ACPI / CPPC: Document CPPC sysfs interface

	Collaborative Processor Performance Control (CPPC)

CPPC defined in the ACPI spec describes a mechanism for the OS to manage the

performance of a logical processor on a contigious and abstract performance

scale. CPPC exposes a set of registers to describe abstract performance scale,

to request performance levels and to measure per-cpu delivered performance.

For more details on CPPC please refer to the ACPI specification at:

http://uefi.org/specifications

Some of the CPPC registers are exposed via sysfs under:

/sys/devices/system/cpu/cpuX/acpi_cppc/

for each cpu X

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$ ls -lR  /sys/devices/system/cpu/cpu0/acpi_cppc/

/sys/devices/system/cpu/cpu0/acpi_cppc/:

total 0

-r--r--r-- 1 root root 65536 Mar  5 19:38 feedback_ctrs

-r--r--r-- 1 root root 65536 Mar  5 19:38 highest_perf

-r--r--r-- 1 root root 65536 Mar  5 19:38 lowest_freq

-r--r--r-- 1 root root 65536 Mar  5 19:38 lowest_nonlinear_perf

-r--r--r-- 1 root root 65536 Mar  5 19:38 lowest_perf

-r--r--r-- 1 root root 65536 Mar  5 19:38 nominal_freq

-r--r--r-- 1 root root 65536 Mar  5 19:38 nominal_perf

-r--r--r-- 1 root root 65536 Mar  5 19:38 reference_perf

-r--r--r-- 1 root root 65536 Mar  5 19:38 wraparound_time

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* highest_perf : Highest performance of this processor (abstract scale).

* nominal_perf : Highest sustained performance of this processor (abstract scale).

* lowest_nonlinear_perf : Lowest performance of this processor with nonlinear

  power savings (abstract scale).

* lowest_perf : Lowest performance of this processor (abstract scale).

* lowest_freq : CPU frequency corresponding to lowest_perf (in MHz).

* nominal_freq : CPU frequency corresponding to nominal_perf (in MHz).

  The above frequencies should only be used to report processor performance in

  freqency instead of abstract scale. These values should not be used for any

  functional decisions.

* feedback_ctrs : Includes both Reference and delivered performance counter.

  Reference counter ticks up proportional to processor's reference performance.

  Delivered counter ticks up proportional to processor's delivered performance.

* wraparound_time: Minimum time for the feedback counters to wraparound (seconds).

* reference_perf : Performance level at which reference performance counter

  accumulates (abstract scale).

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		Computing Average Delivered Performance

Below describes the steps to compute the average performance delivered by taking

two different snapshots of feedback counters at time T1 and T2.

T1: Read feedback_ctrs as fbc_t1

    Wait or run some workload

T2: Read feedback_ctrs as fbc_t2

delivered_counter_delta = fbc_t2[del] - fbc_t1[del]

reference_counter_delta = fbc_t2[ref] - fbc_t1[ref]

delivered_perf = (refernce_perf x delivered_counter_delta) / reference_counter_delta