Don't offer realtime on Raspberry Pi

[Juerg Haefliger]

Currently the pro client offers a realtime kernel when run on a Raspberry Pi. We get frequent bug reports that this kernel is not working which is expected. The pro client should only offer things that are actually supported on the target platform.

$ cat /proc/device-tree/model
Raspberry Pi 4 Model B Rev 1.2

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 22.04.4 LTS
Release: 22.04
Codename: jammy

$ pro status --all
SERVICE ENTITLED STATUS DESCRIPTION
anbox-cloud yes disabled Scalable Android in the cloud
cc-eal yes n/a Common Criteria EAL2 Provisioning Packages
esm-apps yes enabled Expanded Security Maintenance for Applications
esm-infra yes enabled Expanded Security Maintenance for Infrastructure
fips yes n/a NIST-certified FIPS crypto packages
fips-preview yes disabled Preview of FIPS crypto packages undergoing certification with NIST
fips-updates yes disabled FIPS compliant crypto packages with stable security updates
livepatch yes n/a Canonical Livepatch service
realtime-kernel yes disabled Ubuntu kernel with PREEMPT_RT patches integrated
├ generic yes disabled Generic version of the RT kernel (default)
└ intel-iotg yes n/a RT kernel optimized for Intel IOTG platform
ros yes n/a Security Updates for the Robot Operating System
ros-updates yes n/a All Updates for the Robot Operating System
usg yes disabled Security compliance and audit tools

[Juerg Haefliger (juergh)]

See https://discourse.ubuntu.com/t/installing-a-real-time-ubuntu-kernel-on-a-raspberry-pi/34833/2

[Renan Rodrigo (renanrodrigo)]

Hello, Juerg,
thanks for bringing this to our attention.

Currently, the definitions for the realtime-kernel service on Pro define it as available on amd64 and arm64.
Should we disable it for PIs only or for all arm64?
Removing arm64 is quite easy, but detecting if you are running a particular platform is not something we have yet - we would need to work on how to do it.

[JP Meijers (jpm)]

I'm testing the RT kernel on a couple of Raspi 5's. If one omits the `--variant=raspi` it breaks the system, because the generic RT kernel is installed.

$ pro --version
32.3~24.04

$ cat /proc/device-tree/model
Raspberry Pi 5 Model B Rev 1.0

$ sudo pro enable realtime-kernel
One moment, checking your subscription first
The Real-time kernel is an Ubuntu kernel with PREEMPT_RT patches integrated.

This will change your kernel. To revert to your original kernel, you will need
to make the change manually.

Do you want to continue? [ default = Yes ]: (Y/n)
Configuring APT access to Real-time kernel
Updating Real-time kernel package lists
Updating standard Ubuntu package lists
Installing Real-time kernel packages
Unexpected APT error.
Failed running command 'apt-get install --assume-yes --allow-downgrades -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" ubuntu-realtime' [exit(100)]. Message: E: Sub-process /usr/bin/dpkg returned an error code (1)

See /var/log/ubuntu-advantage.log
Could not enable Real-time kernel.

After this the dpkg error prevents any packages from being installed or removed, always ending with this error message:

dpkg: error processing package linux-image-6.8.1-1001-realtime (--configure):
 installed linux-image-6.8.1-1001-realtime package post-installation script subp
rocess returned error exit status 1