HWMON Subsystem

The Hardware Monitoring (HWMON) subsystem provides a unified interface for monitoring hardware sensors: temperature, voltage, current, power, fan speed, and humidity.

HWMON Basics

flowchart TB
    subgraph Sensors["Sensor Types"]
        TEMP["Temperature<br/>(°C)"]
        VOLT["Voltage<br/>(mV)"]
        CURR["Current<br/>(mA)"]
        PWR["Power<br/>(μW)"]
        FAN["Fan Speed<br/>(RPM)"]
        HUMID["Humidity<br/>(%RH)"]
    end

    subgraph HWMON["HWMON Core"]
        HWDEV["hwmon_device"]
        ATTRS["Sensor Attributes"]
        SYSFS["/sys/class/hwmon/"]
    end

    subgraph Apps["Applications"]
        SENSORS["sensors<br/>(lm-sensors)"]
        THERMAL["Thermal Zone"]
        CUSTOM["Custom Monitor"]
    end

    TEMP --> HWDEV
    VOLT --> HWDEV
    CURR --> HWDEV
    PWR --> HWDEV
    FAN --> HWDEV
    HUMID --> HWDEV

    HWDEV --> ATTRS
    ATTRS --> SYSFS

    SYSFS --> SENSORS
    SYSFS --> THERMAL
    SYSFS --> CUSTOM

    style Sensors fill:#738f99,stroke:#0277bd
    style HWMON fill:#7a8f73,stroke:#2e7d32
    style Apps fill:#8f8a73,stroke:#f9a825

Sensor Types and Units

Type	sysfs Prefix	Unit	Scale
Temperature	`temp`	°C	millidegree (1000 = 1°C)
Voltage	`in`	V	millivolt
Current	`curr`	A	milliamp
Power	`power`	W	microwatt
Fan Speed	`fan`	RPM	1 RPM
Humidity	`humidity`	%RH	milli %RH (1000 = 1%)
Intrusion	`intrusion`	-	boolean

Linux HWMON Architecture

flowchart TB
    subgraph Driver["HWMON Driver"]
        CHIP["hwmon_chip_info"]
        CHAN["hwmon_channel_info"]
        OPS["hwmon_ops"]
    end

    subgraph Core["HWMON Core"]
        REG["hwmon_device_register_with_info()"]
        DEV["hwmon_device"]
    end

    subgraph Interface["sysfs Interface"]
        TEMP1["temp1_input"]
        TEMP1C["temp1_crit"]
        TEMP1L["temp1_label"]
        IN0["in0_input"]
        FAN1["fan1_input"]
    end

    CHIP --> REG
    CHAN --> CHIP
    OPS --> CHIP
    REG --> DEV
    DEV --> TEMP1
    DEV --> TEMP1C
    DEV --> TEMP1L
    DEV --> IN0
    DEV --> FAN1

    style Driver fill:#738f99,stroke:#0277bd
    style Core fill:#7a8f73,stroke:#2e7d32
    style Interface fill:#8f8a73,stroke:#f9a825

Key Structures

hwmon_chip_info

#include <linux/hwmon.h>

struct hwmon_chip_info {
    const struct hwmon_ops *ops;
    const struct hwmon_channel_info * const *info;
};

hwmon_channel_info

struct hwmon_channel_info {
    enum hwmon_sensor_types type;
    const u32 *config;  /* Array of attribute configs */
};

/* Sensor types */
enum hwmon_sensor_types {
    hwmon_chip,
    hwmon_temp,
    hwmon_in,
    hwmon_curr,
    hwmon_power,
    hwmon_energy,
    hwmon_humidity,
    hwmon_fan,
    hwmon_pwm,
    hwmon_intrusion,
    hwmon_max,
};

hwmon_ops

struct hwmon_ops {
    umode_t (*is_visible)(const void *drvdata,
                          enum hwmon_sensor_types type,
                          u32 attr, int channel);
    int (*read)(struct device *dev, enum hwmon_sensor_types type,
                u32 attr, int channel, long *val);
    int (*read_string)(struct device *dev, enum hwmon_sensor_types type,
                       u32 attr, int channel, const char **str);
    int (*write)(struct device *dev, enum hwmon_sensor_types type,
                 u32 attr, int channel, long val);
};

Channel Configuration Flags

Temperature Attributes

/* Temperature sensor attributes */
HWMON_T_INPUT        /* Current temperature */
HWMON_T_MAX          /* Maximum threshold */
HWMON_T_MAX_HYST     /* Maximum hysteresis */
HWMON_T_MIN          /* Minimum threshold */
HWMON_T_MIN_HYST     /* Minimum hysteresis */
HWMON_T_CRIT         /* Critical threshold */
HWMON_T_CRIT_HYST    /* Critical hysteresis */
HWMON_T_EMERGENCY    /* Emergency threshold */
HWMON_T_EMERGENCY_HYST
HWMON_T_LOWEST       /* Historical lowest */
HWMON_T_HIGHEST      /* Historical highest */
HWMON_T_LABEL        /* Sensor label */
HWMON_T_ALARM        /* Alarm status */
HWMON_T_FAULT        /* Fault status */
HWMON_T_OFFSET       /* Temperature offset */

Voltage/Current/Power Attributes

/* Voltage (in) */
HWMON_I_INPUT, HWMON_I_MIN, HWMON_I_MAX, HWMON_I_LABEL,
HWMON_I_ALARM, HWMON_I_CRIT, HWMON_I_LCRIT

/* Current (curr) */
HWMON_C_INPUT, HWMON_C_MIN, HWMON_C_MAX, HWMON_C_LABEL,
HWMON_C_ALARM, HWMON_C_CRIT, HWMON_C_LCRIT

/* Power */
HWMON_P_INPUT, HWMON_P_MIN, HWMON_P_MAX, HWMON_P_LABEL,
HWMON_P_ALARM, HWMON_P_CRIT

Fan Attributes

HWMON_F_INPUT        /* Current RPM */
HWMON_F_MIN          /* Minimum RPM */
HWMON_F_MAX          /* Maximum RPM */
HWMON_F_TARGET       /* Target RPM */
HWMON_F_LABEL        /* Fan label */
HWMON_F_ALARM        /* Alarm status */
HWMON_F_FAULT        /* Fault status */
HWMON_F_DIV          /* Fan divisor */
HWMON_F_PULSES       /* Pulses per revolution */

sysfs Interface

# List HWMON devices
ls /sys/class/hwmon/

# Read sensor values
cat /sys/class/hwmon/hwmon0/name
cat /sys/class/hwmon/hwmon0/temp1_input   # millidegrees
cat /sys/class/hwmon/hwmon0/temp1_label
cat /sys/class/hwmon/hwmon0/in0_input     # millivolts
cat /sys/class/hwmon/hwmon0/fan1_input    # RPM

# Set thresholds (if writable)
echo 80000 > /sys/class/hwmon/hwmon0/temp1_max  # 80°C max

# Using lm-sensors
sensors
sensors -j  # JSON output

sysfs Attribute Naming

<type><index>_<attribute>

Examples:
temp1_input    - Temperature sensor 1 input
temp2_max      - Temperature sensor 2 maximum
in0_input      - Voltage sensor 0 input
curr1_input    - Current sensor 1 input
fan1_input     - Fan 1 speed

Common Use Cases

System Temperature Monitoring

# Find CPU temperature
for hwmon in /sys/class/hwmon/hwmon*; do
    name=$(cat $hwmon/name 2>/dev/null)
    if [ "$name" = "coretemp" ] || [ "$name" = "k10temp" ]; then
        echo "CPU Temperature:"
        for temp in $hwmon/temp*_input; do
            val=$(cat $temp)
            echo "  $(basename $temp): $((val/1000))°C"
        done
    fi
done

Thermal Zone Integration

HWMON devices can be thermal zone sources:

thermal_zones {
    cpu_thermal {
        polling-delay = <1000>;
        polling-delay-passive = <100>;

        thermal-sensors = <&temp_sensor 0>;

        trips {
            cpu_alert: cpu-alert {
                temperature = <80000>;  /* 80°C */
                hysteresis = <2000>;
                type = "passive";
            };
            cpu_crit: cpu-crit {
                temperature = <100000>; /* 100°C */
                hysteresis = <2000>;
                type = "critical";
            };
        };
    };
};

Device Tree Binding

temp_sensor: sensor@48 {
    compatible = "vendor,temp-sensor";
    reg = <0x48>;
    #thermal-sensor-cells = <1>;
};

hwmon@4a {
    compatible = "vendor,power-monitor";
    reg = <0x4a>;
    shunt-resistor-micro-ohms = <10000>;  /* 10 mΩ */
};

Summary

HWMON provides unified sensor monitoring interface
Uses specific units (millidegree, millivolt, etc.)
hwmon_chip_info defines sensor configuration
sysfs attributes follow <type><n>_<attr> naming
Can integrate with thermal framework
lm-sensors provides userspace tools