_%H%M%S").img && \
tar -xvf rt-install.tar.gz && sudo cp -r rt-install/* / && \
sudo sed -i 's/$/ dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0/' /boot/firmware/cmdline.txt