Display flicker and Ethernet disconnect via Hagibis Thunderbolt dock (UCSI/PD instability)

[muthi.hib]

Hi everyone! I'm pretty new to Linux—I've only been using it for about a week. I did try Ubuntu once, but that was over a decade ago. I've put this report together with some help from ChatGPT to make sure it's neat and clear.

System:
- Kubuntu 24.04.3 LTS (Kernel: 6.14.0-1015-oem)
- Acer Swift SF314-510G (BIOS V1.13)
- Intel i5-1135G7, Iris Xe (TGL GT2), DG1 (Iris Xe MAX)
- Dock: Hagibis Thunderbolt 100W USB-C
- Ethernet (Realtek RTL8156, driver r8152)
- Display connected via dock (DisplayPort Alt Mode)

Problem:
- External display flickers or briefly loses signal.
- Ethernet interface (r8152) drops connection periodically.
- Dock re-enumerates as new USB devices every few minutes.

dmesg excerpts:
ucsi_acpi USBC000:00: con1: Firmware bug: duplicate plug altmode SVID 0x8087
ucsi_acpi USBC000:00: failed to reset PPM!
r8152 2-1.4:1.0 enx00e04c680051: Tx status -71
usb 2-1: USB disconnect / reconnect loop (GenesysLogic + Realtek hub)

Reproduction:
1. Connect Hagibis Thunderbolt dock via USB-C.
2. Attach monitor and Ethernet cable to dock.
3. After ~10–30 minutes, display flickers and Ethernet reconnects.
4. Same hardware is stable on Windows 11.

Analysis:
- UCSI ACPI PD controller loses sync, causing Type-C re-enumeration.
- When PD renegotiates, all downstream USB devices reset.
- i915 + ucsi_acpi + r8152 interaction likely root cause.
- BIOS and Thunderbolt firmware already up to date.

Workarounds tested:
- i915.enable_psr=0 i915.enable_fbc=0 i915.enable_dc=0 → partial improvement
- usbcore.autosuspend=-1 → reduces frequency
- r8152 disable_pll_power_down=1 → stabilizes Ethernet
- Direct HDMI (no dock) → stable

Expected:
Stable display and Ethernet when docked.