to shift to lower voltage settings and reduce the power cost of executing
a given number of instructions. (Without voltage adjustment, it's rare
for cpufreq to save much power; the cost-per-instruction must go down.)
-
-
-/sys/devices/.../power/state files
-==================================
-For now you can also test some of this functionality using sysfs.
-
- DEPRECATED: USE "power/state" ONLY FOR DRIVER TESTING, AND
- AVOID USING dev->power.power_state IN DRIVERS.
-
- THESE WILL BE REMOVED. IF THE "power/state" FILE GETS REPLACED,
- IT WILL BECOME SOMETHING COUPLED TO THE BUS OR DRIVER.
-
-In each device's directory, there is a 'power' directory, which contains
-at least a 'state' file. The value of this field is effectively boolean,
-PM_EVENT_ON or PM_EVENT_SUSPEND.
-
- * Reading from this file displays a value corresponding to
- the power.power_state.event field. All nonzero values are
- displayed as "2", corresponding to a low power state; zero
- is displayed as "0", corresponding to normal operation.
-
- * Writing to this file initiates a transition using the
- specified event code number; only '0', '2', and '3' are
- accepted (without a newline); '2' and '3' are both
- mapped to PM_EVENT_SUSPEND.
-
-On writes, the PM core relies on that recorded event code and the device/bus
-capabilities to determine whether it uses a partial suspend() or resume()
-sequence to change things so that the recorded event corresponds to the
-numeric parameter.
-
- - If the bus requires the irqs-disabled suspend_late()/resume_early()
- phases, writes fail because those operations are not supported here.
-
- - If the recorded value is the expected value, nothing is done.
-
- - If the recorded value is nonzero, the device is partially resumed,
- using the bus.resume() and/or class.resume() methods.
-
- - If the target value is nonzero, the device is partially suspended,
- using the class.suspend() and/or bus.suspend() methods and the
- PM_EVENT_SUSPEND message.
-
-Drivers have no way to tell whether their suspend() and resume() calls
-have come through the sysfs power/state file or as part of entering a
-system sleep state, except that when accessed through sysfs the normal
-parent/child sequencing rules are ignored. Drivers (such as bus, bridge,
-or hub drivers) which expose child devices may need to enforce those rules
-on their own.