linux/Documentation/power/pm_qos_interface.txt

PM quality of Service interface.

This interface provides a kernel and user mode interface for registering
performance expectations by drivers, subsystems and user space applications on
one of the parameters.

Currently we have {cpu_dma_latency, network_latency, network_throughput} as the
initial set of pm_qos parameters.

The infrastructure exposes multiple misc device nodes one per implemented
parameter.  The set of parameters implement is defined by pm_qos_power_init()
and pm_qos_params.h.  This is done because having the available parameters
being runtime configurable or changeable from a driver was seen as too easy to
abuse.

For each parameter a list of performance requirements is maintained along with
an aggregated target value.  The aggregated target value is updated with
changes to the requirement list or elements of the list.  Typically the
aggregated target value is simply the max or min of the requirement values held
in the parameter list elements.

From kernel mode the use of this interface is simple:
pm_qos_add_requirement(param_id, name, target_value):
Will insert a named element in the list for that identified PM_QOS parameter
with the target value.  Upon change to this list the new target is recomputed
and any registered notifiers are called only if the target value is now
different.

pm_qos_update_requirement(param_id, name, new_target_value):
Will search the list identified by the param_id for the named list element and
then update its target value, calling the notification tree if the aggregated
target is changed.  with that name is already registered.

pm_qos_remove_requirement(param_id, name):
Will search the identified list for the named element and remove it, after
removal it will update the aggregate target and call the notification tree if
the target was changed as a result of removing the named requirement.


From user mode:
Only processes can register a pm_qos requirement.  To provide for automatic
cleanup for process the interface requires the process to register its
parameter requirements in the following way:

To register the default pm_qos target for the specific parameter, the process
must open one of /dev/[cpu_dma_latency, network_latency, network_throughput]

As long as the device node is held open that process has a registered
requirement on the parameter.  The name of the requirement is "process_<PID>"
derived from the current->pid from within the open system call.

To change the requested target value the process needs to write a s32 value to
the open device node.  This translates to a pm_qos_update_requirement call.

To remove the user mode request for a target value simply close the device
node.