Naming and data format standards for sysfs files

The libsensors library offers an interface to the raw sensors data through the sysfs interface. Since lm-sensors 3.0.0, libsensors is completely chip-independent. It assumes that all the kernel drivers implement the standard sysfs interface described in this document. This makes adding or updating support for any given chip very easy, as libsensors, and applications using it, do not need to be modified. This is a major improvement compared to lm-sensors 2.

Note that motherboards vary widely in the connections to sensor chips. There is no standard that ensures, for example, that the second temperature sensor is connected to the CPU, or that the second fan is on the CPU. Also, some values reported by the chips need some computation before they make full sense. For example, most chips can only measure voltages between 0 and +4V. Other voltages are scaled back into that range using external resistors. Since the values of these resistors can change from motherboard to motherboard, the conversions cannot be hard coded into the driver and have to be done in user space.

For this reason, even if we aim at a chip-independent libsensors, it will still require a configuration file (e.g. /etc/sensors.conf) for proper values conversion, labeling of inputs and hiding of unused inputs.

An alternative method that some programs use is to access the sysfs files directly. This document briefly describes the standards that the drivers follow, so that an application program can scan for entries and access this data in a simple and consistent way. That said, such programs will have to implement conversion, labeling and hiding of inputs. For this reason, it is still not recommended to bypass the library.

Each chip gets its own directory in the sysfs /sys/devices tree. To find all sensor chips, it is easier to follow the device symlinks from /sys/class/hwmon/hwmon*.

Up to lm-sensors 3.0.0, libsensors looks for hardware monitoring attributes in the "physical" device directory. Since lm-sensors 3.0.1, attributes found in the hwmon "class" device directory are also supported. Complex drivers (e.g. drivers for multifunction chips) may want to use this possibility to avoid namespace pollution. The only drawback will be that older versions of libsensors won't support the driver in question.

All sysfs values are fixed point numbers.

There is only one value per file, unlike the older /proc specification. The common scheme for files naming is: <type><number>_<item>. Usual types for sensor chips are "in" (voltage), "temp" (temperature) and "fan" (fan). Usual items are "input" (measured value), "max" (high threshold, "min" (low threshold). Numbering usually starts from 1, except for voltages which start from 0 (because most data sheets use this). A number is always used for elements that can be present more than once, even if there is a single element of the given type on the specific chip. Other files do not refer to a specific element, so they have a simple name, and no number.

Alarms are direct indications read from the chips. The drivers do NOT make comparisons of readings to thresholds. This allows violations between readings to be caught and alarmed. The exact definition of an alarm (for example, whether a threshold must be met or must be exceeded to cause an alarm) is chip-dependent.

When setting values of hwmon sysfs attributes, the string representation of the desired value must be written, note that strings which are not a number are interpreted as 0! For more on how written strings are interpreted see the "sysfs attribute writes interpretation" section at the end of this file.

Attribute access

Hardware monitoring sysfs attributes are displayed by unrestricted userspace applications. For this reason, all standard ABI attributes shall be world readable. Writeable standard ABI attributes shall be writeable only for privileged users.

======= =========================================== [0-*] denotes any positive number starting from 0 [1-*] denotes any positive number starting from 1 RO read only value WO write only value RW read/write value ======= ===========================================

Read/write values may be read-only for some chips, depending on the hardware implementation.

All entries (except name) are optional, and should only be created in a given driver if the chip has the feature.

See Documentation/ABI/testing/sysfs-class-hwmon for a complete description of the attributes.

Global attributes

name The chip name.

label A descriptive label that allows to uniquely identify a device within the system.

update_interval The interval at which the chip will update readings.

Voltages

in[0-*]_min Voltage min value.

in[0-*]_lcrit Voltage critical min value.

in[0-*]_max Voltage max value.

in[0-*]_crit Voltage critical max value.

in[0-*]_input Voltage input value.

in[0-*]_average Average voltage

in[0-*]_lowest Historical minimum voltage

in[0-*]_highest Historical maximum voltage

in[0-*]_reset_history Reset inX_lowest and inX_highest

in_reset_history Reset inX_lowest and inX_highest for all sensors

in[0-*]_label Suggested voltage channel label.

in[0-*]_enable Enable or disable the sensors.

cpu[0-*]_vid CPU core reference voltage.

vrm Voltage Regulator Module version number.

in[0-*]_rated_min Minimum rated voltage.

in[0-*]_rated_max Maximum rated voltage.

Also see the Alarms section for status flags associated with voltages.

Fans

fan[1-*]_min Fan minimum value

fan[1-*]_max Fan maximum value

fan[1-*]_input Fan input value.

fan[1-*]_div Fan divisor.

fan[1-*]_pulses Number of tachometer pulses per fan revolution.

fan[1-*]_target Desired fan speed

fan[1-*]_label Suggested fan channel label.

fan[1-*]_enable Enable or disable the sensors.

Also see the Alarms section for status flags associated with fans.

PWM

pwm[1-*] Pulse width modulation fan control.

pwm[1-*]_enable Fan speed control method.

pwm[1-*]_mode direct current or pulse-width modulation.

pwm[1-*]_freq Base PWM frequency in Hz.

pwm[1-*]_auto_channels_temp Select which temperature channels affect this PWM output in auto mode.

pwm[1-*]_auto_point[1-*]_pwm / pwm[1-*]_auto_point[1-*]_temp / pwm[1-*]_auto_point[1-*]_temp_hyst Define the PWM vs temperature curve.

temp[1-*]_auto_point[1-*]_pwm / temp[1-*]_auto_point[1-*]_temp / temp[1-*]_auto_point[1-*]_temp_hyst Define the PWM vs temperature curve.

There is a third case where trip points are associated to both PWM output channels and temperature channels: the PWM values are associated to PWM output channels while the temperature values are associated to temperature channels. In that case, the result is determined by the mapping between temperature inputs and PWM outputs. When several temperature inputs are mapped to a given PWM output, this leads to several candidate PWM values. The actual result is up to the chip, but in general the highest candidate value (fastest fan speed) wins.

Temperatures

temp[1-*]_type Sensor type selection.

temp[1-*]_max Temperature max value.

temp[1-*]_min Temperature min value.

temp[1-*]_max_hyst Temperature hysteresis value for max limit.

temp[1-*]_min_hyst Temperature hysteresis value for min limit.

temp[1-*]_input Temperature input value.

temp[1-*]_crit Temperature critical max value, typically greater than corresponding temp_max values.

temp[1-*]_crit_hyst Temperature hysteresis value for critical limit.

temp[1-*]_emergency Temperature emergency max value, for chips supporting more than two upper temperature limits.

temp[1-*]_emergency_hyst Temperature hysteresis value for emergency limit.

temp[1-*]_lcrit Temperature critical min value, typically lower than corresponding temp_min values.

temp[1-*]_lcrit_hyst Temperature hysteresis value for critical min limit.

temp[1-*]_offset Temperature offset which is added to the temperature reading by the chip.

temp[1-*]_label Suggested temperature channel label.

temp[1-*]_lowest Historical minimum temperature

temp[1-*]_highest Historical maximum temperature

temp[1-*]_reset_history Reset temp_lowest and temp_highest

temp_reset_history Reset temp_lowest and temp_highest for all sensors

temp[1-*]_enable Enable or disable the sensors.

temp[1-*]_rated_min Minimum rated temperature.

temp[1-*]_rated_max Maximum rated temperature.

Some chips measure temperature using external thermistors and an ADC, and report the temperature measurement as a voltage. Converting this voltage back to a temperature (or the other way around for limits) requires mathematical functions not available in the kernel, so the conversion must occur in user space. For these chips, all temp* files described above should contain values expressed in millivolt instead of millidegree Celsius. In other words, such temperature channels are handled as voltage channels by the driver.

Also see the Alarms section for status flags associated with temperatures.

Currents

curr[1-*]_max Current max value.

curr[1-*]_min Current min value.

curr[1-*]_lcrit Current critical low value

curr[1-*]_crit Current critical high value.

curr[1-*]_input Current input value.

curr[1-*]_average Average current use.

curr[1-*]_lowest Historical minimum current.

curr[1-*]_highest Historical maximum current.

curr[1-*]_reset_history Reset currX_lowest and currX_highest

WO

curr_reset_history Reset currX_lowest and currX_highest for all sensors.

curr[1-*]_enable Enable or disable the sensors.

curr[1-*]_rated_min Minimum rated current.

curr[1-*]_rated_max Maximum rated current.

Also see the Alarms section for status flags associated with currents.

Power

power[1-*]_average Average power use.

power[1-*]_average_interval Power use averaging interval.

power[1-*]_average_interval_max Maximum power use averaging interval.

power[1-*]_average_interval_min Minimum power use averaging interval.

power[1-*]_average_highest Historical average maximum power use

power[1-*]_average_lowest Historical average minimum power use

power[1-*]_average_max A poll notification is sent to power[1-*]_average when power use rises above this value.

power[1-*]_average_min A poll notification is sent to power[1-*]_average when power use sinks below this value.

power[1-*]_input Instantaneous power use.

power[1-*]_input_highest Historical maximum power use

power[1-*]_input_lowest Historical minimum power use.

power[1-*]_reset_history Reset input_highest, input_lowest, average_highest and average_lowest.

power[1-*]_accuracy Accuracy of the power meter.

power[1-*]_cap If power use rises above this limit, the system should take action to reduce power use.

power[1-*]_cap_hyst Margin of hysteresis built around capping and notification.

power[1-*]_cap_max Maximum cap that can be set.

power[1-*]_cap_min Minimum cap that can be set.

power[1-*]_max Maximum power.

power[1-*]_crit Critical maximum power.

			If power rises to or above this limit, the
			system is expected take drastic action to reduce
			power consumption, such as a system shutdown or
			a forced powerdown of some devices.

			Unit: microWatt

			RW

power[1-*]_enable Enable or disable the sensors.

			When disabled the sensor read will return
			-ENODATA.

			- 1: Enable
			- 0: Disable

			RW

power[1-*]_rated_min Minimum rated power.

			Unit: microWatt

			RO

power[1-*]_rated_max Maximum rated power.

			Unit: microWatt

			RO

Also see the Alarms section for status flags associated with power readings.

Energy

energy[1-*]_input Cumulative energy use

			Unit: microJoule

			RO

energy[1-*]_enable Enable or disable the sensors.

			When disabled the sensor read will return
			-ENODATA.

			- 1: Enable
			- 0: Disable

			RW

Humidity

humidity[1-*]_input Humidity.

humidity[1-*]_enable Enable or disable the sensors.

humidity[1-*]_rated_min Minimum rated humidity.

humidity[1-*]_rated_max Maximum rated humidity.

Alarms

Each channel or limit may have an associated alarm file, containing a boolean value. 1 means than an alarm condition exists, 0 means no alarm.

Usually a given chip will either use channel-related alarms, or limit-related alarms, not both. The driver should just reflect the hardware implementation.

Each input channel may have an associated fault file. This can be used to notify open diodes, unconnected fans etc. where the hardware supports it. When this boolean has value 1, the measurement for that channel should not be trusted.

fan[1-*]_fault / temp[1-*]_fault Input fault condition.

Some chips also offer the possibility to get beeped when an alarm occurs:

beep_enable Master beep enable.

in[0-*]_beep, curr[1-*]_beep, fan[1-*]_beep, temp[1-*]_beep, Channel beep.

In theory, a chip could provide per-limit beep masking, but no such chip was seen so far.

Old drivers provided a different, non-standard interface to alarms and beeps. These interface files are deprecated, but will be kept around for compatibility reasons:

alarms Alarm bitmask.

beep_mask Bitmask for beep.

Intrusion detection

intrusion[0-*]_alarm Chassis intrusion detection.

intrusion[0-*]_beep Chassis intrusion beep.

Average sample configuration

Devices allowing for reading {in,power,curr,temp}_average values may export attributes for controlling number of samples used to compute average.

sysfs attribute writes interpretation

hwmon sysfs attributes always contain numbers, so the first thing to do is to convert the input to a number, there are 2 ways todo this depending whether the number can be negative or not::

unsigned long u = simple_strtoul(buf, NULL, 10);
long s = simple_strtol(buf, NULL, 10);

With buf being the buffer with the user input being passed by the kernel. Notice that we do not use the second argument of strto[u]l, and thus cannot tell when 0 is returned, if this was really 0 or is caused by invalid input. This is done deliberately as checking this everywhere would add a lot of code to the kernel.

Notice that it is important to always store the converted value in an unsigned long or long, so that no wrap around can happen before any further checking.

After the input string is converted to an (unsigned) long, the value should be checked if its acceptable. Be careful with further conversions on the value before checking it for validity, as these conversions could still cause a wrap around before the check. For example do not multiply the result, and only add/subtract if it has been divided before the add/subtract.

What to do if a value is found to be invalid, depends on the type of the sysfs attribute that is being set. If it is a continuous setting like a tempX_max or inX_max attribute, then the value should be clamped to its limits using clamp_val(value, min_limit, max_limit). If it is not continuous like for example a tempX_type, then when an invalid value is written, -EINVAL should be returned.

Example1, temp1_max, register is a signed 8 bit value (-128 - 127 degrees)::

long v = simple_strtol(buf, NULL, 10) / 1000;
v = clamp_val(v, -128, 127);
/* write v to register */

Example2, fan divider setting, valid values 2, 4 and 8::

unsigned long v = simple_strtoul(buf, NULL, 10);

switch (v) {
case 2: v = 1; break;
case 4: v = 2; break;
case 8: v = 3; break;
default:
	return -EINVAL;
}
/* write v to register */