linux/Documentation/cpu-freq/user-guide.txt
Mattia Dongili 9c9a43ed27 [CPUFREQ] return error when failing to set minfreq
I just stumbled on this bug/feature, this is how to reproduce it:

# echo 450000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
# echo 450000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
# echo powersave > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
# cpufreq-info -p
450000 450000 powersave
# echo 1800000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq ; echo $?
0
# cpufreq-info -p
450000 450000 powersave

Here it is. The kernel refuses to set a min_freq higher than the
max_freq but it allows a max_freq lower than min_freq (lowering min_freq
also).

This behaviour is pretty straightforward (but undocumented) and it
doesn't return an error altough failing to accomplish the requested
action (set min_freq).
The problem (IMO) is basically that userspace is not allowed to set a
full policy atomically while the kernel always does that thus it must
enforce an ordering on operations.

The attached patch returns -EINVAL if trying to increase frequencies
starting from scaling_min_freq and documents the correct ordering of writes.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Dominik Brodowski <linux at dominikbrodowski.net>
Signed-off-by: Dave Jones <davej@redhat.com>

--
2006-07-31 18:37:05 -04:00

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CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
U S E R G U I D E
Dominik Brodowski <linux@brodo.de>
Clock scaling allows you to change the clock speed of the CPUs on the
fly. This is a nice method to save battery power, because the lower
the clock speed, the less power the CPU consumes.
Contents:
---------
1. Supported Architectures and Processors
1.1 ARM
1.2 x86
1.3 sparc64
1.4 ppc
1.5 SuperH
2. "Policy" / "Governor"?
2.1 Policy
2.2 Governor
3. How to change the CPU cpufreq policy and/or speed
3.1 Preferred interface: sysfs
3.2 Deprecated interfaces
1. Supported Architectures and Processors
=========================================
1.1 ARM
-------
The following ARM processors are supported by cpufreq:
ARM Integrator
ARM-SA1100
ARM-SA1110
1.2 x86
-------
The following processors for the x86 architecture are supported by cpufreq:
AMD Elan - SC400, SC410
AMD mobile K6-2+
AMD mobile K6-3+
AMD mobile Duron
AMD mobile Athlon
AMD Opteron
AMD Athlon 64
Cyrix Media GXm
Intel mobile PIII and Intel mobile PIII-M on certain chipsets
Intel Pentium 4, Intel Xeon
Intel Pentium M (Centrino)
National Semiconductors Geode GX
Transmeta Crusoe
Transmeta Efficeon
VIA Cyrix 3 / C3
various processors on some ACPI 2.0-compatible systems [*]
[*] Only if "ACPI Processor Performance States" are available
to the ACPI<->BIOS interface.
1.3 sparc64
-----------
The following processors for the sparc64 architecture are supported by
cpufreq:
UltraSPARC-III
1.4 ppc
-------
Several "PowerBook" and "iBook2" notebooks are supported.
1.5 SuperH
----------
The following SuperH processors are supported by cpufreq:
SH-3
SH-4
2. "Policy" / "Governor" ?
==========================
Some CPU frequency scaling-capable processor switch between various
frequencies and operating voltages "on the fly" without any kernel or
user involvement. This guarantees very fast switching to a frequency
which is high enough to serve the user's needs, but low enough to save
power.
2.1 Policy
----------
On these systems, all you can do is select the lower and upper
frequency limit as well as whether you want more aggressive
power-saving or more instantly available processing power.
2.2 Governor
------------
On all other cpufreq implementations, these boundaries still need to
be set. Then, a "governor" must be selected. Such a "governor" decides
what speed the processor shall run within the boundaries. One such
"governor" is the "userspace" governor. This one allows the user - or
a yet-to-implement userspace program - to decide what specific speed
the processor shall run at.
3. How to change the CPU cpufreq policy and/or speed
====================================================
3.1 Preferred Interface: sysfs
------------------------------
The preferred interface is located in the sysfs filesystem. If you
mounted it at /sys, the cpufreq interface is located in a subdirectory
"cpufreq" within the cpu-device directory
(e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
cpuinfo_min_freq : this file shows the minimum operating
frequency the processor can run at(in kHz)
cpuinfo_max_freq : this file shows the maximum operating
frequency the processor can run at(in kHz)
scaling_driver : this file shows what cpufreq driver is
used to set the frequency on this CPU
scaling_available_governors : this file shows the CPUfreq governors
available in this kernel. You can see the
currently activated governor in
scaling_governor, and by "echoing" the name of another
governor you can change it. Please note
that some governors won't load - they only
work on some specific architectures or
processors.
scaling_min_freq and
scaling_max_freq show the current "policy limits" (in
kHz). By echoing new values into these
files, you can change these limits.
NOTE: when setting a policy you need to
first set scaling_max_freq, then
scaling_min_freq.
If you have selected the "userspace" governor which allows you to
set the CPU operating frequency to a specific value, you can read out
the current frequency in
scaling_setspeed. By "echoing" a new frequency into this
you can change the speed of the CPU,
but only within the limits of
scaling_min_freq and scaling_max_freq.
3.2 Deprecated Interfaces
-------------------------
Depending on your kernel configuration, you might find the following
cpufreq-related files:
/proc/cpufreq
/proc/sys/cpu/*/speed
/proc/sys/cpu/*/speed-min
/proc/sys/cpu/*/speed-max
These are files for deprecated interfaces to cpufreq, which offer far
less functionality. Because of this, these interfaces aren't described
here.