nixos-servers/docs/plans/pn51-stability.md

# ASUS PN51 Stability Testing

## Overview

Two ASUS PN51-E1 mini PCs (Ryzen 7 5700U) purchased years ago but shelved due to stability issues. Revisiting them to potentially add to the homelab.

## Hardware

| | pn01 (10.69.12.60) | pn02 (10.69.12.61) |
|---|---|---|
| **CPU** | AMD Ryzen 7 5700U (8C/16T) | AMD Ryzen 7 5700U (8C/16T) |
| **RAM** | 2x 32GB DDR4 SO-DIMM (64GB) | 1x 32GB DDR4 SO-DIMM (32GB) |
| **Storage** | 1TB NVMe | 1TB Samsung 870 EVO (SATA SSD) |
| **BIOS** | 0508 (2023-11-08) | Updated 2026-02-21 (latest from ASUS) |

## Original Issues

- **pn01**: Would boot but freeze randomly after some time. No console errors, completely unresponsive. memtest86 passed.
- **pn02**: Had trouble booting — would start loading kernel from installer USB then instantly reboot. When it did boot, would also freeze randomly.

## Debugging Steps

### 2026-02-21: Initial Setup

1. **Disabled fTPM** (labeled "Security Device" in ASUS BIOS) on both units
   - AMD Ryzen 5000 series had a known fTPM bug causing random hard freezes with no console output
   - Both units booted the NixOS installer successfully after this change
2. Installed NixOS on both, added to repo as `pn01` and `pn02` on VLAN 12
3. Configured monitoring (node-exporter, promtail, nixos-exporter)

### 2026-02-21: pn02 First Freeze

- pn02 froze approximately 1 hour after boot
- All three Prometheus targets went down simultaneously — hard freeze, not graceful shutdown
- Journal on next boot: `system.journal corrupted or uncleanly shut down`
- Kernel warnings from boot log before freeze:
  - **TSC clocksource unstable**: `Marking clocksource 'tsc' as unstable because the skew is too large` — TSC skewing ~3.8ms over 500ms relative to HPET watchdog
  - **AMD PSP error**: `psp gfx command LOAD_TA(0x1) failed and response status is (0x7)` — Platform Security Processor failing to load trusted application
- pn01 did not show these warnings on this particular boot, but has shown them historically (see below)

### 2026-02-21: pn02 BIOS Update

- Updated pn02 BIOS to latest version from ASUS website
- **TSC still unstable** after BIOS update — same ~3.8ms skew
- **PSP LOAD_TA still failing** after BIOS update
- Monitoring back up, letting it run to see if freeze recurs

### 2026-02-22: TSC/PSP Confirmed on Both Units

- Checked kernel logs after ~9 hours uptime — both units still running
- **pn01 now shows TSC unstable and PSP LOAD_TA failure** on this boot (same ~3.8ms TSC skew, same PSP error)
- pn01 had these same issues historically when tested years ago — the earlier clean boot was just lucky TSC calibration timing
- **Conclusion**: TSC instability and PSP LOAD_TA are platform-level quirks of the PN51-E1 / Ryzen 5700U, present on both units
- The kernel handles TSC instability gracefully (falls back to HPET), and PSP LOAD_TA is non-fatal
- Neither issue is likely the cause of the hard freezes — the fTPM bug remains the primary suspect

### 2026-02-22: Stress Test (1 hour)

- Ran `stress-ng --cpu 16 --vm 2 --vm-bytes 8G --timeout 1h` on both units
- CPU temps peaked at ~85°C, settled to ~80°C sustained (throttle limit is 105°C)
- Both survived the full hour with no freezes, no MCE errors, no kernel issues
- No concerning log entries during or after the test

### 2026-02-22: TSC Runtime Switch Test

- Attempted to switch clocksource back to TSC at runtime on pn01:
  ```
  echo tsc > /sys/devices/system/clocksource/clocksource0/current_clocksource
  ```
- Kernel watchdog immediately reverted to HPET — TSC skew is ongoing, not just a boot-time issue
- **Conclusion**: TSC is genuinely unstable on the PN51-E1 platform. HPET is the correct clocksource.
- For virtualization (Incus), this means guest VMs will use HPET-backed timing. Performance impact is minimal for typical server workloads (DNS, monitoring, light services) but would matter for latency-sensitive applications.

### 2026-02-22: BIOS Tweaks (Both Units)

- Disabled ErP Ready on both (EU power efficiency mode — aggressively cuts power in idle)
- Disabled WiFi and Bluetooth in BIOS on both
- **TSC still unstable** after these changes — same ~3.8ms skew on both units
- ErP/power states are not the cause of the TSC issue

### 2026-02-22: pn02 Second Freeze

- pn02 froze again ~5.5 hours after boot (at idle, not under load)
- All Prometheus targets down simultaneously — same hard freeze pattern
- Last log entry was normal nix-daemon activity — zero warning/error logs before crash
- Survived the 1h stress test earlier but froze at idle later — not thermal
- pn01 remains stable throughout
- **Action**: Blacklisted `amdgpu` kernel module on pn02 (`boot.blacklistedKernelModules = [ "amdgpu" ]`) to eliminate GPU/PSP firmware interactions as a cause. No console output but managed via SSH.

## Benign Kernel Errors (Both Units)

These appear on both units and can be ignored:
- `clocksource: Marking clocksource 'tsc' as unstable` — TSC skew vs HPET, kernel falls back gracefully. Platform-level quirk on PN51-E1, not always reproducible on every boot.
- `psp gfx command LOAD_TA(0x1) failed` — AMD PSP firmware error, non-fatal. Present on both units across all BIOS versions.
- `pcie_mp2_amd: amd_sfh_hid_client_init failed err -95` — AMD Sensor Fusion Hub, no sensors connected
- `Bluetooth: hci0: Reading supported features failed` — Bluetooth init quirk
- `Serial bus multi instantiate pseudo device driver INT3515:00: error -ENXIO` — unused serial bus device
- `snd_hda_intel: no codecs found` — no audio device connected, headless server
- `ata2.00: supports DRM functions and may not be fully accessible` — Samsung SSD DRM quirk (pn02 only)

## Next Steps

- Monitor pn02 with amdgpu blacklisted — if stable, try the less impactful `amdgpu.runpm=0 amdgpu.dpm=0` kernel params instead
- If pn02 still freezes without amdgpu, likely a hardware defect on this unit
- pn01 continues to be stable — keep monitoring
- Once stable: add second RAM stick back to pn02, reinstall with NVMe
- Evaluate for Incus hypervisor use (see `nixos-hypervisor.md`)