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+Power Capping Framework
+==================================
+
+The power capping framework provides a consistent interface between the kernel
+and the user space that allows power capping drivers to expose the settings to
+user space in a uniform way.
+
+Terminology
+=========================
+The framework exposes power capping devices to user space via sysfs in the
+form of a tree of objects. The objects at the root level of the tree represent
+'control types', which correspond to different methods of power capping.  For
+example, the intel-rapl control type represents the Intel "Running Average
+Power Limit" (RAPL) technology, whereas the 'idle-injection' control type
+corresponds to the use of idle injection for controlling power.
+
+Power zones represent different parts of the system, which can be controlled and
+monitored using the power capping method determined by the control type the
+given zone belongs to. They each contain attributes for monitoring power, as
+well as controls represented in the form of power constraints.  If the parts of
+the system represented by different power zones are hierarchical (that is, one
+bigger part consists of multiple smaller parts that each have their own power
+controls), those power zones may also be organized in a hierarchy with one
+parent power zone containing multiple subzones and so on to reflect the power
+control topology of the system.  In that case, it is possible to apply power
+capping to a set of devices together using the parent power zone and if more
+fine grained control is required, it can be applied through the subzones.
+
+
+Example sysfs interface tree:
+
+/sys/devices/virtual/powercap
+??? intel-rapl
+    ??? intel-rapl:0
+    ?   ??? constraint_0_name
+    ?   ??? constraint_0_power_limit_uw
+    ?   ??? constraint_0_time_window_us
+    ?   ??? constraint_1_name
+    ?   ??? constraint_1_power_limit_uw
+    ?   ??? constraint_1_time_window_us
+    ?   ??? device -> ../../intel-rapl
+    ?   ??? energy_uj
+    ?   ??? intel-rapl:0:0
+    ?   ?   ??? constraint_0_name
+    ?   ?   ??? constraint_0_power_limit_uw
+    ?   ?   ??? constraint_0_time_window_us
+    ?   ?   ??? constraint_1_name
+    ?   ?   ??? constraint_1_power_limit_uw
+    ?   ?   ??? constraint_1_time_window_us
+    ?   ?   ??? device -> ../../intel-rapl:0
+    ?   ?   ??? energy_uj
+    ?   ?   ??? max_energy_range_uj
+    ?   ?   ??? name
+    ?   ?   ??? enabled
+    ?   ?   ??? power
+    ?   ?   ?   ??? async
+    ?   ?   ?   []
+    ?   ?   ??? subsystem -> ../../../../../../class/power_cap
+    ?   ?   ??? uevent
+    ?   ??? intel-rapl:0:1
+    ?   ?   ??? constraint_0_name
+    ?   ?   ??? constraint_0_power_limit_uw
+    ?   ?   ??? constraint_0_time_window_us
+    ?   ?   ??? constraint_1_name
+    ?   ?   ??? constraint_1_power_limit_uw
+    ?   ?   ??? constraint_1_time_window_us
+    ?   ?   ??? device -> ../../intel-rapl:0
+    ?   ?   ??? energy_uj
+    ?   ?   ??? max_energy_range_uj
+    ?   ?   ??? name
+    ?   ?   ??? enabled
+    ?   ?   ??? power
+    ?   ?   ?   ??? async
+    ?   ?   ?   []
+    ?   ?   ??? subsystem -> ../../../../../../class/power_cap
+    ?   ?   ??? uevent
+    ?   ??? max_energy_range_uj
+    ?   ??? max_power_range_uw
+    ?   ??? name
+    ?   ??? enabled
+    ?   ??? power
+    ?   ?   ??? async
+    ?   ?   []
+    ?   ??? subsystem -> ../../../../../class/power_cap
+    ?   ??? enabled
+    ?   ??? uevent
+    ??? intel-rapl:1
+    ?   ??? constraint_0_name
+    ?   ??? constraint_0_power_limit_uw
+    ?   ??? constraint_0_time_window_us
+    ?   ??? constraint_1_name
+    ?   ??? constraint_1_power_limit_uw
+    ?   ??? constraint_1_time_window_us
+    ?   ??? device -> ../../intel-rapl
+    ?   ??? energy_uj
+    ?   ??? intel-rapl:1:0
+    ?   ?   ??? constraint_0_name
+    ?   ?   ??? constraint_0_power_limit_uw
+    ?   ?   ??? constraint_0_time_window_us
+    ?   ?   ??? constraint_1_name
+    ?   ?   ??? constraint_1_power_limit_uw
+    ?   ?   ??? constraint_1_time_window_us
+    ?   ?   ??? device -> ../../intel-rapl:1
+    ?   ?   ??? energy_uj
+    ?   ?   ??? max_energy_range_uj
+    ?   ?   ??? name
+    ?   ?   ??? enabled
+    ?   ?   ??? power
+    ?   ?   ?   ??? async
+    ?   ?   ?   []
+    ?   ?   ??? subsystem -> ../../../../../../class/power_cap
+    ?   ?   ??? uevent
+    ?   ??? intel-rapl:1:1
+    ?   ?   ??? constraint_0_name
+    ?   ?   ??? constraint_0_power_limit_uw
+    ?   ?   ??? constraint_0_time_window_us
+    ?   ?   ??? constraint_1_name
+    ?   ?   ??? constraint_1_power_limit_uw
+    ?   ?   ??? constraint_1_time_window_us
+    ?   ?   ??? device -> ../../intel-rapl:1
+    ?   ?   ??? energy_uj
+    ?   ?   ??? max_energy_range_uj
+    ?   ?   ??? name
+    ?   ?   ??? enabled
+    ?   ?   ??? power
+    ?   ?   ?   ??? async
+    ?   ?   ?   []
+    ?   ?   ??? subsystem -> ../../../../../../class/power_cap
+    ?   ?   ??? uevent
+    ?   ??? max_energy_range_uj
+    ?   ??? max_power_range_uw
+    ?   ??? name
+    ?   ??? enabled
+    ?   ??? power
+    ?   ?   ??? async
+    ?   ?   []
+    ?   ??? subsystem -> ../../../../../class/power_cap
+    ?   ??? uevent
+    ??? power
+    ?   ??? async
+    ?   []
+    ??? subsystem -> ../../../../class/power_cap
+    ??? enabled
+    ??? uevent
+
+The above example illustrates a case in which the Intel RAPL technology,
+available in Intel® IA-64 and IA-32 Processor Architectures, is used. There is one
+control type called intel-rapl which contains two power zones, intel-rapl:0 and
+intel-rapl:1, representing CPU packages.  Each of these power zones contains
+two subzones, intel-rapl:j:0 and intel-rapl:j:1 (j = 0, 1), representing the
+"core" and the "uncore" parts of the given CPU package, respectively.  All of
+the zones and subzones contain energy monitoring attributes (energy_uj,
+max_energy_range_uj) and constraint attributes (constraint_*) allowing controls
+to be applied (the constraints in the 'package' power zones apply to the whole
+CPU packages and the subzone constraints only apply to the respective parts of
+the given package individually). Since Intel RAPL doesn't provide instantaneous
+power value, there is no power_uw attribute.
+
+In addition to that, each power zone contains a name attribute, allowing the
+part of the system represented by that zone to be identified.
+For example:
+
+cat /sys/class/power_cap/intel-rapl/intel-rapl:0/name
+package-0
+
+The Intel RAPL technology allows two constraints, short term and long term,
+with two different time windows to be applied to each power zone.  Thus for
+each zone there are 2 attributes representing the constraint names, 2 power
+limits and 2 attributes representing the sizes of the time windows. Such that,
+constraint_j_* attributes correspond to the jth constraint (j = 0,1).
+
+For example:
+	constraint_0_name
+	constraint_0_power_limit_uw
+	constraint_0_time_window_us
+	constraint_1_name
+	constraint_1_power_limit_uw
+	constraint_1_time_window_us
+
+Power Zone Attributes
+=================================
+Monitoring attributes
+----------------------
+
+energy_uj (rw): Current energy counter in micro joules. Write "0" to reset.
+If the counter can not be reset, then this attribute is read only.
+
+max_energy_range_uj (ro): Range of the above energy counter in micro-joules.
+
+power_uw (ro): Current power in micro watts.
+
+max_power_range_uw (ro): Range of the above power value in micro-watts.
+
+name (ro): Name of this power zone.
+
+It is possible that some domains have both power ranges and energy counter ranges;
+however, only one is mandatory.
+
+Constraints
+----------------
+constraint_X_power_limit_uw (rw): Power limit in micro watts, which should be
+applicable for the time window specified by "constraint_X_time_window_us".
+
+constraint_X_time_window_us (rw): Time window in micro seconds.
+
+constraint_X_name (ro): An optional name of the constraint
+
+constraint_X_max_power_uw(ro): Maximum allowed power in micro watts.
+
+constraint_X_min_power_uw(ro): Minimum allowed power in micro watts.
+
+constraint_X_max_time_window_us(ro): Maximum allowed time window in micro seconds.
+
+constraint_X_min_time_window_us(ro): Minimum allowed time window in micro seconds.
+
+Except power_limit_uw and time_window_us other fields are optional.
+
+Common zone and control type attributes
+----------------------------------------
+enabled (rw): Enable/Disable controls at zone level or for all zones using
+a control type.
+
+Power Cap Client Driver Interface
+==================================
+The API summary:
+
+Call powercap_register_control_type() to register control type object.
+Call powercap_register_zone() to register a power zone (under a given
+control type), either as a top-level power zone or as a subzone of another
+power zone registered earlier.
+The number of constraints in a power zone and the corresponding callbacks have
+to be defined prior to calling powercap_register_zone() to register that zone.
+
+To Free a power zone call powercap_unregister_zone().
+To free a control type object call powercap_unregister_control_type().
+Detailed API can be generated using kernel-doc on include/linux/powercap.h.