Raspbian lite with fully preemptive real-time kernel. This repo follows the official methodology to cross compile the kernel as explained here https://www.raspberrypi.com/documentation/computers/linux_kernel.html
.
In addition to building the kernel, this repo also offers a raspbian lite sd card image with the built kernel ready to be used on a raspberry pi.
The fully preempt rt model is enabled inside Dockerfile
using ./scripts/config --enable
. The fully preempt rt model can be enabled only if virtualization is disabled.
sdcard
imageClone this repository and run make
. It will create a folder build
with a zipped sdcard image. Default values:
Dockerfile
)https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/
https://downloads.raspberrypi.org/raspios_lite_arm64/images/
you can run make [platform]
where platform
is either Pi1
, Pi2
, PiZero
, PiCM1
, Pi3
, Pi4i
, Pi400
, PiZero2
, PiCM3
, or PiCM4
.
First you need to figure out where the SD card is mounted. Either check dmesg
right after you plugged in the SD card or check changes to /dev/
before and after plugging in the SD card. I'm using a USB adapter for an SD card and I can access it at /dev/sda
(just an example). Do not assume that you can access the SD card at the same location.
Next unzip the image from the build
subfolder and then use dd
to write the image to the SD card. Make sure you figured out the correct location of the SD card in /dev/
(very important).
Here is an example of how to dd
the card image:
sudo dd if=build/2022-01-28-raspios-bullseye-arm64-lite.img of=/dev/sdcard status=progress
Please replace /dev/sdcard
in the above example with the right location of your SD card.
Once you wrote the image to the SD card, place the card into the raspberry pi. Power the pi and ssh into it (ssh is enabled by default). Run uname -a
and it should print SMP
and PREEMPT_RT
, indicating that the RT patch is applied to the kernel:
pi@raspberrypi:~ $ uname -a
Linux raspberrypi 5.10.95-rt61-rc1-v8+ #1 SMP PREEMPT_RT Mon Mar 28 13:28:11 CEST 2022 aarch64 GNU/Linux
For Raspberry Pi processors that have only 1 core, such as the Pi Zero W, The SMP flag (Symetrical multiprocessing) is deactivated because it needs at least 2 cores to work.
You may benchmark the real-time capabilities of the pi by running rt-tests
. Install rt-tests
:
sudo apt update
sudo apt install rt-tests
Here is an example of benchmarking latency:
pi@raspberrypi:~ $ sudo cyclictest --histogram=US
WARN: cyclictest was not built with the numa option
# /dev/cpu_dma_latency set to 0us
policy: other/other: loadavg: 0.53 0.62 0.39 2/166 1463
T: 0 ( 1463) P: 0 I:1000 C: 16114 Min: 11 Act: 42 Avg: 66 Max: 459
which shows a maximum latency of 459us
(roughly half a millisecond). Values vary depending on CPU load.
Please note that cyclictest does not put any load on the cpu, the results are useless if the test is not done while your RT application runs, or while an equivalent stress-test is running. You might want to read the cyclictest documentation here to learn how to build your tests.
Run make custom
. You will get a shell to a container. Then run:
cd /rpi-rt-kernel/linux
make menuconfig
Username pi
password raspberry
dwc_otg.fiq_enable=0
dwc_otg.fiq_fsm_enable=0