[Regression] (550.54.14) Native HDMI 2.1 signals i.e above 4k60Hz won't work on 8 bit output (hdmi_deepcolor=0)

RTX 4090, LG C2 42", KDE Plasma 6 Wayland on Fedora 40. I’ve seen this issue here and there on some previous driver versions too, X11 and wayland where going above HDMI 2.0 limits result in a blank screen (TV going “No signal”). Interestinly enough enabling hdmi_deepcolor=1 allows me to go 4k120hz so I think the issue resurfaced with this changelog line:

  • Fixed a bug that caused HDMI FRL displays to flicker or blank when enabling VRR with 8 bits per color channel

As KDE Plasma has VRR and I’ve set it to “Always”. I also think drivers prior to like 545 had similar but unrelated issue with HDMI 2.1 4k120hz as back then I tested them on desktop environments like GNOME and Xfce.

Sidenote: it is unclear to me whether the kernel parameter for 10 bit output is

nvidia_modeset.hdmi_deepcolor=1

or

nvidia-modeset.hdmi_deepcolor=1

so I added both just in case.

Bug report attached
nvidia-bug-report.log.gz (675.9 KB)