ray
62 lines
2.6 KiB
Markdown
62 lines
2.6 KiB
Markdown
# Locked Dummy Plug — BadMatch on xrandr Mode Switch
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## Symptom
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All `xrandr --output HDMI-0 --mode <res>` commands fail with:
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```
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xrandr: Configure crtc 0 failed
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X Error of failed request: BadMatch (invalid parameter attributes)
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Major opcode of failed request: 140 (RANDR)
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Minor opcode of failed request: 7 (RRSetScreenSize)
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```
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Exit code 1. Occurs for every resolution/refresh combination — 4K@60, 1440p@120, 1080p@120 — even though EDID advertises all of them.
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## Root Cause
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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`.
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## Hardware Details
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- **Make/Model:** FUEARAN 4K/120 (labeled as HDMI 2.1 dummy plug)
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- **EDID tag:** `0046554552414e20344b2f313230` (decoded: "FUEARAN 4K/120")
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- **Physical connection:** HDMI-0 (NVIDIA port, X11 connector)
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- **NVIDIA GPU:** RTX 2080 Ti, driver 580.159.03
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- **Xorg mode:** X11 (not Wayland, not KMS capture)
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- **Reports available via xrandr:** 3840x2160@60*, 2560x1440@120, 1920x1080@120, etc.
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- **Actually works:** 3840x2160 at whatever rate it powers up (60Hz via xrandr)
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## Diagnosis
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```bash
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# Test any mode switch — if it fails, plug is locked
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DISPLAY=:0 xrandr --output HDMI-0 --mode 1920x1080 2>&1
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# Check current actual mode
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DISPLAY=:0 xrandr | grep "*"
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# Verify Sunshine captures it correctly despite the lock
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grep "Streaming display:" ~/.config/sunshine/sunshine.log
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# Should show: HDMI-0 with res 3840x2160 offset by 0x0
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```
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## Fix
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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:
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```json
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"prep-cmd": [
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{
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"do": "",
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"undo": "setsid steam steam://close/bigpicture"
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}
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]
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```
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## Alternatives
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1. **Better dummy plug:** "Headless Ghost" or "Dr. HDMI" series — these support actual mode switching via hardware.
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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.
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3. **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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