_read_temp: fall back to hwmon k10temp when thermal_zone0 is absent — max_temp_c is inert on the whole production fleet

[VijitSingh97]

Why

_read_temp reads /sys/class/thermal/thermal_zone0/temp (with the env-only THERMAL_ZONE override). Surveyed 2026-07-10: none of the 8 production rigs has any /sys/class/thermal/thermal_zone* — on both the 7800X3D and the EPYC 7642s, CPU temperature is exposed only through hwmon (k10temp: 72°C on miner-0, 89–92°C Tctl on the loaded EPYCs). Consequence: the #139 watchdog's max_temp_c cutoff is inert fleet-wide (unreadable temp skips thermal — correct fail-safe, but the feature can't be used at all), and tune's hashrate-per-watt temperature sampling (#54) is equally blind on these boards. THERMAL_ZONE can't rescue the watchdog: the timer unit bakes no env, deliberately.

Scope

• _read_temp fallback chain: TUNE_TEMP_CMD (unchanged, test override) → THERMAL_ZONE/thermal_zone0 (unchanged) → first hwmon whose name is k10temp (AMD) or coretemp (Intel), reading temp1_input (same millidegree format, so the awk conversion is shared). Still best-effort: all absent → empty, thermal logic skips, exactly today's behavior.
• Docs (configuration.md max_temp_c row + operations.md watchdog section): name the fallback and add the Tctl caveat — k10temp on EPYC reports Tctl, a control temperature that can sit well above ambient-style expectations (the loaded 7642s idle-of-nothing at ~90 Tctl); operators must check the live reading before choosing a cutoff, which the existing "check what your board reports" guidance already implies.
• Tests: stubbed hwmon tree (right name → read, wrong-name-first → keeps scanning, none → empty); watchdog + tune paths pick it up for free since both call _read_temp.

Acceptance criteria

• On a machine with no thermal_zone but a k10temp hwmon, _read_temp returns the k10temp value; watchdog's max_temp_c becomes functional there.
• No thermal source at all still returns empty (thermal skipped, miner never stopped by a missing sensor).
• Docs carry the Tctl caveat. make lint + make test green (bash 3.2 + Linux).

Related

• watchdog: auto-recover a wedged miner + optional thermal cutoff (timer, opt-in) #139 — the watchdog whose thermal cutoff this unblocks; Auto-tuning: iterative, multi-knob search (follow-up to #46 MVP) #54/tune: measure sustained / thermal steady-state (and reject throttled readings) #62 — tune-time thermal sensing with the same blindness; doctor/bios: detect NPS1 on EPYC — flag a NUMA-per-socket regression (detectable, currently silent) #201 — sibling "check what the BIOS/board actually exposes" work.