Sddm crashes when enabling amdgpu for secondary card in multi-gpu

[Karen Webb]

Background: This is a laptop with dual graphics, an APU with 8610G and an 8670M. By default, radeon driver is used for both devices. SDDM works, but vulkan support is provided by llvmpipe instead of hardware, performance in general is poor, and there is often display corruption when switching vterm from a text console back to sddm or an X/Wayland session or when resuming from sleep.

Issue: When in a 'hybrid' setup with the radeon driver for the 8610G and the amdgpu driver for the 8670M, sddm fails to start.

Reproduction steps: Find a laptop with a pre-Southern-Islands APU and a Southern Islands GPU. Explicitly set the kernel parameters amdgpu.si_support=1 and radeon.si_support=0, call update-initramfs and reboot.

Workarounds:
Use radeon for both devices (default) -> No hardware support for vulkan. Bug on vterm switch/resume from sleep as described above.
Blacklist radeon -> No hardware support for any graphics api. sddm works, and can start X sessions, but wayland sessions (kwin and mutter) fail to start.
Use radeon for the -G gpu and amdgpu for the -M gpu, and use an alternative DM such as gdm3 -> no issues other than the fact I have to use gdm3

ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: sddm 0.20.0-2ubuntu4.2
ProcVersionSignature: Ubuntu 6.8.0-39.39-generic 6.8.8
Uname: Linux 6.8.0-39-generic x86_64
ApportVersion: 2.28.1-0ubuntu3
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Thu Jul 25 13:20:34 2024
InstallationDate: Installed on 2024-05-08 (78 days ago)
InstallationMedia: Kubuntu 24.04 LTS "Noble Numbat" - Release amd64 (20240425.1)
SourcePackage: sddm
UpgradeStatus: No upgrade log present (probably fresh install)