===============================
  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:

   * CPU latency QoS.
   * The per-device PM QoS framework provides the API to manage the

     per-device latency constraints and PM QoS flags.

  The latency unit used in the PM QoS framework is the microsecond (usec).

  
  1. PM QoS framework

  ===================

  A global list of CPU latency QoS requests is maintained along with an aggregated
  (effective) target value.  The aggregated target value is updated with changes
  to the request list or elements of the list.  For CPU latency QoS, the
  aggregated target value is simply the min of the request values held in the 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 space the use of this interface is simple:

  void cpu_latency_qos_add_request(handle, target_value):
    Will insert an element into the CPU latency QoS list 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 cpu_latency_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 cpu_latency_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 cpu_latency_qos_limit():
    Returns the aggregated value for the CPU latency QoS.
  
  int cpu_latency_qos_request_active(handle):
    Returns if the request is still active, i.e. it has not been removed from the
    CPU latency QoS list.

  int cpu_latency_qos_add_notifier(notifier):
    Adds a notification callback function to the CPU latency QoS. The callback is
    called when the aggregated value for the CPU latency QoS is changed.

  int cpu_latency_qos_remove_notifier(notifier):
    Removes the notification callback function from the CPU latency QoS.

  From user space:

  The infrastructure exposes one device node, /dev/cpu_dma_latency, for the CPU
  latency QoS.

  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 as follows.

  To register the default PM QoS target for the CPU latency QoS, the process must
  open /dev/cpu_dma_latency.

  
  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, it can write a hex string for the value
  using the 10 char long format e.g. "0x12345678".  This translates to a
  cpu_latency_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 and flags framework

  ================================================

  For each device, there are three lists of PM QoS requests. Two of them are
  maintained along with the aggregated targets of resume latency and active
  state latency tolerance (in microseconds) and the third one is for PM QoS flags.
  Values are updated in response to changes of the request list.

  The target values of resume latency and active state latency tolerance are
  simply the minimum of the request values held in the parameter list elements.
  The PM QoS flags aggregate value is a gather (bitwise OR) of all list elements'

  values.  One device PM QoS flag is defined currently: PM_QOS_FLAG_NO_POWER_OFF.

  Note: The aggregated target values are implemented in such a way 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, type, 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, type):

    Returns the aggregated value for a given device's constraints list.

  enum pm_qos_flags_status dev_pm_qos_flags(device, mask)

    Check PM QoS flags of the given device against the given mask of flags.
    The meaning of the return values is as follows:
  
  	PM_QOS_FLAGS_ALL:
  		All flags from the mask are set
  	PM_QOS_FLAGS_SOME:
  		Some flags from the mask are set
  	PM_QOS_FLAGS_NONE:
  		No flags from the mask are set
  	PM_QOS_FLAGS_UNDEFINED:
  		The device's PM QoS structure has not been initialized
  		or the list of requests is empty.

  int dev_pm_qos_add_ancestor_request(dev, handle, type, value)

    Add a PM QoS request for the first direct ancestor of the given device whose
    power.ignore_children flag is unset (for DEV_PM_QOS_RESUME_LATENCY requests)
    or whose power.set_latency_tolerance callback pointer is not NULL (for
    DEV_PM_QOS_LATENCY_TOLERANCE requests).

  
  int dev_pm_qos_expose_latency_limit(device, value)

    Add a request to the device's PM QoS list of resume latency constraints and
    create a sysfs attribute pm_qos_resume_latency_us under the device's power
    directory allowing user space to manipulate that request.

  
  void dev_pm_qos_hide_latency_limit(device)

    Drop the request added by dev_pm_qos_expose_latency_limit() from the device's
    PM QoS list of resume latency constraints and remove sysfs attribute
    pm_qos_resume_latency_us from the device's power directory.

  
  int dev_pm_qos_expose_flags(device, value)

    Add a request to the device's PM QoS list of flags and create sysfs attribute
    pm_qos_no_power_off under the device's power directory allowing user space to
    change the value of the PM_QOS_FLAG_NO_POWER_OFF flag.

  
  void dev_pm_qos_hide_flags(device)

    Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS
    list of flags and remove sysfs attribute pm_qos_no_power_off from the device's
    power directory.

  
  Notification mechanisms:

  The per-device PM QoS framework has a per-device notification tree.

  int dev_pm_qos_add_notifier(device, notifier, type):

    Adds a notification callback function for the device for a particular request
    type.

    The callback is called when the aggregated value of the device constraints
    list is changed.

  int dev_pm_qos_remove_notifier(device, notifier, type):

    Removes the notification callback function for the device.

  
  Active state latency tolerance

  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  
  This device PM QoS type is used to support systems in which hardware may switch
  to energy-saving operation modes on the fly.  In those systems, if the operation
  mode chosen by the hardware attempts to save energy in an overly aggressive way,
  it may cause excess latencies to be visible to software, causing it to miss
  certain protocol requirements or target frame or sample rates etc.
  
  If there is a latency tolerance control mechanism for a given device available
  to software, the .set_latency_tolerance callback in that device's dev_pm_info
  structure should be populated.  The routine pointed to by it is should implement
  whatever is necessary to transfer the effective requirement value to the
  hardware.
  
  Whenever the effective latency tolerance changes for the device, its
  .set_latency_tolerance() callback will be executed and the effective value will
  be passed to it.  If that value is negative, which means that the list of
  latency tolerance requirements for the device is empty, the callback is expected
  to switch the underlying hardware latency tolerance control mechanism to an
  autonomous mode if available.  If that value is PM_QOS_LATENCY_ANY, in turn, and
  the hardware supports a special "no requirement" setting, the callback is
  expected to use it.  That allows software to prevent the hardware from
  automatically updating the device's latency tolerance in response to its power
  state changes (e.g. during transitions from D3cold to D0), which generally may
  be done in the autonomous latency tolerance control mode.
  
  If .set_latency_tolerance() is present for the device, sysfs attribute
  pm_qos_latency_tolerance_us will be present in the devivce's power directory.
  Then, user space can use that attribute to specify its latency tolerance
  requirement for the device, if any.  Writing "any" to it means "no requirement,
  but do not let the hardware control latency tolerance" and writing "auto" to it
  allows the hardware to be switched to the autonomous mode if there are no other
  requirements from the kernel side in the device's list.
  
  Kernel code can use the functions described above along with the
  DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update
  latency tolerance requirements for devices.