forked from rrcarlosr/Jetpack
223 lines
6.1 KiB
Plaintext
223 lines
6.1 KiB
Plaintext
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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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1.6 Blackfin
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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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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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Intel PXA
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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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All SuperH processors supporting rate rounding through the clock
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framework are supported by cpufreq.
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1.6 Blackfin
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------------
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The following Blackfin processors are supported by cpufreq:
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BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
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BF531, BF532, BF533, Rev 0.3 or higher
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BF534, BF536, BF537, Rev 0.2 or higher
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BF561, Rev 0.3 or higher
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BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher
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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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cpuinfo_transition_latency The time it takes on this CPU to
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switch between two frequencies in nano
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seconds. If unknown or known to be
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that high that the driver does not
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work with the ondemand governor, -1
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(CPUFREQ_ETERNAL) will be returned.
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Using this information can be useful
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to choose an appropriate polling
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frequency for a kernel governor or
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userspace daemon. Make sure to not
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switch the frequency too often
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resulting in performance loss.
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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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cpuinfo_cur_freq : Current frequency of the CPU as obtained from
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the hardware, in KHz. This is the frequency
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the CPU actually runs at.
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scaling_available_frequencies : List of available frequencies, in KHz.
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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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affected_cpus : List of Online CPUs that require software
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coordination of frequency.
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related_cpus : List of Online + Offline CPUs that need software
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coordination of frequency.
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scaling_cur_freq : Current frequency of the CPU as determined by
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the governor and cpufreq core, in KHz. This is
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the frequency the kernel thinks the CPU runs
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at.
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bios_limit : If the BIOS tells the OS to limit a CPU to
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lower frequencies, the user can read out the
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maximum available frequency from this file.
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This typically can happen through (often not
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intended) BIOS settings, restrictions
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triggered through a service processor or other
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BIOS/HW based implementations.
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This does not cover thermal ACPI limitations
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which can be detected through the generic
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thermal driver.
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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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