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Documentation / cpu-freq / intel-pstate.txt

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Based on kernel version 4.3. Page generated on 2015-11-02 12:44 EST.

1	Intel P-state driver
2	--------------------
4	This driver provides an interface to control the P state selection for
5	SandyBridge+ Intel processors.  The driver can operate two different
6	modes based on the processor model, legacy mode and Hardware P state (HWP)
7	mode.
9	In legacy mode, the Intel P-state implements two internal governors,
10	performance and powersave, that differ from the general cpufreq governors of
11	the same name (the general cpufreq governors implement target(), whereas the
12	internal Intel P-state governors implement setpolicy()).  The internal
13	performance governor sets the max_perf_pct and min_perf_pct to 100; that is,
14	the governor selects the highest available P state to maximize the performance
15	of the core.  The internal powersave governor selects the appropriate P state
16	based on the current load on the CPU.
18	In HWP mode P state selection is implemented in the processor
19	itself. The driver provides the interfaces between the cpufreq core and
20	the processor to control P state selection based on user preferences
21	and reporting frequency to the cpufreq core.  In this mode the
22	internal Intel P-state governor code is disabled.
24	In addition to the interfaces provided by the cpufreq core for
25	controlling frequency the driver provides sysfs files for
26	controlling P state selection. These files have been added to
27	/sys/devices/system/cpu/intel_pstate/
29	      max_perf_pct: limits the maximum P state that will be requested by
30	      the driver stated as a percentage of the available performance. The
31	      available (P states) performance may be reduced by the no_turbo
32	      setting described below.
34	      min_perf_pct: limits the minimum P state that will be  requested by
35	      the driver stated as a percentage of the max (non-turbo)
36	      performance level.
38	      no_turbo: limits the driver to selecting P states below the turbo
39	      frequency range.
41	      turbo_pct: displays the percentage of the total performance that
42	      is supported by hardware that is in the turbo range.  This number
43	      is independent of whether turbo has been disabled or not.
45	      num_pstates: displays the number of pstates that are supported
46	      by hardware.  This number is independent of whether turbo has
47	      been disabled or not.
49	For contemporary Intel processors, the frequency is controlled by the
50	processor itself and the P-states exposed to software are related to
51	performance levels.  The idea that frequency can be set to a single
52	frequency is fiction for Intel Core processors. Even if the scaling
53	driver selects a single P state the actual frequency the processor
54	will run at is selected by the processor itself.
56	For legacy mode debugfs files have also been added to allow tuning of
57	the internal governor algorythm. These files are located at
58	/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode.
60	      deadband
61	      d_gain_pct
62	      i_gain_pct
63	      p_gain_pct
64	      sample_rate_ms
65	      setpoint
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