Documentation/cpu-freq/intel-pstate.txt

   1 Intel P-state driver
   2 --------------------
   3
   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 and Hardware P state (HWP)
   7 mode.
   8
   9 In legacy mode the driver implements a scaling driver with an internal
  10 governor for Intel Core processors.  The driver follows the same model
  11 as the Transmeta scaling driver (longrun.c) and implements the
  12 setpolicy() instead of target().  Scaling drivers that implement
  13 setpolicy() are assumed to implement internal governors by the cpufreq
  14 core. All the logic for selecting the current P state is contained
  15 within the driver; no external governor is used by the cpufreq core.
  16
  17 In HWP mode P state selection is implemented in the processor
  18 itself. The driver provides the interfaces between the cpufreq core and
  19 the processor to control P state selection based on user preferences
  20 and reporting frequency to the cpufreq core.  In this mode the
  21 internal governor code is disabled.
  22
  23 In addtion to the interfaces provided by the cpufreq core for
  24 controlling frequency the driver provides sysfs files for
  25 controlling 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
  48 For contemporary Intel processors, the frequency is controlled by the
  49 processor itself and the P-states exposed to software are related to
  50 performance levels.  The idea that frequency can be set to a single
  51 frequency is fiction for Intel Core processors. Even if the scaling
  52 driver selects a single P state the actual frequency the processor
  53 will run at is selected by the processor itself.
  54
  55 For legacy mode debugfs files have also been added to allow tuning of
  56 the 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