linux-next/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.

Two different PM QoS frameworks are available:
1. PM QoS classes for cpu_dma_latency, network_latency, network_throughput.
2. the per-device PM QoS framework provides the API to manage the per-device latency
constraints.

Each parameters have defined units:
 * latency: usec
 * timeout: usec
 * throughput: kbs (kilo bit / sec)


1. PM QoS framework

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 requests is maintained along with
an aggregated target value.  The aggregated target value is updated with
changes to the request list or elements of the list.  Typically the
aggregated target value is simply the max or min of the request values held
in the parameter list elements.
Note: the aggregated target value is implemented as an atomic variable so that
reading the aggregated value does not require any locking mechanism.


From kernel mode the use of this interface is simple:

void pm_qos_add_request(handle, param_class, target_value):
Will insert an element into the list for that identified PM QoS class 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.
Clients of pm_qos need to save the returned handle for future use in other
pm_qos API functions.

void pm_qos_update_request(handle, new_target_value):
Will update the list element pointed to by the handle with the new target value
and recompute the new aggregated target, calling the notification tree if the
target is changed.

void pm_qos_remove_request(handle):
Will remove the element.  After removal it will update the aggregate target and
call the notification tree if the target was changed as a result of removing
the request.

int pm_qos_request(param_class):
Returns the aggregated value for a given PM QoS class.

int pm_qos_request_active(handle):
Returns if the request is still active, i.e. it has not been removed from a
PM QoS class constraints list.

int pm_qos_add_notifier(param_class, notifier):
Adds a notification callback function to the PM QoS class. The callback is
called when the aggregated value for the PM QoS class is changed.

int pm_qos_remove_notifier(int param_class, notifier):
Removes the notification callback function for the PM QoS class.


From user mode:
Only processes can register a pm_qos request.  To provide for automatic
cleanup of a process, the interface requires the process to register its
parameter requests 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
request on the parameter.

To change the requested target value the process needs to write an s32 value to
the open device node.  Alternatively the user mode program could write a hex
string for the value using 10 char long format e.g. "0x12345678".  This
translates to a pm_qos_update_request call.

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


2. PM QoS per-device latency framework

For each device a list of performance requests is maintained along with
an aggregated target value.  The aggregated target value is updated with
changes to the request list or elements of the list.  Typically the
aggregated target value is simply the max or min of the request values held
in the parameter list elements.
Note: the aggregated target value is implemented as an atomic variable so that
reading the aggregated value does not require any locking mechanism.


From kernel mode the use of this interface is the following:

int dev_pm_qos_add_request(device, handle, value):
Will insert an element into the list for that identified device 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.
Clients of dev_pm_qos need to save the handle for future use in other
dev_pm_qos API functions.

int dev_pm_qos_update_request(handle, new_value):
Will update the list element pointed to by the handle with the new target value
and recompute the new aggregated target, calling the notification trees if the
target is changed.

int dev_pm_qos_remove_request(handle):
Will remove the element.  After removal it will update the aggregate target and
call the notification trees if the target was changed as a result of removing
the request.

s32 dev_pm_qos_read_value(device):
Returns the aggregated value for a given device's constraints list.


Notification mechanisms:
The per-device PM QoS framework has 2 different and distinct notification trees:
a per-device notification tree and a global notification tree.

int dev_pm_qos_add_notifier(device, notifier):
Adds a notification callback function for the device.
The callback is called when the aggregated value of the device constraints list
is changed.

int dev_pm_qos_remove_notifier(device, notifier):
Removes the notification callback function for the device.

int dev_pm_qos_add_global_notifier(notifier):
Adds a notification callback function in the global notification tree of the
framework.
The callback is called when the aggregated value for any device is changed.

int dev_pm_qos_remove_global_notifier(notifier):
Removes the notification callback function from the global notification tree
of the framework.


From user mode:
No API for user space access to the per-device latency constraints is provided
yet - still under discussion.