Documentation / cpufreq: add intel-pstate.txt

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

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

This 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.

Intel SandyBridge+ processors are supported.

New sysfs files for controlling P state selection 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.

      min_perf_pct: limits the minimum P state that will be  requested by
      the driver stated as a percentage of the available performance.

      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.

New debugfs files have also been added to /sys/kernel/debug/pstate_snb/

      deadband
      d_gain_pct
      i_gain_pct
      p_gain_pct
      sample_rate_ms
      setpoint