66 lines
2.9 KiB
Plaintext
66 lines
2.9 KiB
Plaintext
Intel P-state driver
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This driver provides an interface to control the P state selection for
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SandyBridge+ Intel processors. The driver can operate two different
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modes based on the processor model, legacy mode and Hardware P state (HWP)
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mode.
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In legacy mode, the Intel P-state implements two internal governors,
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performance and powersave, that differ from the general cpufreq governors of
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the same name (the general cpufreq governors implement target(), whereas the
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internal Intel P-state governors implement setpolicy()). The internal
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performance governor sets the max_perf_pct and min_perf_pct to 100; that is,
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the governor selects the highest available P state to maximize the performance
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of the core. The internal powersave governor selects the appropriate P state
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based on the current load on the CPU.
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In HWP mode P state selection is implemented in the processor
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itself. The driver provides the interfaces between the cpufreq core and
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the processor to control P state selection based on user preferences
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and reporting frequency to the cpufreq core. In this mode the
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internal Intel P-state governor code is disabled.
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In addition to the interfaces provided by the cpufreq core for
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controlling frequency the driver provides sysfs files for
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controlling P state selection. These files have been added to
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/sys/devices/system/cpu/intel_pstate/
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max_perf_pct: limits the maximum P state that will be requested by
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the driver stated as a percentage of the available performance. The
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available (P states) performance may be reduced by the no_turbo
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setting described below.
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min_perf_pct: limits the minimum P state that will be requested by
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the driver stated as a percentage of the max (non-turbo)
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performance level.
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no_turbo: limits the driver to selecting P states below the turbo
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frequency range.
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turbo_pct: displays the percentage of the total performance that
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is supported by hardware that is in the turbo range. This number
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is independent of whether turbo has been disabled or not.
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num_pstates: displays the number of pstates that are supported
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by hardware. This number is independent of whether turbo has
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been disabled or not.
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For contemporary Intel processors, the frequency is controlled by the
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processor itself and the P-states exposed to software are related to
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performance levels. The idea that frequency can be set to a single
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frequency is fiction for Intel Core processors. Even if the scaling
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driver selects a single P state the actual frequency the processor
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will run at is selected by the processor itself.
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For legacy mode debugfs files have also been added to allow tuning of
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the internal governor algorythm. These files are located at
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/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode.
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deadband
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d_gain_pct
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i_gain_pct
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p_gain_pct
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sample_rate_ms
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setpoint
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