[linux-block.git] / Documentation / hwmon / dell-smm-hwmon.rst

.. SPDX-License-Identifier: GPL-2.0-or-later

.. include:: <isonum.txt>

Kernel driver dell-smm-hwmon
============================

:Copyright: |copy| 2002-2005 Massimo Dal Zotto <dz@debian.org>
:Copyright: |copy| 2019 Giovanni Mascellani <gio@debian.org>

Description
-----------

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).

 __ https://github.com/vitorafsr/i8kutils

``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
                                        Celsius.
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
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
: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.

 __ https://github.com/clopez/dellfan/

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
--------------------------

.. 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
             ``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
-------------

.. 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 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:

=============== ==================================
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

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
                                          tolerance

``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.

Limitations
-----------

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.
Commit	Line	Data
4a1288f1 GM	1	.. SPDX-License-Identifier: GPL-2.0-or-later
	2
	3	.. include:: <isonum.txt>
	4
	5	Kernel driver dell-smm-hwmon
	6	============================
	7
	8	:Copyright: \|copy\| 2002-2005 Massimo Dal Zotto <dz@debian.org>
	9	:Copyright: \|copy\| 2019 Giovanni Mascellani <gio@debian.org>
	10
	11	Description
	12	-----------
	13
	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).
	20
	21	__ https://github.com/vitorafsr/i8kutils
	22
	23	``sysfs`` interface
	24	-------------------
	25
	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:
	31
	32	=============================== ======= =======================================
	33	Name Perm Description
	34	=============================== ======= =======================================
	35	fan[1-3]_input RO Fan speed in RPM.
	36	fan[1-3]_label RO Fan label.
b1986c8e AW	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
4a1288f1 GM	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
	45	Celsius.
	46	temp[1-10]_label RO Temperature sensor label.
	47	=============================== ======= =======================================
	48
41cd4373 AW	49	Due to the nature of the SMM interface, each pwmX attribute controls
	50	fan number X.
	51
4a1288f1 GM	52	Disabling automatic BIOS fan control
	53	------------------------------------
	54
	55	On some laptops the BIOS automatically sets fan speed every few
	56	seconds. Therefore the fan speed set by mean of this driver is quickly
	57	overwritten.
	58
	59	There is experimental support for disabling automatic BIOS fan
	60	control, at least on laptops where the corresponding SMM command is
	61	known, by writing the value ``1`` in the attribute ``pwm1_enable``
	62	(writing ``2`` enables automatic BIOS control again). Even if you have
	63	more than one fan, all of them are set to either enabled or disabled
	64	automatic fan control at the same time and, notwithstanding the name,
	65	``pwm1_enable`` sets automatic control for all fans.
	66
	67	If ``pwm1_enable`` is not available, then it means that SMM codes for
	68	enabling and disabling automatic BIOS fan control are not whitelisted
	69	for your hardware. It is possible that codes that work for other
	70	laptops actually work for yours as well, or that you have to discover
	71	new codes.
	72
	73	Check the list ``i8k_whitelist_fan_control`` in file
	74	``drivers/hwmon/dell-smm-hwmon.c`` in the kernel tree: as a first
	75	attempt you can try to add your machine and use an already-known code
	76	pair. If, after recompiling the kernel, you see that ``pwm1_enable``
	77	is present and works (i.e., you can manually control the fan speed),
	78	then please submit your finding as a kernel patch, so that other users
	79	can benefit from it. Please see
	80	:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
	81	for information on submitting patches.
	82
	83	If no known code works on your machine, you need to resort to do some
	84	probing, because unfortunately Dell does not publish datasheets for
	85	its SMM. You can experiment with the code in `this repository`__ to
	86	probe the BIOS on your machine and discover the appropriate codes.
	87
	88	__ https://github.com/clopez/dellfan/
	89
	90	Again, when you find new codes, we'd be happy to have your patches!
	91
e0d3f7cb AW	92	``thermal`` interface
	93	---------------------------
	94
	95	The driver also exports the fans as thermal cooling devices with
	96	``type`` set to ``dell-smm-fan[1-3]``. This allows for easy fan control
	97	using one of the thermal governors.
	98
4a1288f1 GM	99	Module parameters
	100	-----------------
	101
	102	* force:bool
	103	Force loading without checking for supported
	104	models. (default: 0)
	105
	106	* ignore_dmi:bool
	107	Continue probing hardware even if DMI data does not
	108	match. (default: 0)
	109
	110	* restricted:bool
	111	Allow fan control only to processes with the
	112	``CAP_SYS_ADMIN`` capability set or processes run
	113	as root when using the legacy ``/proc/i8k``
	114	interface. In this case normal users will be able
	115	to read temperature and fan status but not to
	116	control the fan. If your notebook is shared with
	117	other users and you don't trust them you may want
	118	to use this option. (default: 1, only available
	119	with ``CONFIG_I8K``)
	120
	121	* power_status:bool
	122	Report AC status in ``/proc/i8k``. (default: 0,
	123	only available with ``CONFIG_I8K``)
	124
	125	* fan_mult:uint
	126	Factor to multiply fan speed with. (default:
	127	autodetect)
	128
	129	* fan_max:uint
	130	Maximum configurable fan speed. (default:
	131	autodetect)
	132
	133	Legacy ``/proc`` interface
	134	--------------------------
	135
	136	.. warning:: This interface is obsolete and deprecated and should not
	137	used in new applications. This interface is only
	138	available when kernel is compiled with option
	139	``CONFIG_I8K``.
	140
	141	The information provided by the kernel driver can be accessed by
	142	simply reading the ``/proc/i8k`` file. For example::
	143
	144	$ cat /proc/i8k
	145	1.0 A17 2J59L02 52 2 1 8040 6420 1 2
	146
	147	The fields read from ``/proc/i8k`` are::
	148
	149	1.0 A17 2J59L02 52 2 1 8040 6420 1 2
	150	\| \| \| \| \| \| \| \| \| \|
	151	\| \| \| \| \| \| \| \| \| +------- 10. buttons status
	152	\| \| \| \| \| \| \| \| +--------- 9. AC status
	153	\| \| \| \| \| \| \| +-------------- 8. fan0 RPM
	154	\| \| \| \| \| \| +------------------- 7. fan1 RPM
	155	\| \| \| \| \| +--------------------- 6. fan0 status
	156	\| \| \| \| +----------------------- 5. fan1 status
	157	\| \| \| +-------------------------- 4. temp0 reading (Celsius)
	158	\| \| +---------------------------------- 3. Dell service tag (later known as 'serial number')
	159	\| +-------------------------------------- 2. BIOS version
	160	+------------------------------------------ 1. /proc/i8k format version
	161
	162	A negative value, for example -22, indicates that the BIOS doesn't
163	return the corresponding information. This is normal on some
164	models/BIOSes.
165
166	For performance reasons the ``/proc/i8k`` doesn't report by default
167	the AC status since this SMM call takes a long time to execute and is
168	not really needed. If you want to see the ac status in ``/proc/i8k``
169	you must explictitly enable this option by passing the
170	``power_status=1`` parameter to insmod. If AC status is not
171	available -1 is printed instead.
172
173	The driver provides also an ioctl interface which can be used to
174	obtain the same information and to control the fan status. The ioctl
175	interface can be accessed from C programs or from shell using the
176	i8kctl utility. See the source file of ``i8kutils`` for more
177	information on how to use the ioctl interface.
ec3db1ec AW	178
	179	SMM Interface
	180	-------------
	181
	182	.. warning:: The SMM interface was reverse-engineered by trial-and-error
	183	since Dell did not provide any Documentation,
	184	please keep that in mind.
	185
	186	The driver uses the SMM interface to send commands to the system BIOS.
	187	This interface is normally used by Dell's 32-bit diagnostic program or
	188	on newer notebook models by the buildin BIOS diagnostics.
	189	The SMM is triggered by writing to the special ioports ``0xb2`` and ``0x84``,
	190	and may cause short hangs when the BIOS code is taking too long to
	191	execute.
	192
	193	The SMM handler inside the system BIOS looks at the contents of the
	194	``eax``, ``ebx``, ``ecx``, ``edx``, ``esi`` and ``edi`` registers.
	195	Each register has a special purpose:
	196
	197	=============== ==================================
	198	Register Purpose
	199	=============== ==================================
	200	eax Holds the command code before SMM,
	201	holds the first result after SMM.
	202	ebx Holds the arguments.
	203	ecx Unknown, set to 0.
	204	edx Holds the second result after SMM.
	205	esi Unknown, set to 0.
	206	edi Unknown, set to 0.
	207	=============== ==================================
	208
	209	The SMM handler can signal a failure by either:
	210
	211	- setting the lower sixteen bits of ``eax`` to ``0xffff``
	212	- not modifying ``eax`` at all
	213	- setting the carry flag
	214
	215	SMM command codes
	216	-----------------
	217
	218	=============== ======================= ================================================
	219	Command Code Command Name Description
	220	=============== ======================= ================================================
	221	``0x0025`` Get Fn key status Returns the Fn key pressed after SMM:
	222
	223	- 9th bit in ``eax`` indicates Volume up
	224	- 10th bit in ``eax`` indicates Volume down
	225	- both bits indicate Volume mute
	226
	227	``0xa069`` Get power status Returns current power status after SMM:
	228
	229	- 1st bit in ``eax`` indicates Battery connected
	230	- 3th bit in ``eax`` indicates AC connected
	231
	232	``0x00a3`` Get fan state Returns current fan state after SMM:
	233
	234	- 1st byte in ``eax`` holds the current
	235	fan state (0 - 2 or 3)
	236
	237	``0x01a3`` Set fan state Sets the fan speed:
	238
	239	- 1st byte in ``ebx`` holds the fan number
	240	- 2nd byte in ``ebx`` holds the desired
	241	fan state (0 - 2 or 3)
242
243	``0x02a3`` Get fan speed Returns the current fan speed in RPM:
244
245	- 1st byte in ``ebx`` holds the fan number
246	- 1st word in ``eax`` holds the current
247	fan speed in RPM (after SMM)
248
249	``0x03a3`` Get fan type Returns the fan type:
250
251	- 1st byte in ``ebx`` holds the fan number
252	- 1st byte in ``eax`` holds the
253	fan type (after SMM):
254
255	- 5th bit indicates docking fan
256	- 1 indicates Processor fan
257	- 2 indicates Motherboard fan
258	- 3 indicates Video fan
259	- 4 indicates Power supply fan
260	- 5 indicates Chipset fan
261	- 6 indicates other fan type
262
263	``0x04a3`` Get nominal fan speed Returns the nominal RPM in each fan state:
264
265	- 1st byte in ``ebx`` holds the fan number
266	- 2nd byte in ``ebx`` holds the fan state
267	in question (0 - 2 or 3)
268	- 1st word in ``eax`` holds the nominal
269	fan speed in RPM (after SMM)
270
271	``0x05a3`` Get fan speed tolerance Returns the speed tolerance for each fan state:
272
273	- 1st byte in ``ebx`` holds the fan number
274	- 2nd byte in ``ebx`` holds the fan state
275	in question (0 - 2 or 3)
276	- 1st byte in ``eax`` returns the speed
277	tolerance
278
279	``0x10a3`` Get sensor temperature Returns the measured temperature:
280
281	- 1st byte in ``ebx`` holds the sensor number
282	- 1st byte in ``eax`` holds the measured
283	temperature (after SMM)
284
285	``0x11a3`` Get sensor type Returns the sensor type:
286
287	- 1st byte in ``ebx`` holds the sensor number
288	- 1st byte in ``eax`` holds the
289	temperature type (after SMM):
290
291	- 1 indicates CPU sensor
292	- 2 indicates GPU sensor
293	- 3 indicates SODIMM sensor
294	- 4 indicates other sensor type
295	- 5 indicates Ambient sensor
296	- 6 indicates other sensor type
297
298	``0xfea3`` Get SMM signature Returns Dell signature if interface
299	is supported (after SMM):
300
301	- ``eax`` holds 1145651527
302	(0x44494147 or "DIAG")
303	- ``edx`` holds 1145392204
304	(0x44454c4c or "DELL")
305
306	``0xffa3`` Get SMM signature Same as ``0xfea3``, check both.
307	=============== ======================= ================================================
308
309	There are additional commands for enabling (``0x31a3`` or ``0x35a3``) and
310	disabling (``0x30a3`` or ``0x34a3``) automatic fan speed control.
311	The commands are however causing severe sideeffects on many machines, so
312	they are not used by default.
313
314	On several machines (Inspiron 3505, Precision 490, Vostro 1720, ...), the
315	fans supports a 4th "magic" state, which signals the BIOS that automatic
316	fan control should be enabled for a specific fan.
317	However there are also some machines who do support a 4th regular fan state too,
318	but in case of the "magic" state, the nominal RPM reported for this state is a
319	placeholder value, which however is not always detectable.
320
321	Firmware Bugs
322	-------------
323
324	The SMM calls can behave erratic on some machines:
325
326	======================================================= =================
327	Firmware Bug Affected Machines
328	======================================================= =================
329	Reading of fan states return spurious errors. Precision 490
330
331	Reading of fan types causes erratic fan behaviour. Studio XPS 8000
332
333	Studio XPS 8100
334
335	Inspiron 580
336
c8e5e37a AW	337	Inspiron 3505
c8e5e37a AW	338
ec3db1ec AW	339	Fan-related SMM calls take too long (about 500ms). Inspiron 7720
	340
	341	Vostro 3360
	342
	343	XPS 13 9333
	344
	345	XPS 15 L502X
	346	======================================================= =================
	347
	348	In case you experience similar issues on your Dell machine, please
	349	submit a bugreport on bugzilla to we can apply workarounds.
	350
	351	Limitations
	352	-----------
	353
	354	The SMM calls can take too long to execute on some machines, causing
	355	short hangs and/or audio glitches.
	356	Also the fan state needs to be restored after suspend, as well as
	357	the automatic mode settings.
	358	When reading a temperature sensor, values above 127 degrees indicate
	359	a BIOS read error or a deactivated sensor.