intel_pstate: Add support for HWP

[mirror_ubuntu-artful-kernel.git] / Documentation / cpu-freq / intel-pstate.txt

Intel P-state driver
--------------------

This driver provides an interface to control the P state selection for
SandyBridge+ Intel processors.  The driver can operate two different
modes based on the processor model legacy and Hardware P state (HWP)
mode.

In legacy mode the driver implements a scaling driver with an internal
governor for Intel Core processors.  The driver follows the same model
as the Transmeta scaling driver (longrun.c) and implements the
setpolicy() instead of target().  Scaling drivers that implement
setpolicy() are assumed to implement internal governors by the cpufreq
core. All the logic for selecting the current P state is contained
within the driver; no external governor is used by the cpufreq core.

In HWP mode P state selection is implemented in the processor
itself. The driver provides the interfaces between the cpufreq core and
the processor to control P state selection based on user preferences
and reporting frequency to the cpufreq core.  In this mode the
internal governor code is disabled.

In addtion to the interfaces provided by the cpufreq core for
controlling frequency the driver provides sysfs files for
controlling P state selection. These files have been added to
/sys/devices/system/cpu/intel_pstate/

      max_perf_pct: limits the maximum P state that will be requested by
      the driver stated as a percentage of the available performance. The
      available (P states) performance may be reduced by the no_turbo
      setting described below.

      min_perf_pct: limits the minimum P state that will be  requested by
      the driver stated as a percentage of the max (non-turbo)
      performance level.

      no_turbo: limits the driver to selecting P states below the turbo
      frequency range.

For contemporary Intel processors, the frequency is controlled by the
processor itself and the P-states exposed to software are related to
performance levels.  The idea that frequency can be set to a single
frequency is fiction for Intel Core processors. Even if the scaling
driver selects a single P state the actual frequency the processor
will run at is selected by the processor itself.

For legacy mode debugfs files have also been added to allow tuning of
the internal governor algorythm. These files are located at
/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode.

      deadband
      d_gain_pct
      i_gain_pct
      p_gain_pct
      sample_rate_ms
      setpoint