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