Before bisecting, I investigated the PCI issue:
https://bugs.launchpad.net/bugs/1902795

I've faced the PCI issue before the AGP one, but it was less critical.
I've submitted a patch that may fix some issues (with drawback of being
non-optimal on platforms were PCI graphics are known to already work)
but not everything is fixed. To be clear: that does not fix PCI GPU
failure on K8 / K10 neither AGP-as-PCI failure, but that is at least a
step forward, and would help specialists to investigate more the issue.
At this point I may have reached my skill cap on such topic.

Because PCI and AGP-as-PCI behaves differently and the later code may have 
specific bugs, I opened an issue dedicated to track bugs when AGP cards are 
driven as PCI ones:
https://bugs.launchpad.net/bugs/1902981

I did not bisected anything after having identified the breakage was
introduced by 5.9-rc1, and I directly tried to revert commit
ba806f98f868ce107aa9c453fef751de9980e4af that disabled AGP at kernel
build. I reverted this commit over the 5.10-rc2 tag from the torvalds
branch and both the ATI Radeon HD 4670 on the K10 computer and the ATI
Radeon X1950 PRO on the AMD K8 computer started to work again
immediately. Game performance was as expected and desktop experience was
really smooth like we can expect from such aging but high end hardware
from its time.

It looks like I misread your first comment, I believed you were asking
me to try the 5.10-rc1 build like if that commit was already reverted
(the only action that seems to be able to fix the issue at this point).

After this commit is reverted, I have not yet noticed the other issues
I've reported (disk IO seems to be OK, I have not tested audio yet), so
the other ones may have been collateral damage of the AGP/PCI one.

So, you were right, the regression was introduced by the "drm/radeon:
disable AGP by default" commit. It sounds too early to disable without
alternative some hardware that was still sold as brand new in 2012 (the
ATI Radeon HD 4670 AGP was).

-- 
You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1899304

Title:
  AGP disablement leaves GPUs without working alternative (PCI fallback
  is broken), makes very-capable ATI TeraScale GPUs unusable

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  This system runs Ubuntu 20.04, freshly installed 3~4 months ago (July
  2020).

  There is two kernels available on this system:

  - 5.4.0-47-generic
  - 5.4.0-48-generic

  With kernel 5.4.0-47-generic:

  - GNOME shell loads properly on Radeon X1950 PRO,
  - Unvanquished game runs on ATI Radeon X1950 PRO at 70 fps on 1280×720 
resolution,
  - Unvanquished game runs on ATI Radeon 9500 at 40 fps on 640×480 resolution.

  Everything looks consistent with the limits and the age of the
  hardware.

  With kernel 5.4.0-48-generic:

  - GNOME Shell never finish to load on Radeon X1950 PRO, either a grey screen 
is displayed and keyboard shortcuts does not respond, or the top bar is stuck 
between the center and the top of the screen and the shell does not respond, or 
the top bar on the top of the screen but the shell does not respond, to get a 
desktop I run `sudo systemctl stop display-manager` then `startx 
/usr/bin/lxsession` from a TTY. Running GNOME Shell with startx or from a lone 
xterm started with startx leads to same issues.
  - Unvanquished game runs on ATI Radeon X1950 PRO at 7 fps on 1280×720 
resolution,
  - Unvanquished game runs on ATI Radeon 9500 at 3 fps on 640×480 resolution.

  Note: for unknown reasons, GNOME Shell loads properly on the ATI
  Radeon 9500 but not on the Radeon X1950 PRO.

  Everything is slow. When the game is running, a very high load is
  reported by htop, which does not look like the experience seen on the
  5.4.0-47-generic kernel. When the game is running cycling between
  windows using Alt-Tab takes a lot of second while it's immediate on
  5.4.0-47-generic kernel. Also, even without the game running or on a
  lightweight desktop like LXDE, cycling windows is not smooth and
  window refreshing is slow enough to be noticeable.

  About the hardware, note that:

  - the CPU only has one core, no hyperthreading (AMD Athlon 64 FX for socket 
939),
  - the GPUs are AGP ones using R300 and R500 technology (pre-TeraScale),
  - there is 3GB of DDR RAM,
  - there is no on-disk swap but zram-based swap in compressed ram is used,
  - system is stored on and boots from an USB 3.1 key plugged on an USB 2.0 
port,
  - /tmp is a tmpfs ram disk,
  - CPU is set to performance profile,
  - the install is not really messy and not many packages are installed, this 
USB key is purposed for hardware/system testing and to diagnose such issues.

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: linux-image-5.4.0-48-generic 5.4.0-48.52
  ProcVersionSignature: Ubuntu 5.4.0-48.52-generic 5.4.60
  Uname: Linux 5.4.0-48-generic x86_64
  ApportVersion: 2.20.11-0ubuntu27.9
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  illwieckz   7503 F.... pulseaudio
  CasperMD5CheckResult: skip
  Date: Sun Oct 11 01:47:48 2020
  InstallationDate: Installed on 2020-07-09 (93 days ago)
  InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
  IwConfig:
   enp0s11   no wireless extensions.

   lo        no wireless extensions.
  MachineType: MSI MS-6702E
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=fr_FR.UTF-8
   SHELL=/bin/bash
  ProcFB: 0 radeondrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-48-generic 
root=UUID=10314d0c-ec6b-4f7f-b926-ed8b80185331 ro
  PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No 
PulseAudio daemon running, or not running as session daemon.
  RelatedPackageVersions:
   linux-restricted-modules-5.4.0-48-generic N/A
   linux-backports-modules-5.4.0-48-generic  N/A
   linux-firmware                            1.187.3
  RfKill:

  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 10/12/2006
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: 080011
  dmi.board.asset.tag: To Be Filled By O.E.M.
  dmi.board.name: MS-6702E
  dmi.board.vendor: MSI
  dmi.board.version: 1.0
  dmi.chassis.asset.tag: To Be Filled By O.E.M.
  dmi.chassis.type: 3
  dmi.chassis.vendor: To Be Filled By O.E.M.
  dmi.chassis.version: To Be Filled By O.E.M.
  dmi.modalias: 
dmi:bvnAmericanMegatrendsInc.:bvr080011:bd10/12/2006:svnMSI:pnMS-6702E:pvr1.0:rvnMSI:rnMS-6702E:rvr1.0:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
  dmi.product.family: To Be Filled By O.E.M.
  dmi.product.name: MS-6702E
  dmi.product.sku: To Be Filled By O.E.M.
  dmi.product.version: 1.0
  dmi.sys.vendor: MSI

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