NVIDIA fix: results on the RTX 5080 PC (2026-10-01)

PC: RTX 5080 (GB203, 10de:2c02) + AMD Granite Ridge iGPU, MSI MAG 341C OLED on the NVIDIA DP-1 (3440x1440, 175 Hz), CachyOS, Limine, kernel 7.2.8-1-cachyos, nvidia-open 615.71.09, gamescope 3.16.31-1, gamescope-session-cachyos 1.1.6-1.

Result: the kernel-parameter fix does not cure the picture

Baseline (before): gaming mode corrupted without HDR; with HDR it worked but not at 175 Hz and not full screen.
After apply + reboot (modeset = Y, fbdev = Y, NVIDIA modules in the initramfs): flickers heavily, artifacts, still no 175 Hz. modeset/fbdev were already Y live before the fix (driver 615), so the fix changed little.
Cause is not the parameters: NVIDIA's open bug "display modes above 2560x1440@120 with HDR cause flickering/corruption within gamescope-session" (internal bug 5240452, April 2025, no fix as of March 2026; 555 up to 595+, RTX 3070 Ti/4090/5090; corruption is display-side only): https://forums.developer.nvidia.com/t/display-modes-above-2560x1440p-120hz-with-hdr-enabled-cause-flickering-corruption-within-gamescope-session/295314

Bugs found in the fix on this PC (fixed in commit a5e9015 on feature/nvidia-gaming-fix)

CachyOS writes Limine's line as KERNEL_CMDLINE[default]+="..."; the sed only matched =, so apply stopped with "Couldn't add the NVIDIA parameters".
The kernel treats nvidia_drm.modeset=1 and nvidia-drm.modeset=1 alike; the check only knew the hyphen.
tests/nvidia-hardware-test.sh did not set SCRIPT_DIR, so patch_file read /patches/....

What gamescope did (journal, gaming mode)

Picks the RTX 5080 (card1), DP-1, mode list has 3440x1440@175/144/100. It selected @60 by default; with -W 3440 -H 1440 -r 175 added (a copy of /usr/lib/steamos/gamescope-session, whose last line is -O ... \ so args need a copy) it selected @175 and did not crash, but the picture was the same. Steam then sets the Xwayland size to 1920x1080 (not full screen).
vkGetPhysicalDeviceFormatProperties2 returned zero modifiers for 2 16-bit formats: harmless here.
A gamescope SIGABRT (double free or corruption) at session exit, backtrace through libnvidia-eglcore and libvulkan_radeon.
The PC froze once and was reset after a gaming-mode session (no kernel error logged).

What works: nested gamescope inside KDE (Wayland), borderless

gamescope -W 3440 -H 1440 -r 175 -f ...: artifacts once the window is focused and fullscreen (unfocused it is fine). Hypothesis, not proven: KWin's direct scanout of the fullscreen window.
gamescope -W 3440 -H 1440 -r 175 -b -e --hdr-enabled -- steam -gamepadui (borderless): clean, full screen, about 175 Hz.
So on NVIDIA the separate gamescope session (the SteamOS conversion) is blocked by the driver bug; a nested borderless gamescope works. Whether Steamify gets a nested option for NVIDIA is the user's decision.

Cause confirmed: KWin's direct scanout; a session without Plasma works (2026-10-01, later)

A TTY session of only kwin_wayland --drm + gamescope -b + steam -gamepadui (no Plasma shell, no panel) artifacted too (KWin hands the covering window straight to the display).
Same session with KWIN_DRM_NO_DIRECT_SCANOUT=1 (exists in KWin 6.7.5's libkwin; not in the kwin_wayland binary's strings): NO artifacts, full screen, about 175 Hz (gamescope logs Changed refresh to: 174.962hz). Animations felt a bit clunky (cause unknown: an extra composition step without scanout, no CAP_SYS_NICE for gamescope, no VRR, nested frame pacing; other switches to try: KWIN_DRM_DISABLE_TRIPLE_BUFFERING, KWIN_DRM_PREFER_COLOR_DEPTH).
Launcher used for the test: ~/.local/bin/steamify-nvidia-session-test (run from a free TTY; KWin as the compositor, gamescope -W 3440 -H 1440 -r 175 -b -e --hdr-enabled -- steam -gamepadui, a watchdog ends it after 90 s). Lesson: the first version's timeout left KWin/gamescope/Steam running (the cleanup came after a pipeline that waits for them); pkill -f '<pattern>' in a command that contains the pattern kills the caller.
Open: a real session entry (/usr/share/wayland-sessions/, needs sudo) and the product decision: an NVIDIA session mode in Steamify instead of the gamescope DRM session. Not built, not in the product repo.

State of the PC after the test (to undo)

The fix is applied (Limine file backup /etc/default/limine.bak-gamescope-wizard, early-load file, pacman hook): undo in INSTRUCTIONS.md.
User override of the gaming session: ~/.config/systemd/user/gamescope-session.service.d/steamify-nvidia-test.conf and ~/.local/share/steamify/nvidia-test/.

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