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e2f74f355e
This interface is mainly intended (and implemented) for ACPI _PPC BIOS
frequency limitations, but other cpufreq drivers can also use it for
similar use-cases.
Why is this needed:
Currently it's not obvious why cpufreq got limited.
People see cpufreq/scaling_max_freq reduced, but this could have
happened by:
- any userspace prog writing to scaling_max_freq
- thermal limitations
- hardware (_PPC in ACPI case) limitiations
Therefore export bios_limit (in kHz) to:
- Point the user that it's the BIOS (broken or intended) which limits
frequency
- Export it as a sysfs interface for userspace progs.
While this was a rarely used feature on laptops, there will appear
more and more server implemenations providing "Green IT" features like
allowing the service processor to limit the frequency. People want
to know about HW/BIOS frequency limitations.
All ACPI P-state driven cpufreq drivers are covered with this patch:
- powernow-k8
- powernow-k7
- acpi-cpufreq
Tested with a patched DSDT which limits the first two cores (_PPC returns 1)
via _PPC, exposed by bios_limit:
# echo 2200000 >cpu2/cpufreq/scaling_max_freq
# cat cpu*/cpufreq/scaling_max_freq
2600000
2600000
2200000
2200000
# #scaling_max_freq shows general user/thermal/BIOS limitations
# cat cpu*/cpufreq/bios_limit
2600000
2600000
2800000
2800000
# #bios_limit only shows the HW/BIOS limitation
CC: Pallipadi Venkatesh <venkatesh.pallipadi@intel.com>
CC: Len Brown <lenb@kernel.org>
CC: davej@codemonkey.org.uk
CC: linux@dominikbrodowski.net
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Dave Jones <davej@redhat.com>
225 lines
6.1 KiB
Plaintext
225 lines
6.1 KiB
Plaintext
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 CPUs that require software coordination
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of frequency.
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related_cpus : List of CPUs that need some sort of frequency
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coordination, whether software or hardware.
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scaling_driver : Hardware driver for cpufreq.
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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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