hwe 20.04 kernel 5.11 series hang at graphical login leaving laptop unusable

[Corneille]

The 5.11-series 20.04 HWE kernels are not usable on my laptop. They boot up to graphical login. Then the screen goes black for a second every few seconds. When i try to log in the machine seems to hang and the desktop never appears...

The kern.log shows a lot of these errors:
"[drm:amdgpu_pci_remove [amdgpu]] *ERROR* Hotplug removal is not supported"

Many call traces appear as well, involving [amdgpu] and [ttm].

I have reverted to using the 5.8 kernels.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-generic-hwe-20.04 5.11.0.34.36~20.04.13
ProcVersionSignature: Ubuntu 5.8.0-63.71~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-63-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: GNOME-Flashback:GNOME
Date: Wed Sep 8 09:58:49 2021
InstallationDate: Installed on 2014-07-16 (2610 days ago)
InstallationMedia: Ubuntu-GNOME 14.04 LTS "Trusty Tahr" - Release amd64 (20140416.2)
SourcePackage: linux-meta-hwe-5.11
UpgradeStatus: Upgraded to focal on 2021-02-24 (195 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Mike Ferreira (mafoelffen)]

Mine is a desktop with NVidia and has occurred since the update to linux-generic-5.11.-00.34.

But the same machine works fine on Mainline 5.11.1

[Corneille (cvdp)]

I'm now using a workaround. I added "module_blacklist=amdgpu" to the kernel command line in /etc/default/grub.
The kernel 5.11-series now runs fine, albeit without hybrid amd graphics.

I have put the relevant bit of kern.log when amdgpu is not blacklisted in the attachment.