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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
+1.6 Blackfin
+
+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
+
+
+
+1. Supported Architectures and Processors
+=========================================
+
+1.1 ARM
+-------
+
+The following ARM processors are supported by cpufreq:
+
+ARM Integrator
+ARM-SA1100
+ARM-SA1110
+Intel PXA
+
+
+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
+----------
+
+All SuperH processors supporting rate rounding through the clock
+framework are supported by cpufreq.
+
+1.6 Blackfin
+------------
+
+The following Blackfin processors are supported by cpufreq:
+
+BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
+BF531, BF532, BF533, Rev 0.3 or higher
+BF534, BF536, BF537, Rev 0.2 or higher
+BF561, Rev 0.3 or higher
+BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher
+
+
+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)
+cpuinfo_transition_latency The time it takes on this CPU to
+ switch between two frequencies in nano
+ seconds. If unknown or known to be
+ that high that the driver does not
+ work with the ondemand governor, -1
+ (CPUFREQ_ETERNAL) will be returned.
+ Using this information can be useful
+ to choose an appropriate polling
+ frequency for a kernel governor or
+ userspace daemon. Make sure to not
+ switch the frequency too often
+ resulting in performance loss.
+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.
+
+cpuinfo_cur_freq : Current frequency of the CPU as obtained from
+ the hardware, in KHz. This is the frequency
+ the CPU actually runs at.
+
+scaling_available_frequencies : List of available frequencies, in KHz.
+
+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.
+
+affected_cpus : List of Online CPUs that require software
+ coordination of frequency.
+
+related_cpus : List of Online + Offline CPUs that need software
+ coordination of frequency.
+
+scaling_driver : Hardware driver for cpufreq.
+
+scaling_cur_freq : Current frequency of the CPU as determined by
+ the governor and cpufreq core, in KHz. This is
+ the frequency the kernel thinks the CPU runs
+ at.
+
+bios_limit : If the BIOS tells the OS to limit a CPU to
+ lower frequencies, the user can read out the
+ maximum available frequency from this file.
+ This typically can happen through (often not
+ intended) BIOS settings, restrictions
+ triggered through a service processor or other
+ BIOS/HW based implementations.
+ This does not cover thermal ACPI limitations
+ which can be detected through the generic
+ thermal driver.
+
+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.