dell-smm-hwmon: Add documentation

.. 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.

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.

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

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!

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.

   coretemp

   da9052

   da9055

   dell-smm-hwmon

   dme1737

   ds1621

   ds620