Linux/Documentation/power/pm_qos_interface.txt

  1 PM Quality Of Service Interface.
  2 
  3 This interface provides a kernel and user mode interface for registering
  4 performance expectations by drivers, subsystems and user space applications on
  5 one of the parameters.
  6 
  7 Two different PM QoS frameworks are available:
  8 1. PM QoS classes for cpu_dma_latency, network_latency, network_throughput.
  9 2. the per-device PM QoS framework provides the API to manage the per-device latency
 10 constraints.
 11 
 12 Each parameters have defined units:
 13  * latency: usec
 14  * timeout: usec
 15  * throughput: kbs (kilo bit / sec)
 16 
 17 
 18 1. PM QoS framework
 19 
 20 The infrastructure exposes multiple misc device nodes one per implemented
 21 parameter.  The set of parameters implement is defined by pm_qos_power_init()
 22 and pm_qos_params.h.  This is done because having the available parameters
 23 being runtime configurable or changeable from a driver was seen as too easy to
 24 abuse.
 25 
 26 For each parameter a list of performance requests is maintained along with
 27 an aggregated target value.  The aggregated target value is updated with
 28 changes to the request list or elements of the list.  Typically the
 29 aggregated target value is simply the max or min of the request values held
 30 in the parameter list elements.
 31 Note: the aggregated target value is implemented as an atomic variable so that
 32 reading the aggregated value does not require any locking mechanism.
 33 
 34 
 35 From kernel mode the use of this interface is simple:
 36 
 37 void pm_qos_add_request(handle, param_class, target_value):
 38 Will insert an element into the list for that identified PM QoS class with the
 39 target value.  Upon change to this list the new target is recomputed and any
 40 registered notifiers are called only if the target value is now different.
 41 Clients of pm_qos need to save the returned handle for future use in other
 42 pm_qos API functions.
 43 
 44 void pm_qos_update_request(handle, new_target_value):
 45 Will update the list element pointed to by the handle with the new target value
 46 and recompute the new aggregated target, calling the notification tree if the
 47 target is changed.
 48 
 49 void pm_qos_remove_request(handle):
 50 Will remove the element.  After removal it will update the aggregate target and
 51 call the notification tree if the target was changed as a result of removing
 52 the request.
 53 
 54 int pm_qos_request(param_class):
 55 Returns the aggregated value for a given PM QoS class.
 56 
 57 int pm_qos_request_active(handle):
 58 Returns if the request is still active, i.e. it has not been removed from a
 59 PM QoS class constraints list.
 60 
 61 int pm_qos_add_notifier(param_class, notifier):
 62 Adds a notification callback function to the PM QoS class. The callback is
 63 called when the aggregated value for the PM QoS class is changed.
 64 
 65 int pm_qos_remove_notifier(int param_class, notifier):
 66 Removes the notification callback function for the PM QoS class.
 67 
 68 
 69 From user mode:
 70 Only processes can register a pm_qos request.  To provide for automatic
 71 cleanup of a process, the interface requires the process to register its
 72 parameter requests in the following way:
 73 
 74 To register the default pm_qos target for the specific parameter, the process
 75 must open one of /dev/[cpu_dma_latency, network_latency, network_throughput]
 76 
 77 As long as the device node is held open that process has a registered
 78 request on the parameter.
 79 
 80 To change the requested target value the process needs to write an s32 value to
 81 the open device node.  Alternatively the user mode program could write a hex
 82 string for the value using 10 char long format e.g. "0x12345678".  This
 83 translates to a pm_qos_update_request call.
 84 
 85 To remove the user mode request for a target value simply close the device
 86 node.
 87 
 88 
 89 2. PM QoS per-device latency framework
 90 
 91 For each device a list of performance requests is maintained along with
 92 an aggregated target value.  The aggregated target value is updated with
 93 changes to the request list or elements of the list.  Typically the
 94 aggregated target value is simply the max or min of the request values held
 95 in the parameter list elements.
 96 Note: the aggregated target value is implemented as an atomic variable so that
 97 reading the aggregated value does not require any locking mechanism.
 98 
 99 
100 From kernel mode the use of this interface is the following:
101 
102 int dev_pm_qos_add_request(device, handle, value):
103 Will insert an element into the list for that identified device with the
104 target value.  Upon change to this list the new target is recomputed and any
105 registered notifiers are called only if the target value is now different.
106 Clients of dev_pm_qos need to save the handle for future use in other
107 dev_pm_qos API functions.
108 
109 int dev_pm_qos_update_request(handle, new_value):
110 Will update the list element pointed to by the handle with the new target value
111 and recompute the new aggregated target, calling the notification trees if the
112 target is changed.
113 
114 int dev_pm_qos_remove_request(handle):
115 Will remove the element.  After removal it will update the aggregate target and
116 call the notification trees if the target was changed as a result of removing
117 the request.
118 
119 s32 dev_pm_qos_read_value(device):
120 Returns the aggregated value for a given device's constraints list.
121 
122 
123 Notification mechanisms:
124 The per-device PM QoS framework has 2 different and distinct notification trees:
125 a per-device notification tree and a global notification tree.
126 
127 int dev_pm_qos_add_notifier(device, notifier):
128 Adds a notification callback function for the device.
129 The callback is called when the aggregated value of the device constraints list
130 is changed.
131 
132 int dev_pm_qos_remove_notifier(device, notifier):
133 Removes the notification callback function for the device.
134 
135 int dev_pm_qos_add_global_notifier(notifier):
136 Adds a notification callback function in the global notification tree of the
137 framework.
138 The callback is called when the aggregated value for any device is changed.
139 
140 int dev_pm_qos_remove_global_notifier(notifier):
141 Removes the notification callback function from the global notification tree
142 of the framework.
143 
144 
145 From user mode:
146 No API for user space access to the per-device latency constraints is provided
147 yet - still under discussion.
148

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