RTL8196E watchdog (rtl819x-wdt)

This is the production driver for the RTL8196E hardware watchdog. It exposes the normal Linux /dev/watchdog interface and resets the SoC when the feeder stops. Its production objective is recovery, not live forensic tracing.

Normal operation

The BusyBox feeder opens /dev/watchdog and pings it every 30 seconds. The driver defaults to a 60-second software timeout and the hardware's maximum window is about 671 seconds (25 kHz timer clock). nowayout remains a read-only boot-time module parameter.

The watchdog also covers kernel panics and machine restarts:

Event	Driver action
feeder stops	watchdog core stops refreshing the hardware
panic() / soft-lockup panic	arm the shortest hardware reset window
restart callback	arm the shortest hardware reset window

The short hardware window is about 1.31 seconds. The reset sequence writes the disable/clear pattern and then zero, which avoids reusing a stale hardware counter state on this silicon.

Compact post-mortem record

On a panic, the driver first arms recovery and then writes one bounded, 108-byte record. It contains uptime, EPC/RA, CP0 cause/status, pending softirqs, WDTCNR, flags, and a sanitized panic reason. The magic word is written last, so a torn write is discarded at the next boot.

The record resides in the dedicated watchdog-crash@1ffd000 no-map page. It is deliberately separate from boothold@1ffe000: the bootloader handoff words and the watchdog record cannot overwrite one another. A missing or invalid memory-region disables only this optional record; the watchdog continues to operate.

At probe, a valid record is emitted as one previous panic: dmesg line and then cleared. Older record formats are discarded rather than decoded.

Quick checks

dmesg | grep -E 'rtl819x-wdt|previous panic'
cat /sys/class/watchdog/watchdog0/identity
cat /sys/class/watchdog/watchdog0/timeout
ls -l /dev/watchdog

The expected probe line identifies driver v1.12, its configured timeout, and whether nowayout is active.

For implementation and audit rationale, see DESIGN.md and AUDIT.md.