Some applications, such as real-time digital audio processing, require a system with minimum latency, usually around 2msec. This requirement can’t usually be met by common or standard linux kernels. To turn a linux kernel into a realtime or lowlatency kernel, a specific kernel configuration should be applied, or extra patches should be integrated (such PREEMPT_RT patch or XENOMAI framework).
I recently tried to snap a Digital Audio Workstation called sushi for my Raspberry pi4. Sushi is part of ELK project, a digital audio OS, XENOMAI based, which runs on x86 and Raspberry pi4. My goal would be to run sushi (+other audio packages needed by sushi, such jackd) on Ubuntu Core, fully confined, so to turn my rpi4 in a headless synthesizer that once connected to a usb piano keyboard just starts to play. The work is still in progress and can be found here.
Coming back to lowlatency kernel … to run my sushi snap on Raspberry Pi, the standard uc18 image is not suitable, the kernel should be replaced with a lowlatency one. I decided to create a new kernel with a lowlatency configuration, and create a new uc image with it.
In case you are interested to build your lowlatency uc image for rpi, follow the steps below:
-
Install lxd, snapcraft and ubuntu-image on your ubuntu host
sudo snap install lxd
lxd init(just leave default values) -
Launch a bionic container
lxc launch ubuntu:18.04 bionic-lowlatency -
Open a bash shell to the bionic container, and clone the git repo
lxc exec bionic-lowlatency /bin/bash -- -
Install snapcraft and ubuntu-image
sudo apt update
sudo apt upgrade
sudo snap install snapcraft --classic
sudo snap install ubuntu-image --classic -
Clone the rapsberry lowlatency kernel repo
git clone https://github.com/dbruno74/linux-raspberrypi-org.git
cd linux-raspberrypi-org -
Look at snap/snapcraft.yaml. Check lowlatency configuration (look for “configuring low latency”) and make modifications if needed
...
kernel:
....
kconfigs:
...
# configuring low latency
- CONFIG_HZ=1000
- CONFIG_HZ_1000=y
- '# CONFIG_HZ_250 is not set'
- CONFIG_IRQ_FORCED_THREADING_DEFAULT=y
- CONFIG_PREEMPT=y
- '# CONFIG_PREEMPT_VOLUNTARY is not set'
-
Snap the kernel
snapcraft --target-arch=armhf -
Create the model assertion (if you don’t know how to do this, go here), name it “pi4-lowlatency-model.json”
{
"type": "model",
"authority-id": "<your-id>",
"brand-id": "<your-id>",
"series": "16",
"model": "lowlatency-pi4",
"architecture": "armhf",
"base": "core18",
"gadget": "pi=18-pi",
"kernel": "linux-raspberrypi-org",
"timestamp": "<timestamp>"
}
-
Sign your model assertion
snap sign -k <your_key> pi4-lowlatency-model.json | tee pi4-lowlatency-model.assert -
Build the image
ubuntu-image snap -i 2G --extra-snaps linux-raspberrypi-org_raspi-5.4.0-1009.9_armhf.snap pi4-lowlatency-model.assert -
Insert a microsd card in your microsd drive and flash the image (I assume the device name of your microsd reader is “mmcblk0”)
sudo dd if=pi.img of=/dev/mmcblk0 bs=32M -
Boot your ubuntu core lowlatency image!