Based on kernel version 3.12. Page generated on 2013-11-13 21:59 EST.
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 and PM QoS flags. 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 and flags framework 90 91 For each device, there are two lists of PM QoS requests. One is maintained 92 along with the aggregated target of latency value and the other is for PM QoS 93 flags. Values are updated in response to changes of the request list. 94 95 Target latency value is simply the minimum of the request values held in the 96 parameter list elements. The PM QoS flags aggregate value is a gather (bitwise 97 OR) of all list elements' values. Two device PM QoS flags are defined currently: 98 PM_QOS_FLAG_NO_POWER_OFF and PM_QOS_FLAG_REMOTE_WAKEUP. 99 100 Note: the aggregated target value is implemented as an atomic variable so that 101 reading the aggregated value does not require any locking mechanism. 102 103 104 From kernel mode the use of this interface is the following: 105 106 int dev_pm_qos_add_request(device, handle, type, value): 107 Will insert an element into the list for that identified device with the 108 target value. Upon change to this list the new target is recomputed and any 109 registered notifiers are called only if the target value is now different. 110 Clients of dev_pm_qos need to save the handle for future use in other 111 dev_pm_qos API functions. 112 113 int dev_pm_qos_update_request(handle, new_value): 114 Will update the list element pointed to by the handle with the new target value 115 and recompute the new aggregated target, calling the notification trees if the 116 target is changed. 117 118 int dev_pm_qos_remove_request(handle): 119 Will remove the element. After removal it will update the aggregate target and 120 call the notification trees if the target was changed as a result of removing 121 the request. 122 123 s32 dev_pm_qos_read_value(device): 124 Returns the aggregated value for a given device's constraints list. 125 126 enum pm_qos_flags_status dev_pm_qos_flags(device, mask) 127 Check PM QoS flags of the given device against the given mask of flags. 128 The 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. 134 135 int dev_pm_qos_add_ancestor_request(dev, handle, value) 136 Add a PM QoS request for the first direct ancestor of the given device whose 137 power.ignore_children flag is unset. 138 139 int dev_pm_qos_expose_latency_limit(device, value) 140 Add a request to the device's PM QoS list of latency constraints and create 141 a sysfs attribute pm_qos_resume_latency_us under the device's power directory 142 allowing user space to manipulate that request. 143 144 void dev_pm_qos_hide_latency_limit(device) 145 Drop the request added by dev_pm_qos_expose_latency_limit() from the device's 146 PM QoS list of latency constraints and remove sysfs attribute pm_qos_resume_latency_us 147 from the device's power directory. 148 149 int dev_pm_qos_expose_flags(device, value) 150 Add a request to the device's PM QoS list of flags and create sysfs attributes 151 pm_qos_no_power_off and pm_qos_remote_wakeup under the device's power directory 152 allowing user space to change these flags' value. 153 154 void dev_pm_qos_hide_flags(device) 155 Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS list 156 of flags and remove sysfs attributes pm_qos_no_power_off and pm_qos_remote_wakeup 157 under the device's power directory. 158 159 Notification mechanisms: 160 The per-device PM QoS framework has 2 different and distinct notification trees: 161 a per-device notification tree and a global notification tree. 162 163 int dev_pm_qos_add_notifier(device, notifier): 164 Adds a notification callback function for the device. 165 The callback is called when the aggregated value of the device constraints list 166 is changed. 167 168 int dev_pm_qos_remove_notifier(device, notifier): 169 Removes the notification callback function for the device. 170 171 int dev_pm_qos_add_global_notifier(notifier): 172 Adds a notification callback function in the global notification tree of the 173 framework. 174 The callback is called when the aggregated value for any device is changed. 175 176 int dev_pm_qos_remove_global_notifier(notifier): 177 Removes the notification callback function from the global notification tree 178 of the framework. 179 180 181 From user mode: 182 No API for user space access to the per-device latency constraints is provided 183 yet - still under discussion.