Locked Dummy Plug — BadMatch on xrandr Mode Switch

Symptom

All xrandr --output HDMI-0 --mode <res> commands fail with:

xrandr: Configure crtc 0 failed
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  7 (RRSetScreenSize)

Exit code 1. Occurs for every resolution/refresh combination — 4K@60, 1440p@120, 1080p@120 — even though EDID advertises all of them.

Root Cause

Cheap HDMI dummy plugs ("FUEARAN 4K/120" and similar unbranded adapters) use a fixed-resolution video signal generator chip that cannot be reconfigured via CRTC/PHY. The EDID is static and lists many modes but the hardware can only output its single native resolution (typically 3840x2160@60). xrandr accepts the mode switch request, passes it to the NVIDIA driver, which tries to reconfigure the CRTC, and the hardware returns BadMatch.

Hardware Details

Make/Model: FUEARAN 4K/120 (labeled as HDMI 2.1 dummy plug)
EDID tag: 0046554552414e20344b2f313230 (decoded: "FUEARAN 4K/120")
Physical connection: HDMI-0 (NVIDIA port, X11 connector)
NVIDIA GPU: RTX 2080 Ti, driver 580.159.03
Xorg mode: X11 (not Wayland, not KMS capture)
Reports available via xrandr: 3840x2160@60*, 2560x1440@120, 1920x1080@120, etc.
Actually works: 3840x2160 at whatever rate it powers up (60Hz via xrandr)

Diagnosis

# Test any mode switch — if it fails, plug is locked
DISPLAY=:0 xrandr --output HDMI-0 --mode 1920x1080 2>&1

# Check current actual mode
DISPLAY=:0 xrandr | grep "*"

# Verify Sunshine captures it correctly despite the lock
grep "Streaming display:" ~/.config/sunshine/sunshine.log
# Should show: HDMI-0 with res 3840x2160 offset by 0x0

Fix

The output_name = 2 fix in sunshine.conf is the primary black-screen fix (ensures Sunshine captures HDMI-0, not a disconnected display). Since the dummy plug can't switch modes, set all app prep-cmds to no-ops:

"prep-cmd": [
  {
    "do": "",
    "undo": "setsid steam steam://close/bigpicture"
  }
]

Alternatives

Better dummy plug: "Headless Ghost" or "Dr. HDMI" series — these support actual mode switching via hardware.
Custom modeline: Generate with cvt 1920 1080 120 and add via xrandr --newmode + xrandr --addmode — may fail if the hardware PHY truly cannot drive different timings.
NVIDIA Virtual Display (NVFBC-based): sudo nvidia-xconfig --virtual=1920x1080 --mode=1920x1080 — only works with capture = nvfbc which is broken on driver 580+.

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