A device may have a list of frequencies and voltages available.
devfreq, a generic DVFS framework can be registered for a device
in order to let the governor provided to devfreq choose an
operating frequency based on the device driver's policy.

Each device may have its own governor and policy. Devfreq can
reevaluate the device state periodically and/or based on the
notification to "nb", a notifier block, of devfreq.

Like some CPUs with CPUfreq, a device may have multiple clocks.
However, because the clock frequencies of a single device are
determined by the single device's state, an instance of devfreq
is attached to a single device and returns a "representative"
clock frequency of the device, which is also attached
to a device by 1-to-1. The device registering devfreq takes the
responsibility to "interpret" the representative frequency and
to set its every clock accordingly with the "target" callback
given to devfreq.

When OPP is used with the devfreq device, it is recommended to
register devfreq's nb to the OPP's notifier head. If OPP is
used with the devfreq device, you may use OPP helper
functions defined in devfreq.h.

source code:

drivers/devfreq/devfreq.c