|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 mode and Hardware P state (HWP)
|In legacy mode, the Intel P-state implements two internal governors,
|performance and powersave, that differ from the general cpufreq governors of
|the same name (the general cpufreq governors implement target(), whereas the
|internal Intel P-state governors implement setpolicy()). The internal
|performance governor sets the max_perf_pct and min_perf_pct to 100; that is,
|the governor selects the highest available P state to maximize the performance
|of the core. The internal powersave governor selects the appropriate P state
|based on the current load on the CPU.
|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 Intel P-state governor code is disabled.
|In addition 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
| 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.
| turbo_pct: displays the percentage of the total performance that
| is supported by hardware that is in the turbo range. This number
| is independent of whether turbo has been disabled or not.
| num_pstates: displays the number of pstates that are supported
| by hardware. This number is independent of whether turbo has
| been disabled or not.
|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.