Here on a PowerMac G5 with a NVIDIA GeForce 6600LE graphics card, most things
run pretty well except for some of the things I wanted to run on the machine
resulted in nouveau GPU lock-ups, and consequently a glitched display
(artifacts only).

When trying to start GDM, SDDM, or Plasma Desktop on the machine, the display
locks up, and I was able to salvage an error message as follows:

[drm] nouveau 0000:0a:00.0: GPU lockup - switching to software fbcon

After a minute or two, the display responds again, but only showing a black
background with several randomly colored pixels, and some white blocks.

However, this graphics card runs LightDM, XFCE, MATE... and most other
applications without such error and lock-ups. I was able to workaround the
issue by using the nouveau.noaccel=1 kernel parameter (except in the case of
GDM, it returned with the same error), but that simply beats the purpose of
owning a graphics card.

However, even with the noaccel parameter in place, I was still experiencing bad
coloring issues, as reported here:
https://bugs.freedesktop.org/show_bug.cgi?id=98630.

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Some version info:

- Linux Kernel 4.8.6 (with 4K paging in case you are wondering)
- Mesa 13.0.0
- Xorg Server 1.18.4

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P.S.

I tried on with a Quadro FX4500, which simply shows the lock up error whenever
attempting to start X. But I couldn't make sure if it's a hardware issue or
not, as the card sparked on the first power up, that's why I have already
returned it...