1Intel P-state driver
2--------------------
3
4This driver provides an interface to control the P state selection for
5SandyBridge+ Intel processors.  The driver can operate two different
6modes based on the processor model, legacy mode and Hardware P state (HWP)
7mode.
8
9In legacy mode, the Intel P-state implements two internal governors,
10performance and powersave, that differ from the general cpufreq governors of
11the same name (the general cpufreq governors implement target(), whereas the
12internal Intel P-state governors implement setpolicy()).  The internal
13performance governor sets the max_perf_pct and min_perf_pct to 100; that is,
14the governor selects the highest available P state to maximize the performance
15of the core.  The internal powersave governor selects the appropriate P state
16based on the current load on the CPU.
17
18In HWP mode P state selection is implemented in the processor
19itself. The driver provides the interfaces between the cpufreq core and
20the processor to control P state selection based on user preferences
21and reporting frequency to the cpufreq core.  In this mode the
22internal Intel P-state governor code is disabled.
23
24In addition to the interfaces provided by the cpufreq core for
25controlling frequency the driver provides sysfs files for
26controlling P state selection. These files have been added to
27/sys/devices/system/cpu/intel_pstate/
28
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.
33
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.
37
38      no_turbo: limits the driver to selecting P states below the turbo
39      frequency range.
40
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.
44
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.
48
49For contemporary Intel processors, the frequency is controlled by the
50processor itself and the P-states exposed to software are related to
51performance levels.  The idea that frequency can be set to a single
52frequency is fiction for Intel Core processors. Even if the scaling
53driver selects a single P state the actual frequency the processor
54will run at is selected by the processor itself.
55
56For legacy mode debugfs files have also been added to allow tuning of
57the internal governor algorythm. These files are located at
58/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode.
59
60      deadband
61      d_gain_pct
62      i_gain_pct
63      p_gain_pct
64      sample_rate_ms
65      setpoint
66