Screen corruption using radeon kernel driver

[Krylov Michael]

Hello! 

After updating my Linux kernel from version 4.19 (Debian 10 version) to
5.10 (packaged with Debian 11), I've noticed that the image
displayed on my older computer, 32-bit Pentium 4 using ATI Radeon X1950
AGP video card is severely corrupted in the graphical (Xorg and Wayland)
mode: all kinds of black and white stripes across the screen, some
letters missing, etc.

I've checked several options (Xorg drivers, Wayland instead of
Xorg, radeon.agpmode=-1 in kernel command line and so on), but the
problem persisted. I've managed to find that the problem was in the
kernel, as everything worked well with 4.19 kernel with everything
else being from Debian 11.

I have managed to find the culprit of that corruption, that is the
commit 33b3ad3788aba846fc8b9a065fe2685a0b64f713 on the linux kernel.
Reverting this commit and building the kernel with that commit reverted
fixes the problem. Disabling HIMEM also gets rid of that problem. But it
also leaves the system with less that 1G of RAM, which is, of course,
undesirable.

Apparently this problem is somewhat known, as I can tell after googling
for the commit id, see this link for example:
https://lkml.org/lkml/2020/1/9/518 

Mageia distro, for example, reverted this commit in the kernel they are
building:

http://sophie.zarb.org/distrib/Mageia/7/i586/by-pkgid/b9193a4f85192bc57f4d770fb9bb399c/files/32

I've reported this bug to Debian bugtracker, checked the recent verion
of the kernel (5.17), bug still persists. Here's a link to the Debian
bug page:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=993670

I'm not sure if reverting this commit is the correct way to go, so if
you need to check any changes/patches that I could apply and test on
the real hardware, I'll be glad to do that (but please keep in mind
that testing could take some time, I don't have access to this computer
24/7, but I'll do my best to respond ASAP).

Thanks in advance!
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