Hi, thank you for your answer and your attention,

This issue is confirmed again.

1. 5.10.0-rc1 does not fix the problem introduced in 5.4.0-48 regarding ATI/AMD AGP GPU.
2. PCI GPUs are broken on AMD K8/K10 platform since years but they work on Intel platform, GPU being ATI/AMD or Nvidia doesn't make a difference, so there is no display fallback if AGP is disabled.
3. Present issue not only reports graphical performance regression, but global performance regression on the computers as a whole: slow disk IO, audio glitches…

Because the commit you talked about seems to be related to Radeon and AGP and I've experienced bugs with PCI nVidia hardware too, I've done more testing.

I previously reported issues with the PCI Nvidia GeForce 8400 GS rev.2 on 5.4.0-48 kernel, but at the time I did not tried if this one worked on 5.40.0-47 on this host computer. This is now done and in fact, this hardware does not work on this host computer on 4.15.0-118-generic kernel from Ubuntu 16.04 LTS (Xenial), either on 4.8.0-36 and 4.4.0-190 (Xenial). I'm not saying such hardware does not work at all, such hardware works on Intel based computers, they work on Intel-based PCIe computer with Ubuntu 20.04 (I have not tested older).

I've tested Nvidia and ATI PCI GPUs on the K8 AGP-based computer, the K10 AGP-based computer, a K8 PCIe based computer, and an Intel based computer. Teh tested PCI ATI GPU is an ATI Radeon HD 4350, a not-so-old GPU (TeraScale generation) with HDMI output.

Either ATI or Nvidia, those GPU works on Intel based computer but not in AMD K8 AGP or K10 AGP and even not on K8 PCIe-based computer.

So, here is the list of GPU tested:

AGP ATI Radeon HD 4670 (RV730 XT, TeraScale 1), HDMI + DVI-I + VGA
AGP ATI Radeon X1950 PRO (RV570, R500), DVI-I + DVI-I
PCI ATI Radeon HD 4350 (RV710, Terascale 1), HDMI + DVI-I + VGA
PCI Nvidia Geforce 8400 GS rev.2 (NV98, Tesla 1.0), DVI-I + VGA

AGP GPUs worked up to 5.4.0-47 and stopped working with 5.4.0-48, they still does not work on 5.10.0-rc1, I reproduced the issue on 5.10.0-rc1 on both K8 and K10 AGP based computers. I don't have access right now to Intel-based AGP computers to test them.

Those are the computers tested:

K8 AGP based: ASRock AM2NF3-VSTA motherboard with AMD Phenom II X4 970 CPU (quad core), Nvidia nForce3 bridge, 16GB DDR2 800MHz, AGP + PCI
K10 AGP based: Dell Optiplex 740 motherboard with AMD Athlon 64 X2 CPU (dual core), Nvidia C51 host bridge, C51 PCI Express bridge, 6GB DDR2 667MHz, PCIe + PCI
K8 PCIe based: MSI MS-6702E motherboard with AMD Athlon 64 3200+ CPU (single core), VIA K8T800Pro, VT8237/8251 bridge, 3GB DDR 400MHz, AGP + PCI
Intel PCIe based: Lenovo ThinkCentre M58 motherboard with Pentium E5200 CPU (dual core), Intel 82801 PCI Bridge, 1GB DDR2 800MHz, PCIe + PCI

On all K8 or K10-based computers with AGP, the screen may displays garbage but host can be accessed through SSH, so I got some logs (more to come).

So to sum it up:

- PCI Graphics are broken in kernel since ages on K8/K10 platform but went probably unnoticed since they work on Intel platform and those GPU are not very commons.
- AGP Graphics regressed starting with 5.4.0-48.

Because PCI graphics seems to be broken for ages on K8/K10 computers, when AGP is disabled in kernel, there is no fallback available to run a display, at all.

On a side note, I'm surprised to hear hardware support like AGP may have been disabled less than six months after the release of an Long Term Support distribution that is expected to be supported for multiple years. Six months is even shorter than the usual support for non-LTS versions.

Also, while PCIe was introduced a long time ago (2004?), AGP lived very lately, as demonstrated by this not-so-obsolete machine featuring a quad core CPU with hardware virtualization, 16GB of RAM and a TeraScale (OpenGL 3.3, OpenCL 1.0 capable) GPU with 1GB of VRAM.

The motherboard is from 2006, GPU is from 2008, and CPU is from end of the year 2010. It seems to be a bit early to end support for them. This is among the best the market had to offer in 2010.

Here can be found some documentation on the hardware:
Motherboard: https://www.asrock.com/mb/nvidia/am2nf3-vsta/
GPU: http://www.hisdigital.com/us/product2-448.shtml
CPU: https://www.cpu-world.com/CPUs/K10/AMD-Phenom%20II%20X4%20970%20Black%20Edition%20-%20HDZ970FBK4DGM%20(HDZ970FBGMBOX).html