PowerOP is a system power parameter management API submitted for discussion. PowerOP writes and reads power "operating points", comprised of arbitrary integer-valued values, called power parameters, that correspond to registers, clocks, dividers, voltage regulators, etc. that may be modified to set a basic power/performance point for the system. The core basically passes an array of integer-valued power parameters (with very little additional structure imposed by the core) to a platform-specific backend that interprets those values and makes the requested adjustments. PowerOP is intended to leave all power policy decisions to higher layers. An optional sysfs representation of power parameters is also available, primarily for diagnostic use. PowerOP can be thought of as a layer below cpufreq that actually accesses the hardware to make cpu frequency, voltage, core bus, and perhaps other modifications to set a power point, leaving cpufreq to manage the interfaces based around the "cpu frequency" abstraction, the policies and governors that select the frequency, its notifiers, and so forth. An example hooking up support for one cpufreq platform to PowerOP is in patch 3/3. Depending on the ability of the hardware to make software-controlled power/performance adjustments, this may be useful to select custom voltages, bus speeds, etc. in desktop/server systems. Various embedded systems have several parameters that can be set. For example, an XScale PXA27x could be considered to have six basic power parameters (mainly cpu run mode and memory and bus dividers) that for the most part should be set in tandem to known good sets of values as validated by the silicon vendor, plus other parameters possible for disabling PLLs during low-speed execution, and so forth. PowerOP is aimed at supporting this kind of system, where the cpu frequency abstraction specifies only part of the operating point that may be managed from software. It also pushes the hardware-level power parameter management down to a level that can be shared with other power management policy frameworks, in use in some embedded systems, that wish to deal with entire operating points as the basic unit of system power management.. There are many ways to tackle those issues, of course, and a new API layer is arguably rather heavyweight. This is one suggested way that tries to minimize disturbing existing power management code. Comments very much appreciated. Patch 2/3 is a desktop-oriented example of PowerOP; embedded examples will follow soon. -- Todd
