Documentation / cpufreq: add 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