blob: 977263cb57a837b6352b7eb93ea277bc819ed453 [file] [log] [blame]
.. SPDX-License-Identifier: GPL-2.0-or-later
.. include:: <isonum.txt>
Kernel driver dell-smm-hwmon
:Copyright: |copy| 2002-2005 Massimo Dal Zotto <>
:Copyright: |copy| 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:
=============================== ======= =======================================
Name Perm Description
=============================== ======= =======================================
fan[1-3]_input RO Fan speed in RPM.
fan[1-3]_label RO Fan label.
fan[1-3]_min RO Minimal Fan speed in RPM
fan[1-3]_max RO Maximal Fan speed in RPM
fan[1-3]_target RO Expected Fan speed in RPM
pwm[1-3] RW Control the fan PWM duty-cycle.
pwm1_enable WO Enable or disable automatic BIOS fan
control (not supported on all laptops,
see below for details).
temp[1-10]_input RO Temperature reading in milli-degrees
temp[1-10]_label RO 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
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
:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
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:
* fan_max:uint
Maximum configurable fan speed. (default:
Legacy ``/proc`` interface
.. warning:: This interface is obsolete and deprecated and should not
used in new applications. This interface is only
available when kernel is compiled with option
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
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
.. warning:: 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 may cause short hangs when the BIOS code is taking too long to
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:
=============== ==================================
Register Purpose
=============== ==================================
eax Holds the command code before SMM,
holds the first result after SMM.
ebx Holds the arguments.
ecx Unknown, set to 0.
edx Holds the second result after SMM.
esi Unknown, set to 0.
edi 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 (legacy SMM interface only)
Legacy SMM Interface
When using the legacy SMM interface, a SMM is triggered by writing the least significant byte
of the command code to the special ioports ``0xb2`` and ``0x84``. This interface is not
described inside the ACPI tables and can thus only be detected by issuing a test SMM call.
WMI SMM Interface
On modern Dell machines, the SMM calls are done over ACPI WMI:
#pragma namespace("\\\\.\\root\\dcim\\sysman\\diagnostics")
[WMI, Provider("Provider_DiagnosticsServices"), Dynamic, Locale("MS\\0x409"),
Description("RunDellDiag"), guid("{F1DDEE52-063C-4784-A11E-8A06684B9B01}")]
class LegacyDiags {
[key, read] string InstanceName;
[read] boolean Active;
[WmiMethodId(1), Implemented, read, write, Description("Legacy Method ")]
void Execute([in, out] uint32 EaxLen, [in, out, WmiSizeIs("EaxLen") : ToInstance] uint8 EaxVal[],
[in, out] uint32 EbxLen, [in, out, WmiSizeIs("EbxLen") : ToInstance] uint8 EbxVal[],
[in, out] uint32 EcxLen, [in, out, WmiSizeIs("EcxLen") : ToInstance] uint8 EcxVal[],
[in, out] uint32 EdxLen, [in, out, WmiSizeIs("EdxLen") : ToInstance] uint8 EdxVal[]);
Some machines support only the WMI SMM interface, while some machines support both interfaces.
The driver automatically detects which interfaces are present and will use the WMI SMM interface
if the legacy SMM interface is not present. The WMI SMM interface is usually slower than the
legacy SMM interface since ACPI methods need to be called in order to trigger a SMM.
SMM command codes
=============== ======================= ================================================
Command Code Command Name Description
=============== ======================= ================================================
``0x0025`` 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
``0xa069`` Get power status Returns current power status after SMM:
- 1st bit in ``eax`` indicates Battery connected
- 3th bit in ``eax`` indicates AC connected
``0x00a3`` Get fan state Returns current fan state after SMM:
- 1st byte in ``eax`` holds the current
fan state (0 - 2 or 3)
``0x01a3`` 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)
``0x02a3`` 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)
``0x03a3`` 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
``0x04a3`` 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)
``0x05a3`` 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
``0x10a3`` 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)
``0x11a3`` 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
``0xfea3`` Get SMM signature Returns Dell signature if interface
is supported (after SMM):
- ``eax`` holds 1145651527
(0x44494147 or "DIAG")
- ``edx`` holds 1145392204
(0x44454c4c or "DELL")
``0xffa3`` 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.