hwmon: (acpi_power_meter) Fix 4.29 MW bug

The ACPI specification says:

"If an error occurs while obtaining the meter reading or if the value
is not available then an Integer with all bits set is returned"

Since the "integer" is 32 bits in case of the ACPI power meter,
userspace will get a power reading of 2^32 * 1000 miliwatts (~4.29 MW)
in case of such an error. This was discovered due to a lm_sensors
bugreport (lm-sensors/lm-sensors#460).
Fix this by returning -ENODATA instead.

Tested-by: <[email protected]>
Fixes: de584af ("hwmon driver for ACPI 4.0 power meters")
Signed-off-by: Armin Wolf <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Guenter Roeck <[email protected]>

Author: Wer-Wolf

drivers/hwmon/acpi_power_meter.c

@@ -31,6 +31,7 @@
 #define POWER_METER_CAN_NOTIFY	(1 << 3)
 #define POWER_METER_IS_BATTERY	(1 << 8)
 #define UNKNOWN_HYSTERESIS	0xFFFFFFFF
+#define UNKNOWN_POWER		0xFFFFFFFF
 
 #define METER_NOTIFY_CONFIG	0x80
 #define METER_NOTIFY_TRIP	0x81
@@ -348,6 +349,9 @@ static ssize_t show_power(struct device *dev,
 	update_meter(resource);
 	mutex_unlock(&resource->lock);
 
+	if (resource->power == UNKNOWN_POWER)
+		return -ENODATA;
+
 	return sprintf(buf, "%llu\n", resource->power * 1000);
 }