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 and Hardware P state (HWP)
7mode.
8
9In legacy mode the driver implements a scaling driver with an internal
10governor for Intel Core processors.  The driver follows the same model
11as the Transmeta scaling driver (longrun.c) and implements the
12setpolicy() instead of target().  Scaling drivers that implement
13setpolicy() are assumed to implement internal governors by the cpufreq
14core. All the logic for selecting the current P state is contained
15within the driver; no external governor is used by the cpufreq core.
16
17In HWP mode P state selection is implemented in the processor
18itself. The driver provides the interfaces between the cpufreq core and
19the processor to control P state selection based on user preferences
20and reporting frequency to the cpufreq core.  In this mode the
21internal governor code is disabled.
22
23In addtion to the interfaces provided by the cpufreq core for
24controlling frequency the driver provides sysfs files for
25controlling P state selection. These files have been added to
26/sys/devices/system/cpu/intel_pstate/
27
28      max_perf_pct: limits the maximum P state that will be requested by
29      the driver stated as a percentage of the available performance. The
30      available (P states) performance may be reduced by the no_turbo
31      setting described below.
32
33      min_perf_pct: limits the minimum P state that will be  requested by
34      the driver stated as a percentage of the max (non-turbo)
35      performance level.
36
37      no_turbo: limits the driver to selecting P states below the turbo
38      frequency range.
39
40      turbo_pct: displays the percentage of the total performance that
41      is supported by hardware that is in the turbo range.  This number
42      is independent of whether turbo has been disabled or not.
43
44      num_pstates: displays the number of pstates that are supported
45      by hardware.  This number is independent of whether turbo has
46      been disabled or not.
47
48For contemporary Intel processors, the frequency is controlled by the
49processor itself and the P-states exposed to software are related to
50performance levels.  The idea that frequency can be set to a single
51frequency is fiction for Intel Core processors. Even if the scaling
52driver selects a single P state the actual frequency the processor
53will run at is selected by the processor itself.
54
55For legacy mode debugfs files have also been added to allow tuning of
56the internal governor algorythm. These files are located at
57/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode.
58
59      deadband
60      d_gain_pct
61      i_gain_pct
62      p_gain_pct
63      sample_rate_ms
64      setpoint
65