General graphics regression using older Radeons on larger/dual displays, screen corruption.
General acceleration/video regression in freedesktop Issue I wish to report!!, to which I have not yet been able to narrow down specific packages, been directed to create bug report here on mesa in first-instance [please point in right direction!]...
From a lot of narrowing-down, this happens for me, on, older "radeon" graphics devices (for example "RS480" on one amd64 machine), or an ATI Radeon AGP card on another, completely different chipset/series of amd64 system. These both use the "radeon" driver BUT the problem is clearly NOT just the driver.
AND only happens when I plug in 2 monitors (takes overall screen size over 2048 pixel wide). AND only happens to me with ubuntu 19.10 and 20.04 variants (including mint-20). BUT was perfectly fine in ubuntu 18.04 and seems ok with debian-10 live as well.
Does not seem to make any difference which desktop variant (e.g xubuntu xfce, Mint-20-Cinnamon, Mint-20-XFCE, ...).
NOTE the lightdm login screen may work fine, both displays fine), but when log in, content scrambled all over the place, mouse cursor overlay however works perfectly fine... NOTE can login with 1 monitor attached, but attach another and auto-configures (or manual configure in xfce), and all chaos ensues at that point instead...
I'm told in #radeon channel A likely good hint:- "old radeons had a pitch limit of 2048 pixels." "the driver tells userspace it's limits, but userspace generally ignores them and tries anyway"" I tried a patch for some different radeon patch/bug wrt a different issue on amdgpu and that didn't help.
I've tried on 20.04 reverting the complete x.org package-set including drivers, full set of mesa libraries+packages (which needed older versions of other tools), older linux kernel, to their ubuntu 18.04 versions (that definitely worked fine!), and I still get the issue occurring, there must be something else compiled-in to other packages, other libraries (gtk??) or otherwise that are triggering the fault, that I do not know...!.
This is proving hard to track down, but I'm sure it will be affecting some using older graphics devices with multi-head (or, presumably, with a 'big monitor > 2048 pixels wide' too).
I'm happy to try some fixes/changes, different bootable linux images, etc. I'd like bug pointed in the right direction, put on correct package, .... We shouldn't be this garbled-display regression, there should be a least some kind of warning or disable acceleration or so, bearing in mind this always USED to work fine.
With many thanks,