CPU temperature always returns empty on MediaTek MT7981 (ImmortalWrt 24.10.1

[meotao]

Environment

Item	Value
Device	BeeconMini SEED AC2
SoC	MediaTek MT7981B (Filogic 820)
OS	ImmortalWrt 24.10.1 r33048-cc720ea55a71
Kernel	6.6.86
quickstart binary	/usr/sbin/quickstart (served at 127.0.0.1:3038)
autocore	version 42

---

Bug description

The CPU temperature widget on the QuickStart homepage always shows 0 (or blank).

The temperature API endpoint returns an empty result object:

# From browser console (authenticated session)
fetch('/cgi-bin/luci/istore/system/cpu/temperature/')
  .then(r => r.json()).then(d => console.log(JSON.stringify(d)))

# Response:
{"result":{}}

---

Root cause analysis

The quickstart binary implements three temperature strategies, tried in order. All three fail on this platform.

Strategy 1 — thermalZoneTemperature

The binary reads /sys/class/thermal/thermal_zone*/type to find a zone whose type contains "cpu", then constructs the temperature path using the type value (not the directory name):

Step 1: read /sys/class/thermal/thermal_zone0/type  → "cpu-thermal"  ✓
Step 2: read /sys/class/thermal/cpu-thermal/temp    → does not exist ✗

On this device, /sys/class/thermal/ only contains thermal_zone0 as a directory entry. There is no symlink or directory named cpu-thermal, so step 2 fails:

$ ls -la /sys/class/thermal/
lrwxrwxrwx  thermal_zone0 -> ../../devices/virtual/thermal/thermal_zone0

$ cat /sys/class/thermal/thermal_zone0/type
cpu-thermal

$ cat /sys/class/thermal/cpu-thermal/temp
cat: can't open '/sys/class/thermal/cpu-thermal/temp': No such file or directory

The actual temperature is readable at:

$ cat /sys/class/thermal/thermal_zone0/temp
62767   # millidegrees Celsius → 62.7°C

sysfs does not permit creating symlinks or directories inside /sys/class/thermal/:

$ ln -sf /sys/class/thermal/thermal_zone0 /sys/class/thermal/cpu-thermal
ln: /sys/class/thermal/cpu-thermal: Operation not permitted

Strategy 2 — hwmonTemperature

Same pattern: the binary reads /sys/class/hwmon/hwmon*/name, finds a match, then constructs the path using the name value:

$ cat /sys/class/hwmon/hwmon1/name
cpu_thermal

$ cat /sys/class/hwmon/hwmon1/temp1_input
62918   # correct value ✓

$ cat /sys/class/hwmon/cpu_thermal/temp1_input
# does not exist ✗

Again, no cpu_thermal directory exists under /sys/class/hwmon/, and sysfs rejects any attempt to create one:

$ ln -sf /sys/class/hwmon/hwmon1 /sys/class/hwmon/cpu_thermal
ln: /sys/class/hwmon/cpu_thermal: Operation not permitted

Strategy 3 — autocoreTemperature

The binary calls ubus call luci getTempInfo, which returns a human-readable string:

$ ubus call luci getTempInfo
{ "tempinfo": "CPU: 63.1°C" }

The binary apparently cannot parse a numeric temperature from this string format, so this strategy also returns empty.

---

Verification

The ubus interface itself and the underlying /sbin/tempinfo script work correctly:

$ /sbin/tempinfo
CPU: 62.9°C

$ ubus call luci getTempInfo
{ "tempinfo": "CPU: 63.1°C" }

$ ubus call luci getCPUInfo
{ "cpuinfo": "ARMv8 Processor rev 4 (v8l) x 2" }

Note: /sbin/cpuinfo intentionally skips temperature for mediatek/* targets:

# from /sbin/cpuinfo:
case "$DISTRIB_TARGET" in
"ipq40xx"/*|"ipq806x"/*|"mediatek"/*|"qualcommax"/*) ;;  # no cpu_temp set

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Suggested fix

The binary's temperature discovery logic assumes that after finding a thermal zone or hwmon node by name, the path can be constructed from that name string. On platforms where the kernel only exposes numbered entries (thermal_zone0, hwmon1) without corresponding named symlinks, this breaks.

Option A — Use the directory path directly, not the name value

When iterating /sys/class/thermal/thermal_zone*/type, once a matching zone is found, read the temperature from the same directory (thermal_zone0/temp), not from a path derived from the type string.

Similarly for hwmon: once a matching hwmon*/name is found, read hwmon*/temp1_input from the same hwmon* directory.

Option B — Fall back to thermal_zone0/temp when named path fails

If the constructed path (e.g., /sys/class/thermal/cpu-thermal/temp) does not exist, fall back to reading directly from the zone that had the matching type.

Option C — Parse temperature from ubus call luci getTempInfo output

The autocoreTemperature strategy already has the correct data ("CPU: 63.1°C"). Extracting the numeric value with a simple regex (e.g., (\d+\.?\d*)°C) would make this strategy work on any platform supported by autocore.

Option C is the most robust since it delegates platform-specific logic to autocore, which already handles the per-target differences correctly.

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Additional info

Confirmed via strings /usr/sbin/quickstart:

/sys/class/thermal/thermal_zone%v/type
/sys/class/thermal/%v/temp
/sys/class/hwmon/hwmon%d/name
/sys/class/hwmon/hwmon%d/temp%d_input
thermalZoneTemperature default
hwmonTemperature default
hwmonTemperature candidate

The %v in /sys/class/thermal/%v/temp is filled with the type value (cpu-thermal), not the zone index — this is the source of the mismatch.