Kernel driver dell-smm-hwmon


© 2002-2005 Massimo Dal Zotto <>


© 2019 Giovanni Mascellani <>


On many Dell laptops the System Management Mode (SMM) BIOS can be queried for the status of fans and temperature sensors. Userspace utilities like sensors can be used to return the readings. The userspace suite i8kutils can also be used to read the sensors and automatically adjust fan speed (please notice that it currently uses the deprecated /proc/i8k interface).

sysfs interface

Temperature sensors and fans can be queried and set via the standard hwmon interface on sysfs, under the directory /sys/class/hwmon/hwmonX for some value of X (search for the X such that /sys/class/hwmon/hwmonX/name has content dell_smm). A number of other attributes can be read or written:






Fan speed in RPM.



Fan label.



Minimal Fan speed in RPM



Maximal Fan speed in RPM



Expected Fan speed in RPM



Control the fan PWM duty-cycle.



Enable or disable automatic BIOS fan control (not supported on all laptops, see below for details).



Temperature reading in milli-degrees Celsius.



Temperature sensor label.

Due to the nature of the SMM interface, each pwmX attribute controls fan number X.

Disabling automatic BIOS fan control

On some laptops the BIOS automatically sets fan speed every few seconds. Therefore the fan speed set by mean of this driver is quickly overwritten.

There is experimental support for disabling automatic BIOS fan control, at least on laptops where the corresponding SMM command is known, by writing the value 1 in the attribute pwm1_enable (writing 2 enables automatic BIOS control again). Even if you have more than one fan, all of them are set to either enabled or disabled automatic fan control at the same time and, notwithstanding the name, pwm1_enable sets automatic control for all fans.

If pwm1_enable is not available, then it means that SMM codes for enabling and disabling automatic BIOS fan control are not whitelisted for your hardware. It is possible that codes that work for other laptops actually work for yours as well, or that you have to discover new codes.

Check the list i8k_whitelist_fan_control in file drivers/hwmon/dell-smm-hwmon.c in the kernel tree: as a first attempt you can try to add your machine and use an already-known code pair. If, after recompiling the kernel, you see that pwm1_enable is present and works (i.e., you can manually control the fan speed), then please submit your finding as a kernel patch, so that other users can benefit from it. Please see Documentation/process/submitting-patches.rst for information on submitting patches.

If no known code works on your machine, you need to resort to do some probing, because unfortunately Dell does not publish datasheets for its SMM. You can experiment with the code in this repository to probe the BIOS on your machine and discover the appropriate codes.

Again, when you find new codes, we'd be happy to have your patches!

thermal interface

The driver also exports the fans as thermal cooling devices with type set to dell-smm-fan[1-3]. This allows for easy fan control using one of the thermal governors.

Module parameters

  • force:bool

    Force loading without checking for supported models. (default: 0)

  • ignore_dmi:bool

    Continue probing hardware even if DMI data does not match. (default: 0)

  • restricted:bool

    Allow fan control only to processes with the CAP_SYS_ADMIN capability set or processes run as root when using the legacy /proc/i8k interface. In this case normal users will be able to read temperature and fan status but not to control the fan. If your notebook is shared with other users and you don't trust them you may want to use this option. (default: 1, only available with CONFIG_I8K)

  • power_status:bool

    Report AC status in /proc/i8k. (default: 0, only available with CONFIG_I8K)

  • fan_mult:uint

    Factor to multiply fan speed with. (default: autodetect)

  • fan_max:uint

    Maximum configurable fan speed. (default: autodetect)

Legacy /proc interface


This interface is obsolete and deprecated and should not used in new applications. This interface is only available when kernel is compiled with option CONFIG_I8K.

The information provided by the kernel driver can be accessed by simply reading the /proc/i8k file. For example:

$ cat /proc/i8k
1.0 A17 2J59L02 52 2 1 8040 6420 1 2

The fields read from /proc/i8k are:

1.0 A17 2J59L02 52 2 1 8040 6420 1 2
|   |   |       |  | | |    |    | |
|   |   |       |  | | |    |    | +------- 10. buttons status
|   |   |       |  | | |    |    +--------- 9.  AC status
|   |   |       |  | | |    +-------------- 8.  fan0 RPM
|   |   |       |  | | +------------------- 7.  fan1 RPM
|   |   |       |  | +--------------------- 6.  fan0 status
|   |   |       |  +----------------------- 5.  fan1 status
|   |   |       +-------------------------- 4.  temp0 reading (Celsius)
|   |   +---------------------------------- 3.  Dell service tag (later known as 'serial number')
|   +-------------------------------------- 2.  BIOS version
+------------------------------------------ 1.  /proc/i8k format version

A negative value, for example -22, indicates that the BIOS doesn't return the corresponding information. This is normal on some models/BIOSes.

For performance reasons the /proc/i8k doesn't report by default the AC status since this SMM call takes a long time to execute and is not really needed. If you want to see the ac status in /proc/i8k you must explictitly enable this option by passing the power_status=1 parameter to insmod. If AC status is not available -1 is printed instead.

The driver provides also an ioctl interface which can be used to obtain the same information and to control the fan status. The ioctl interface can be accessed from C programs or from shell using the i8kctl utility. See the source file of i8kutils for more information on how to use the ioctl interface.

SMM Interface


The SMM interface was reverse-engineered by trial-and-error since Dell did not provide any Documentation, please keep that in mind.

The driver uses the SMM interface to send commands to the system BIOS. This interface is normally used by Dell's 32-bit diagnostic program or on newer notebook models by the buildin BIOS diagnostics. The SMM is triggered by writing to the special ioports 0xb2 and 0x84, and may cause short hangs when the BIOS code is taking too long to execute.

The SMM handler inside the system BIOS looks at the contents of the eax, ebx, ecx, edx, esi and edi registers. Each register has a special purpose:




Holds the command code before SMM, holds the first result after SMM.


Holds the arguments.


Unknown, set to 0.


Holds the second result after SMM.


Unknown, set to 0.


Unknown, set to 0.

The SMM handler can signal a failure by either:

  • setting the lower sixteen bits of eax to 0xffff

  • not modifying eax at all

  • setting the carry flag

SMM command codes

Command Code

Command Name



Get Fn key status

Returns the Fn key pressed after SMM:

  • 9th bit in eax indicates Volume up

  • 10th bit in eax indicates Volume down

  • both bits indicate Volume mute


Get power status

Returns current power status after SMM:

  • 1st bit in eax indicates Battery connected

  • 3th bit in eax indicates AC connected


Get fan state

Returns current fan state after SMM:

  • 1st byte in eax holds the current fan state (0 - 2 or 3)


Set fan state

Sets the fan speed:

  • 1st byte in ebx holds the fan number

  • 2nd byte in ebx holds the desired fan state (0 - 2 or 3)


Get fan speed

Returns the current fan speed in RPM:

  • 1st byte in ebx holds the fan number

  • 1st word in eax holds the current fan speed in RPM (after SMM)


Get fan type

Returns the fan type:

  • 1st byte in ebx holds the fan number

  • 1st byte in eax holds the fan type (after SMM):

    • 5th bit indicates docking fan

    • 1 indicates Processor fan

    • 2 indicates Motherboard fan

    • 3 indicates Video fan

    • 4 indicates Power supply fan

    • 5 indicates Chipset fan

    • 6 indicates other fan type


Get nominal fan speed

Returns the nominal RPM in each fan state:

  • 1st byte in ebx holds the fan number

  • 2nd byte in ebx holds the fan state in question (0 - 2 or 3)

  • 1st word in eax holds the nominal fan speed in RPM (after SMM)


Get fan speed tolerance

Returns the speed tolerance for each fan state:

  • 1st byte in ebx holds the fan number

  • 2nd byte in ebx holds the fan state in question (0 - 2 or 3)

  • 1st byte in eax returns the speed tolerance


Get sensor temperature

Returns the measured temperature:

  • 1st byte in ebx holds the sensor number

  • 1st byte in eax holds the measured temperature (after SMM)


Get sensor type

Returns the sensor type:

  • 1st byte in ebx holds the sensor number

  • 1st byte in eax holds the temperature type (after SMM):

    • 1 indicates CPU sensor

    • 2 indicates GPU sensor

    • 3 indicates SODIMM sensor

    • 4 indicates other sensor type

    • 5 indicates Ambient sensor

    • 6 indicates other sensor type


Get SMM signature

Returns Dell signature if interface is supported (after SMM):

  • eax holds 1145651527 (0x44494147 or "DIAG")

  • edx holds 1145392204 (0x44454c4c or "DELL")


Get SMM signature

Same as 0xfea3, check both.

There are additional commands for enabling (0x31a3 or 0x35a3) and disabling (0x30a3 or 0x34a3) automatic fan speed control. The commands are however causing severe sideeffects on many machines, so they are not used by default.

On several machines (Inspiron 3505, Precision 490, Vostro 1720, ...), the fans supports a 4th "magic" state, which signals the BIOS that automatic fan control should be enabled for a specific fan. However there are also some machines who do support a 4th regular fan state too, but in case of the "magic" state, the nominal RPM reported for this state is a placeholder value, which however is not always detectable.

Firmware Bugs

The SMM calls can behave erratic on some machines:

Firmware Bug

Affected Machines

Reading of fan states return spurious errors.

Precision 490

Reading of fan types causes erratic fan behaviour.

Studio XPS 8000

Studio XPS 8100

Inspiron 580

Inspiron 3505

Fan-related SMM calls take too long (about 500ms).

Inspiron 7720

Vostro 3360

XPS 13 9333

XPS 15 L502X

In case you experience similar issues on your Dell machine, please submit a bugreport on bugzilla to we can apply workarounds.


The SMM calls can take too long to execute on some machines, causing short hangs and/or audio glitches. Also the fan state needs to be restored after suspend, as well as the automatic mode settings. When reading a temperature sensor, values above 127 degrees indicate a BIOS read error or a deactivated sensor.