Radeon driver drops DP signal when switching to framebuffer in 5.8 kernel

[DuckHook]

The 5.8 kernel series in Groovy (20.10) drops the video signal to DisplayPort output on my AMD Radeon HD 7950 card. GRUB comes up, but the signal is dropped when switching to the framebuffer (I believe).

Falling back to the 5.4 kernel series works. So does nomodeset.

However, a LiveUSB does not. Nor do any of the 5.6 to 5.10 kernels.

With no video output, ssh-ing into the dark machine generates the following:

GPU specs:
[code]
duckhook@Zeus:~$ sudo lspci -vk | grep -iA20 vga

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Tahiti PRO [Radeon HD 7950/8950 OEM / R9 280] (prog-if 00 [VGA controller])
 Subsystem: Hightech Information System Ltd. Tahiti PRO [Radeon HD 7950/8950 OEM / R9 280]
 Flags: bus master, fast devsel, latency 0, IRQ 56
 Memory at d0000000 (64-bit, prefetchable) [size=256M]
 Memory at fbe00000 (64-bit, non-prefetchable) [size=256K]
 I/O ports at e000 [size=256]
 Expansion ROM at 000c0000 [disabled] [size=128K]
 Capabilities: [48] Vendor Specific Information: Len=08 <?>
 Capabilities: [50] Power Management version 3
 Capabilities: [58] Express Legacy Endpoint, MSI 00
 Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+
 Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
 Capabilities: [150] Advanced Error Reporting
 Capabilities: [270] Secondary PCI Express
 Capabilities: [2b0] Address Translation Service (ATS)
 Capabilities: [2c0] Page Request Interface (PRI)
 Capabilities: [2d0] Process Address Space ID (PASID)
 Kernel driver in use: radeon
 Kernel modules: radeon, amdgpu
[/code]
A comparison between a working session under 5.4 vs a dark session under 5.8 yields the difference of these two lines in journalctl:
[code]
duckhook@Zeus:~$ sudo journalctl -b | grep -i radeon | grep kernel

…
Nov 15 18:49:04 Zeus kernel: [drm:radeon_dp_link_train_cr [radeon]] *ERROR* clock recovery tried 5 times
Nov 15 18:49:04 Zeus kernel: [drm:radeon_dp_link_train_cr [radeon]] *ERROR* clock recovery failed
[/code]
…

The full log is attached.

Through mainline kernel bisecting, I have traced the problem to the following build inflection point:
Kernel 5.5.5 → Good
Kernel 5.5.6 → Bad

This is pretty much the limit of my technical capabilities. I am not knowledgeable enough to produce an actual commit kernel bisection, but playing a layman's hunch (and at the real risk of leading devs astray), the only reference to radeon in the 5.5.6 changelog is the following:

Daniel Vetter (1):
      radeon: insert 10ms sleep in dce5_crtc_load_lut

Clearly, there are many other changes in this changelog dealing with the video subsystem that I am not qualified to comment on.

Please ask if further info is needed. I am happy to supply all that is within my technical capabilities.

[lotuspsychje (lotuspsychje)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. Please execute the following command only once, as it will automatically gather debugging information, in a terminal:
apport-collect 1904647

When reporting bugs in the future please use apport by using 'ubuntu-bug' and the name of the package affected. You can learn more about this functionality at https://wiki.ubuntu.com/ReportingBugs.

[DuckHook (duckhook)]

Thanks for responding and apologies for the missteps with my reporting.

I may be doing this wrong, but I'm afraid that my apport-collect attempt has failed.

Firstly, I can only ssh into the dark session on the guest. Running apport-collect requires a web browser that I must run in my host. This yields:

[quote]
duckhook@Zeus:~$ apport-collect 1904647
The authorization page:
 (https://launchpad.net/+authorize-token?oauth_token=8Gn2LstLhgmFKx5FbkCl&allow_permission=DESKTOP_INTEGRATION)
should be opening in your browser. Use your browser to authorize
this program to access Launchpad on your behalf.
Waiting to hear from Launchpad about your decision...
Package linux-hwe-5.8 not installed and no hook available, ignoring

*** Updating problem report

No additional information collected.

Press any key to continue...

No pending crash reports. Try --help for more information.
[/quote]

Moreover:

[quote]
duckhook@Zeus:~$ apt policy linux-hwe-5.8
N: Unable to locate package linux-hwe-5.8
N: Couldn't find any package by glob 'linux-hwe-5.8'
duckhook@Zeus:~$ apt search linux-hwe-5.9
Sorting... Done
Full Text Search... Done
[/quote]

[DuckHook (duckhook)]

No further action?

[DuckHook (duckhook)]

The problem is not solved, but I have worked around it by connecting through the HDMI port instead of DP.