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.



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.

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 Submitting patches: the essential guide to getting your code into the kernel 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!

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.