linux/Documentation/power/pm_qos_interface.txt
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   1PM Quality Of Service Interface.
   2
   3This interface provides a kernel and user mode interface for registering
   4performance expectations by drivers, subsystems and user space applications on
   5one of the parameters.
   6
   7Two different PM QoS frameworks are available:
   81. PM QoS classes for cpu_dma_latency, network_latency, network_throughput.
   92. the per-device PM QoS framework provides the API to manage the per-device latency
  10constraints and PM QoS flags.
  11
  12Each parameters have defined units:
  13 * latency: usec
  14 * timeout: usec
  15 * throughput: kbs (kilo bit / sec)
  16
  17
  181. PM QoS framework
  19
  20The infrastructure exposes multiple misc device nodes one per implemented
  21parameter.  The set of parameters implement is defined by pm_qos_power_init()
  22and pm_qos_params.h.  This is done because having the available parameters
  23being runtime configurable or changeable from a driver was seen as too easy to
  24abuse.
  25
  26For each parameter a list of performance requests is maintained along with
  27an aggregated target value.  The aggregated target value is updated with
  28changes to the request list or elements of the list.  Typically the
  29aggregated target value is simply the max or min of the request values held
  30in the parameter list elements.
  31Note: the aggregated target value is implemented as an atomic variable so that
  32reading the aggregated value does not require any locking mechanism.
  33
  34
  35From kernel mode the use of this interface is simple:
  36
  37void pm_qos_add_request(handle, param_class, target_value):
  38Will insert an element into the list for that identified PM QoS class with the
  39target value.  Upon change to this list the new target is recomputed and any
  40registered notifiers are called only if the target value is now different.
  41Clients of pm_qos need to save the returned handle for future use in other
  42pm_qos API functions.
  43
  44void pm_qos_update_request(handle, new_target_value):
  45Will update the list element pointed to by the handle with the new target value
  46and recompute the new aggregated target, calling the notification tree if the
  47target is changed.
  48
  49void pm_qos_remove_request(handle):
  50Will remove the element.  After removal it will update the aggregate target and
  51call the notification tree if the target was changed as a result of removing
  52the request.
  53
  54int pm_qos_request(param_class):
  55Returns the aggregated value for a given PM QoS class.
  56
  57int pm_qos_request_active(handle):
  58Returns if the request is still active, i.e. it has not been removed from a
  59PM QoS class constraints list.
  60
  61int pm_qos_add_notifier(param_class, notifier):
  62Adds a notification callback function to the PM QoS class. The callback is
  63called when the aggregated value for the PM QoS class is changed.
  64
  65int pm_qos_remove_notifier(int param_class, notifier):
  66Removes the notification callback function for the PM QoS class.
  67
  68
  69From user mode:
  70Only processes can register a pm_qos request.  To provide for automatic
  71cleanup of a process, the interface requires the process to register its
  72parameter requests in the following way:
  73
  74To register the default pm_qos target for the specific parameter, the process
  75must open one of /dev/[cpu_dma_latency, network_latency, network_throughput]
  76
  77As long as the device node is held open that process has a registered
  78request on the parameter.
  79
  80To change the requested target value the process needs to write an s32 value to
  81the open device node.  Alternatively the user mode program could write a hex
  82string for the value using 10 char long format e.g. "0x12345678".  This
  83translates to a pm_qos_update_request call.
  84
  85To remove the user mode request for a target value simply close the device
  86node.
  87
  88
  892. PM QoS per-device latency and flags framework
  90
  91For each device, there are two lists of PM QoS requests. One is maintained
  92along with the aggregated target of latency value and the other is for PM QoS
  93flags. Values are updated in response to changes of the request list.
  94
  95Target latency value is simply the minimum of the request values held in the
  96parameter list elements.  The PM QoS flags aggregate value is a gather (bitwise
  97OR) of all list elements' values. Two device PM QoS flags are defined currently:
  98PM_QOS_FLAG_NO_POWER_OFF and PM_QOS_FLAG_REMOTE_WAKEUP.
  99
 100Note: the aggregated target value is implemented as an atomic variable so that
 101reading the aggregated value does not require any locking mechanism.
 102
 103
 104From kernel mode the use of this interface is the following:
 105
 106int dev_pm_qos_add_request(device, handle, type, value):
 107Will insert an element into the list for that identified device with the
 108target value.  Upon change to this list the new target is recomputed and any
 109registered notifiers are called only if the target value is now different.
 110Clients of dev_pm_qos need to save the handle for future use in other
 111dev_pm_qos API functions.
 112
 113int dev_pm_qos_update_request(handle, new_value):
 114Will update the list element pointed to by the handle with the new target value
 115and recompute the new aggregated target, calling the notification trees if the
 116target is changed.
 117
 118int dev_pm_qos_remove_request(handle):
 119Will remove the element.  After removal it will update the aggregate target and
 120call the notification trees if the target was changed as a result of removing
 121the request.
 122
 123s32 dev_pm_qos_read_value(device):
 124Returns the aggregated value for a given device's constraints list.
 125
 126enum pm_qos_flags_status dev_pm_qos_flags(device, mask)
 127Check PM QoS flags of the given device against the given mask of flags.
 128The meaning of the return values is as follows:
 129        PM_QOS_FLAGS_ALL: All flags from the mask are set
 130        PM_QOS_FLAGS_SOME: Some flags from the mask are set
 131        PM_QOS_FLAGS_NONE: No flags from the mask are set
 132        PM_QOS_FLAGS_UNDEFINED: The device's PM QoS structure has not been
 133                        initialized or the list of requests is empty.
 134
 135int dev_pm_qos_add_ancestor_request(dev, handle, value)
 136Add a PM QoS request for the first direct ancestor of the given device whose
 137power.ignore_children flag is unset.
 138
 139int dev_pm_qos_expose_latency_limit(device, value)
 140Add a request to the device's PM QoS list of latency constraints and create
 141a sysfs attribute pm_qos_resume_latency_us under the device's power directory
 142allowing user space to manipulate that request.
 143
 144void dev_pm_qos_hide_latency_limit(device)
 145Drop the request added by dev_pm_qos_expose_latency_limit() from the device's
 146PM QoS list of latency constraints and remove sysfs attribute pm_qos_resume_latency_us
 147from the device's power directory.
 148
 149int dev_pm_qos_expose_flags(device, value)
 150Add a request to the device's PM QoS list of flags and create sysfs attributes
 151pm_qos_no_power_off and pm_qos_remote_wakeup under the device's power directory
 152allowing user space to change these flags' value.
 153
 154void dev_pm_qos_hide_flags(device)
 155Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS list
 156of flags and remove sysfs attributes pm_qos_no_power_off and pm_qos_remote_wakeup
 157under the device's power directory.
 158
 159Notification mechanisms:
 160The per-device PM QoS framework has 2 different and distinct notification trees:
 161a per-device notification tree and a global notification tree.
 162
 163int dev_pm_qos_add_notifier(device, notifier):
 164Adds a notification callback function for the device.
 165The callback is called when the aggregated value of the device constraints list
 166is changed.
 167
 168int dev_pm_qos_remove_notifier(device, notifier):
 169Removes the notification callback function for the device.
 170
 171int dev_pm_qos_add_global_notifier(notifier):
 172Adds a notification callback function in the global notification tree of the
 173framework.
 174The callback is called when the aggregated value for any device is changed.
 175
 176int dev_pm_qos_remove_global_notifier(notifier):
 177Removes the notification callback function from the global notification tree
 178of the framework.
 179
 180
 181From user mode:
 182No API for user space access to the per-device latency constraints is provided
 183yet - still under discussion.
 184
 185