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