Hi Andreas--
I do believe that it can be hardware-related, but only to the video card.  I've been running this computer, the same 4K monitor, and the same displayport cable for three years without any problems, except limited graphics performance due to the built-in Intel graphics!  I've got a pile of displayport cables should I want to switch out one of those, and one other 4K monitor as well.

I'm very surprised you can get the Xorg amdgpu driver to work with the amdgpu kernel module.  I guess the newest I've used is 5.4.0 from backports, so conceivably 5.4.7 is different, but keep in mind this is a video card first released in 2015, so rather than newer being better, perhaps newer is less compatible as old cards are (silently?) deprecated in some way!
I'm taking steps to return this thing, and considering a more typical gaming card such as an RX 560 or 570 or an Nvidia 1050 or 1060.  I don't do gaming, but the video editing should be OK with one of those.  The more I look into the AMD cards, the more people seem to have trouble finding drivers that work well, even on Windows.  I might try them one more time.  It's great to support LInux better than Nvidia does in theory, but in practice ... ?  I also never quite figured out how to run the proprietary AMD drivers under Devuan.  I assume that compiling a kernel which matches the Ubuntu version might do it, but that kind of defeats the purpose of buying a card well-supported in Linux!  Downloading an Nvidia driver is easier than that!
Thanks to everybody in DNG who gave me so much good advice, though.  I'm sticking with Devuan no matter what since all my systemd issues are gone!

--Tim

    On Wednesday, February 5, 2020, 6:20:28 PM UTC, Andreas Messer <andi@???> wrote:  

Hi Tim,

On Sat, Feb 01, 2020 at 04:44:07PM +0000, Tim Wallace via Dng wrote:
>  Hi Andreas--
> lspci shows this:01:00.0 VGA compatible controller: Advanced Micro
> Devices, Inc. [AMD/ATI] Cape Verde GL [FirePro W4100]which shows the
> Cape Verde but not the Southern Island.  I think all Cape Verde are
> that, though.

Yes, Cap Verde is Southern Island generation. So we basically have the
same card.

> The old radeon kernel module paired with the Xorg radeon driver is the
> one that gives random annoying flashes. I can almost live with them, but
> under virtualbox win7 which my wife runs once a week it is unbelievably
> flashy!

Maybe it is something completely different: At work I had similar issues
with my monitor attached to the notebook dock. From time to time, the
monitor turned black for short time and immediately recovered the image.
It seemed to align with some other employees switching on or off some
devices in the first place, but in the end turned out to be a loosely
fitted display port plug. Could it be the cable? I also heard that in
some cases a bad or insufficient display port cable led to unstable
monitor operation.

It could be also a DP link training issue
I do believe that it can be hardware-related, but only to the video card.  I've been running this computer, the same 4K monitor, and the same displayport cable for three years without any problems, except limited graphics performance due to the built-in Intel graphics!

>  The newer amdgpu module fails to work with the Xorg amdgpu driver. 
>  This is with ascii, beowulf, and kernels 4.9, 4.19, and 5.4.  I'm able
>  to get them installed, they just won't start.  The Xorg amdgpu version
>  is 18.1, from late 2018, and a 19.1 is out, from 2019, but claims only
>  minor tweaks.  I'm not thinking it's worth bothering to try to compile
>  that.

Hmm strange. I was using standard beowulf Xorg package with self compiled
kernel 5.4.7's amdgpu module here without any issues.

cheers,

Andreas

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