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+.. SPDX-License-Identifier: GPL-2.0
+
+==================================================
+Collaborative Processor Performance Control (CPPC)
+==================================================
+
+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::
+
+ $ 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
+
+* 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).
+
+
+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