This is a fresh attempt to summarize what I've been looking at the past
few weeks, with a nod to Phill W's request. People with more
specialized expertise will likely want to correct this in any number of
ways. Better knowledge is very much welcome.
Fan control, especially on laptops, which was my central interest, is
part of the larger topic of thermal management, which includes active
and passive methods. Fan control is the usual active method. Passive
methods include several technologies for throttling devices, and these
methods have varying effects on performance. Thermal management has
historically focused on the CPU, but has broadened to embrace the GPU,
hard drive, LCD screen, and the entire enclosure. This has become
increasingly important as form factors have shrunk.
Thermal management for a desktop is usually an easier proposition
because the heat sources are not as crammed together, because it's
easier to add fans if needed, and because the thermal management
technologies most commonly used in a desktop are, in general, better
supported, and they are more exposed by the OEM's.
The situation for laptops is, in general, the opposite of everything
just said about desktops.
*Some further orientation and outline of the various approaches*:
BIOS (presumably similar for UEFI): This is the first thing to look at.
The BIOS likely sets up some thermal management, and may present simple
or sophisticated controls in its interface. In other cases the OEM has
decided not to present any user-configurable controls there whatsoever.
When an OS has booted, the BIOS may relinquish all or part of thermal
control to it (commonly via ACPI). But it may continue to exercise
control through SMM (System Management Mode), which temporarily suspends
processing by the OS in a way that is transparent to it, and runs some
SMM BIOS code. By definition then, it can be difficult to know what, if
anything is controlled by SMM. And SMM is platform-dependent, so there
doesn't seem to be a standards-based way for developers to write code
that works across whatever makes and models use SMM.
ACPI (Advanced Configuration and Power Interface): This is the successor
to PNP configuration and APM power management. Under ACPI, power
management is no longer the responsibility of the BIOS via APM, but of
the OS. ACPI is closely related to OSPM (Operating System-directed
configuration and Power Management), which has been described as a
system implementing ACPI. Proper functioning under ACPI requires
support by the hardware, the BIOS, and the OS. The ACPI BIOS loads some
ACPI tables into memory, the most prominent of which is the DSDT
(Differentiated System Description Table). These tables provide
hardware enumeration data and AML (ACPI Machine Language) bytecode. The
OS kernel uses an interpreter (ACPICA -- ACPI Component Architecture) to
run the bytecode and employ the data to set everything up. [A
development note: ACPI was merged into the UEFI Forum in 2013.]
ACPI and Sysfs: The state of the kernel is reflected to user space via a
sysfs, a virtual filesystem mounted at /sys. In Windows-ish terms, this
describes what device drivers are loaded and what their settings are.
But in addition to the original device nodes that describe the kernel
state most concisely, there are also symlinks to many of the original
device nodes, set up in various /sys locations, serving various
purposes. So for thermal management purposes, one might be instructed
to look at the contents of /sys/class/thermal, but a number of the
folders there are symlinks to yet other /sys directories. Some of the
kernel parameters reflected at /sys are writable (or are supposed to be,
or were at one time).
Hwmon - a sysfs extension: This extension to sysfs provides alternate
interfaces under /sys to report or control kernel parameters that may
also be represented elsewhere in /sys. But some user applications are
written such that they rely exclusively on the hwmon interfaces.
ACPI and Procfs - What is now accomplished for thermal management
purposes by the sysfs mounted at /sys, was previously accomplished by
the procfs mounted at /proc. So there is a lot of documentation
regarding, for instance, /proc/acpi/fan and
/proc/acpi/thermal_zone/*/trip_points (see
https://wiki.ubuntu.com/DebuggingACPI), but that information is now
obsolete.
Generic Thermal Management Framework - Much of thermal policy or
decision-making has been handled in the OS by the kernel, but exposed to
some degree to user control via sysfs. The idea under this framework is
reduce the role the kernel plays to that of a facilitator, and leave
policy/decision-making to user-land tools. But worth noting is that such
user tools do not fundamentally add to lower-level methods.
*SOLUTIONS* (check the standard repositories first for any additional
packages you want to download)
BIOS: Start here. This may be all you need to improve your fan
control. Sometimes an updated BIOS is required.
lm-sensors + fancontrol (http://www.lm-sensors.org): This solution
explicitly relies on the hwmon interfaces of /sys, and it only works
with PWM (pulse-width modulated) fan controllers, not with
voltage-regulated controllers (and no, I don't know why on either
count). But its README says this:
Laptops, on the other hand, rarely expose any hardware monitoring
chip. They often have some BIOS and/or ACPI magic to get the CPU
temperature value, but that's about it. For such laptops, the
lm-sensors package is of no use (sensors-detect will not find
anything), and you have to use acpi instead.
ACPI - Editing/creating thermal trip points: Regarding thermal
management, you will see documentation that commands like these should
change the thermal trip point temperature:
$ sudo sh -c "echo 75000 >
/sys/class/thermal/thermal_zone0/trip_point_1_temp"
or
$ echo 75000 | sudo tee trip_point_1_temp
But this does not seem to be supported by recent kernels and yielded
"Permission denied" errors in my tests. I have seen conflicting
information on whether trip points should be editable.
Concerning laptops, there is this statement at
https://01.org/linux-acpi/documentation/debug-how-isolate-linux-acpi-issues:
"Most notebooks also use native fan control instead of ACPI. There are,
however, a couple of notable exceptions: HP/Compaq, Acer, and
Fujitsu-Siemens often use ACPI-based fan-control."
ACPI - Overriding the DSDT table: There is documentation (e.g. Patching
DSDT in recent Linux kernels without recompiling
<http://blog.michael.kuron-germany.de/2011/03/patching-dsdt-in-recent-linux-kernels-without-recompiling/>)
about how to edit the DSDT table that the BIOS presents to the kernel,
and then direct the kernel to use the edited table. One wonders about
that as a method for creating/modifying ACPI thermal trip points. I have
not tried it. One source with more expertise says that this *might*
work, but notes that if a fan is controlled by SMM, this may overrule
something set up in ACPI.
i8kutils (i8kctl
<http://manpages.ubuntu.com/manpages/trusty/man1/i8kctl.1.html> and
i8kmon <http://manpages.ubuntu.com/manpages/trusty/man1/i8kmon.1.html>):
This relies on SMM, and as a platform-dependent solution, the authors
are aiming to support only Dell laptops. But this is probably the best
solution for those.
thinkpad-acpi (http://www.thinkwiki.org/wiki/How_to_control_fan_speed):
This is an extension of the ACPI support provided by the standard
kernel. Note that it does not support all Lenovo laptops. Lenovo
3000's, for instance, are not Thinkpads and are not supported by this
extension.
Asus: See
http://forum.notebookreview.com/asus/705656-fan-control-asus-prime-ux31-ux31a-ux32a-ux32vd.html
and https://gist.github.com/felipec/6169047 and
https://help.ubuntu.com/community/AsusZenbookPrime#Sensors_.28temps_.26_fans.29
as good starting points.
Thermald: See the links following for information about the thermal
daemon new to 14.04. It is an implementation of the Generic Thermal
Management Framework. It controls cooling via
- the Running Average Power Limit (RAPL) driver (Sandybridge upwards)
- the Intel P-state CPU frequency driver (Sandybridge upwards)
- the CPU freq driver
- the Intel PowerClamp driver
- active or passive cooling devices as presented in sysfs (but it cannot
create any new devices; if there is no FAN device, it will not control
the fan)
https://wiki.ubuntu.com/Kernel/PowerManagement/ThermalIssues
http://manpages.ubuntu.com/manpages/trusty/en/man5/thermal-conf.xml.5.html
https://01.org/linux-thermal-daemon/documentation/introduction-thermal-daemon
http://www.linux.com/news/featured-blogs/200-libby-clark/721494-linux-thermal-daemon-monitors-and-controls-temperature-in-tablets-laptops
*MY CURRENT CASE*
I wanted the fan to start at a lower temperature on a Lenovo 3000 C200
laptop.
The BIOS exposes no thermal settings. But someone who previously made a
serious attempt at this on a Lenovo 3000 N200 says that the fan is
controlled by SMM. This makes sense, since the fan trips at the same
temperature in Windows or Lubuntu. It's notable that active cooling is
done via SMM, while passive cooling is via ACPI.
Lm-sensors: It does not find a PWM controller that it can work with.
ACPI: There are only two trip points defined, both passive, and the
lowest of them at 87C. There is no FAN device in /sys/class/thermal,
and attempts to modify trip point settings resulted in "Permission
denied." I still wonder if editing the DSDT table might net me any
gains, but I decided for the time being not to invest any more time,
especially with the prospect that SMM would undo my work.
Thinkpad-acpi: It does not support the Lenovo 3000.
Thermald: I installed this with high hopes, but found out that it cannot
control a fan if an ACPI fan device does not already exist. And since
this laptop does not have a SandyBridge or newer processor to take
advantage of the RAPL or P-state drivers, thermald does not bring much
to the table that ACPI did not already provide.
Further recourse: The only measures I can imagine now are to write
something like i8kutils for this Lenovo platform. Or edit the DSDT
table just to see what happens. I say "imagine" because I will very
likely do neither! One source also mentioned the prospect of
controlling the embedded controller for the fan via hwmon, but I haven't
seen any details on how to accomplish that apart from lm-sensors.
A small bit of consolation: I recall that Speedfan regards 50C as a good
trip point, and thermald will try to keep the CPU under 45C. But I have
read that it's generally OK for laptops to run somewhat hotter than
desktops. This laptop fan kicks on around 68C and seems to hold the
line pretty well, so maybe the thermal management is not as bad as I
first thought.
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