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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
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.
[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.
* Name *
name The chip name.
This should be a short, lowercase string, not containing
spaces nor dashes, representing the chip name. This is
the only mandatory attribute.
I2C devices get this attribute created automatically.
* Voltages *
in[0-*]_min Voltage min value.
Unit: millivolt
in[0-*]_max Voltage max value.
Unit: millivolt
in[0-*]_input Voltage input value.
Unit: millivolt
Voltage measured on the chip pin.
Actual voltage depends on the scaling resistors on the
motherboard, as recommended in the chip datasheet.
This varies by chip and by motherboard.
Because of this variation, values are generally NOT scaled
by the chip driver, and must be done by the application.
However, some drivers (notably lm87 and via686a)
do scale, because of internal resistors built into a chip.
These drivers will output the actual voltage. Rule of
thumb: drivers should report the voltage values at the
"pins" of the chip.
in[0-*]_label Suggested voltage channel label.
Text string
Should only be created if the driver has hints about what
this voltage channel is being used for, and user-space
doesn't. In all other cases, the label is provided by
cpu[0-*]_vid CPU core reference voltage.
Unit: millivolt
Not always correct.
vrm Voltage Regulator Module version number.
RW (but changing it should no more be necessary)
Originally the VRM standard version multiplied by 10, but now
an arbitrary number, as not all standards have a version
Affects the way the driver calculates the CPU core reference
voltage from the vid pins.
Also see the Alarms section for status flags associated with voltages.
* Fans *
fan[1-*]_min Fan minimum value
Unit: revolution/min (RPM)
fan[1-*]_max Fan maximum value
Unit: revolution/min (RPM)
Only rarely supported by the hardware.
fan[1-*]_input Fan input value.
Unit: revolution/min (RPM)
fan[1-*]_div Fan divisor.
Integer value in powers of two (1, 2, 4, 8, 16, 32, 64, 128).
Some chips only support values 1, 2, 4 and 8.
Note that this is actually an internal clock divisor, which
affects the measurable speed range, not the read value.
Desired fan speed
Unit: revolution/min (RPM)
Only makes sense if the chip supports closed-loop fan speed
control based on the measured fan speed.
fan[1-*]_label Suggested fan channel label.
Text string
Should only be created if the driver has hints about what
this fan channel is being used for, and user-space doesn't.
In all other cases, the label is provided by user-space.
Also see the Alarms section for status flags associated with fans.
* PWM *
pwm[1-*] Pulse width modulation fan control.
Integer value in the range 0 to 255
255 is max or 100%.
Fan speed control method:
0: no fan speed control (i.e. fan at full speed)
1: manual fan speed control enabled (using pwm[1-*])
2+: automatic fan speed control enabled
Check individual chip documentation files for automatic mode
pwm[1-*]_mode 0: DC mode (direct current)
1: PWM mode (pulse-width modulation)
pwm[1-*]_freq Base PWM frequency in Hz.
Only possibly available when pwmN_mode is PWM, but not always
present even then.
Select which temperature channels affect this PWM output in
auto mode. Bitfield, 1 is temp1, 2 is temp2, 4 is temp3 etc...
Which values are possible depend on the chip used.
Define the PWM vs temperature curve. Number of trip points is
chip-dependent. Use this for chips which associate trip points
to PWM output channels.
Define the PWM vs temperature curve. Number of trip points is
chip-dependent. Use this for chips which associate trip points
to temperature channels.
* Temperatures *
temp[1-*]_type Sensor type selection.
Integers 1 to 6
1: PII/Celeron Diode
2: 3904 transistor
3: thermal diode
4: thermistor
6: Intel PECI
Not all types are supported by all chips
temp[1-*]_max Temperature max value.
Unit: millidegree Celsius (or millivolt, see below)
temp[1-*]_min Temperature min value.
Unit: millidegree Celsius
Temperature hysteresis value for max limit.
Unit: millidegree Celsius
Must be reported as an absolute temperature, NOT a delta
from the max value.
temp[1-*]_input Temperature input value.
Unit: millidegree Celsius
temp[1-*]_crit Temperature critical value, typically greater than
corresponding temp_max values.
Unit: millidegree Celsius
Temperature hysteresis value for critical limit.
Unit: millidegree Celsius
Must be reported as an absolute temperature, NOT a delta
from the critical value.
Temperature offset which is added to the temperature reading
by the chip.
Unit: millidegree Celsius
Read/Write value.
temp[1-*]_label Suggested temperature channel label.
Text string
Should only be created if the driver has hints about what
this temperature channel is being used for, and user-space
doesn't. In all other cases, the label is provided by
Historical minimum temperature
Unit: millidegree Celsius
Historical maximum temperature
Unit: millidegree Celsius
Reset temp_lowest and temp_highest
Reset temp_lowest and temp_highest for all sensors
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 *
Note that no known chip provides current measurements as of writing,
so this part is theoretical, so to say.
curr[1-*]_max Current max value
Unit: milliampere
curr[1-*]_min Current min value.
Unit: milliampere
curr[1-*]_input Current input value
Unit: milliampere
* Power *
power[1-*]_average Average power use
Unit: microWatt
power[1-*]_average_interval Power use averaging interval. A poll
notification is sent to this file if the
hardware changes the averaging interval.
Unit: milliseconds
power[1-*]_average_interval_max Maximum power use averaging interval
Unit: milliseconds
power[1-*]_average_interval_min Minimum power use averaging interval
Unit: milliseconds
power[1-*]_average_highest Historical average maximum power use
Unit: microWatt
power[1-*]_average_lowest Historical average minimum power use
Unit: microWatt
power[1-*]_average_max A poll notification is sent to
power[1-*]_average when power use
rises above this value.
Unit: microWatt
power[1-*]_average_min A poll notification is sent to
power[1-*]_average when power use
sinks below this value.
Unit: microWatt
power[1-*]_input Instantaneous power use
Unit: microWatt
power[1-*]_input_highest Historical maximum power use
Unit: microWatt
power[1-*]_input_lowest Historical minimum power use
Unit: microWatt
power[1-*]_reset_history Reset input_highest, input_lowest,
average_highest and average_lowest.
power[1-*]_accuracy Accuracy of the power meter.
Unit: Percent
power[1-*]_alarm 1 if the system is drawing more power than the
cap allows; 0 otherwise. A poll notification is
sent to this file when the power use exceeds the
cap. This file only appears if the cap is known
to be enforced by hardware.
power[1-*]_cap If power use rises above this limit, the
system should take action to reduce power use.
A poll notification is sent to this file if the
cap is changed by the hardware. The *_cap
files only appear if the cap is known to be
enforced by hardware.
Unit: microWatt
power[1-*]_cap_hyst Margin of hysteresis built around capping and
Unit: microWatt
power[1-*]_cap_max Maximum cap that can be set.
Unit: microWatt
power[1-*]_cap_min Minimum cap that can be set.
Unit: microWatt
* Energy *
energy[1-*]_input Cumulative energy use
Unit: microJoule
* 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
Channel alarm
0: no alarm
1: alarm
Limit alarm
0: no alarm
1: alarm
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.
Input fault condition
0: no fault occured
1: fault condition
Some chips also offer the possibility to get beeped when an alarm occurs:
beep_enable Master beep enable
0: no beeps
1: beeps
Channel beep
0: disable
1: enable
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.
Integer representation of one to four bytes.
A '1' bit means an alarm.
Chips should be programmed for 'comparator' mode so that
the alarm will 'come back' after you read the register
if it is still valid.
Generally a direct representation of a chip's internal
alarm registers; there is no standard for the position
of individual bits. For this reason, the use of this
interface file for new drivers is discouraged. Use
individual *_alarm and *_fault files instead.
Bits are defined in kernel/include/sensors.h.
beep_mask Bitmask for beep.
Same format as 'alarms' with the same bit locations,
use discouraged for the same reason. Use individual
*_beep files instead.
* Intrusion detection *
Chassis intrusion detection
0: OK
1: intrusion detected
Contrary to regular alarm flags which clear themselves
automatically when read, this one sticks until cleared by
the user. This is done by writing 0 to the file. Writing
other values is unsupported.
Chassis intrusion beep
0: disable
1: enable
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
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 SENSORS_LIMIT(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 = SENSORS_LIMIT(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;
return -EINVAL;
/* write v to register */