1 .. SPDX-License-Identifier: GPL-2.0-or-later
3 .. include:: <isonum.txt>
5 Kernel driver dell-smm-hwmon
6 ============================
8 :Copyright: |copy| 2002-2005 Massimo Dal Zotto <dz@debian.org>
9 :Copyright: |copy| 2019 Giovanni Mascellani <gio@debian.org>
14 On many Dell laptops the System Management Mode (SMM) BIOS can be
15 queried for the status of fans and temperature sensors. Userspace
16 utilities like ``sensors`` can be used to return the readings. The
17 userspace suite `i8kutils`__ can also be used to read the sensors and
18 automatically adjust fan speed (please notice that it currently uses
19 the deprecated ``/proc/i8k`` interface).
21 __ https://github.com/vitorafsr/i8kutils
26 Temperature sensors and fans can be queried and set via the standard
27 ``hwmon`` interface on ``sysfs``, under the directory
28 ``/sys/class/hwmon/hwmonX`` for some value of ``X`` (search for the
29 ``X`` such that ``/sys/class/hwmon/hwmonX/name`` has content
30 ``dell_smm``). A number of other attributes can be read or written:
32 =============================== ======= =======================================
34 =============================== ======= =======================================
35 fan[1-3]_input RO Fan speed in RPM.
36 fan[1-3]_label RO Fan label.
37 fan[1-3]_min RO Minimal Fan speed in RPM
38 fan[1-3]_max RO Maximal Fan speed in RPM
39 fan[1-3]_target RO Expected Fan speed in RPM
40 pwm[1-3] RW Control the fan PWM duty-cycle.
41 pwm1_enable WO Enable or disable automatic BIOS fan
42 control (not supported on all laptops,
43 see below for details).
44 temp[1-10]_input RO Temperature reading in milli-degrees
46 temp[1-10]_label RO Temperature sensor label.
47 =============================== ======= =======================================
49 Disabling automatic BIOS fan control
50 ------------------------------------
52 On some laptops the BIOS automatically sets fan speed every few
53 seconds. Therefore the fan speed set by mean of this driver is quickly
56 There is experimental support for disabling automatic BIOS fan
57 control, at least on laptops where the corresponding SMM command is
58 known, by writing the value ``1`` in the attribute ``pwm1_enable``
59 (writing ``2`` enables automatic BIOS control again). Even if you have
60 more than one fan, all of them are set to either enabled or disabled
61 automatic fan control at the same time and, notwithstanding the name,
62 ``pwm1_enable`` sets automatic control for all fans.
64 If ``pwm1_enable`` is not available, then it means that SMM codes for
65 enabling and disabling automatic BIOS fan control are not whitelisted
66 for your hardware. It is possible that codes that work for other
67 laptops actually work for yours as well, or that you have to discover
70 Check the list ``i8k_whitelist_fan_control`` in file
71 ``drivers/hwmon/dell-smm-hwmon.c`` in the kernel tree: as a first
72 attempt you can try to add your machine and use an already-known code
73 pair. If, after recompiling the kernel, you see that ``pwm1_enable``
74 is present and works (i.e., you can manually control the fan speed),
75 then please submit your finding as a kernel patch, so that other users
76 can benefit from it. Please see
77 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
78 for information on submitting patches.
80 If no known code works on your machine, you need to resort to do some
81 probing, because unfortunately Dell does not publish datasheets for
82 its SMM. You can experiment with the code in `this repository`__ to
83 probe the BIOS on your machine and discover the appropriate codes.
85 __ https://github.com/clopez/dellfan/
87 Again, when you find new codes, we'd be happy to have your patches!
93 Force loading without checking for supported
97 Continue probing hardware even if DMI data does not
101 Allow fan control only to processes with the
102 ``CAP_SYS_ADMIN`` capability set or processes run
103 as root when using the legacy ``/proc/i8k``
104 interface. In this case normal users will be able
105 to read temperature and fan status but not to
106 control the fan. If your notebook is shared with
107 other users and you don't trust them you may want
108 to use this option. (default: 1, only available
112 Report AC status in ``/proc/i8k``. (default: 0,
113 only available with ``CONFIG_I8K``)
116 Factor to multiply fan speed with. (default:
120 Maximum configurable fan speed. (default:
123 Legacy ``/proc`` interface
124 --------------------------
126 .. warning:: This interface is obsolete and deprecated and should not
127 used in new applications. This interface is only
128 available when kernel is compiled with option
131 The information provided by the kernel driver can be accessed by
132 simply reading the ``/proc/i8k`` file. For example::
135 1.0 A17 2J59L02 52 2 1 8040 6420 1 2
137 The fields read from ``/proc/i8k`` are::
139 1.0 A17 2J59L02 52 2 1 8040 6420 1 2
141 | | | | | | | | | +------- 10. buttons status
142 | | | | | | | | +--------- 9. AC status
143 | | | | | | | +-------------- 8. fan0 RPM
144 | | | | | | +------------------- 7. fan1 RPM
145 | | | | | +--------------------- 6. fan0 status
146 | | | | +----------------------- 5. fan1 status
147 | | | +-------------------------- 4. temp0 reading (Celsius)
148 | | +---------------------------------- 3. Dell service tag (later known as 'serial number')
149 | +-------------------------------------- 2. BIOS version
150 +------------------------------------------ 1. /proc/i8k format version
152 A negative value, for example -22, indicates that the BIOS doesn't
153 return the corresponding information. This is normal on some
156 For performance reasons the ``/proc/i8k`` doesn't report by default
157 the AC status since this SMM call takes a long time to execute and is
158 not really needed. If you want to see the ac status in ``/proc/i8k``
159 you must explictitly enable this option by passing the
160 ``power_status=1`` parameter to insmod. If AC status is not
161 available -1 is printed instead.
163 The driver provides also an ioctl interface which can be used to
164 obtain the same information and to control the fan status. The ioctl
165 interface can be accessed from C programs or from shell using the
166 i8kctl utility. See the source file of ``i8kutils`` for more
167 information on how to use the ioctl interface.