Powering down peripherals
The discussion up to now has been about CPUs and how to reduce power consumption when they are running or idling. Now it is time to focus on other parts of the system peripherals and see whether we can achieve power savings here.
In the Linux kernel, this is managed by the runtime power management system or runtime pm for short. It works with drivers that support runtime pm, shutting down those that are not in use and waking them again when they are next needed. It is dynamic and should be transparent to user space. It is up to the device driver to implement the management of the hardware, but typically, it would include turning off the clock to the subsystem, also known as clock gating, and turning off core circuitry where possible.
The runtime power management is exposed via a sysfs interface. Each device has
a subdirectory named power, in which you will find these files:
control: This allows user space to determine whether runtime pm is used...
