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2026-07-12 10:17:17 -04:00
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+# 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
+
+```bash
+# 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:
+
+```json
+"prep-cmd": [
+  {
+    "do": "",
+    "undo": "setsid steam steam://close/bigpicture"
+  }
+]
+```
+
+## Alternatives
+
+1. **Better dummy plug:** "Headless Ghost" or "Dr. HDMI" series — these support actual mode switching via hardware.
+2. **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.
+3. **NVIDIA Virtual Display (NVFBC-based):** `sudo nvidia-xconfig --virtual=1920x1080 --mode=1920x1080` — only works with `capture = nvfbc` which is broken on driver 580+.