Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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 memory_bandwidth. 10 2. the per-device PM QoS framework provides the API to manage the per-device latency 11 constraints and PM QoS flags. 12 13 Each parameters have defined units: 14 * latency: usec 15 * timeout: usec 16 * throughput: kbs (kilo bit / sec) 17 * memory bandwidth: mbs (mega bit / sec) 18 19 20 1. PM QoS framework 21 22 The infrastructure exposes multiple misc device nodes one per implemented 23 parameter. The set of parameters implement is defined by pm_qos_power_init() 24 and pm_qos_params.h. This is done because having the available parameters 25 being runtime configurable or changeable from a driver was seen as too easy to 26 abuse. 27 28 For each parameter a list of performance requests is maintained along with 29 an aggregated target value. The aggregated target value is updated with 30 changes to the request list or elements of the list. Typically the 31 aggregated target value is simply the max or min of the request values held 32 in the parameter list elements. 33 Note: the aggregated target value is implemented as an atomic variable so that 34 reading the aggregated value does not require any locking mechanism. 35 36 37 From kernel mode the use of this interface is simple: 38 39 void pm_qos_add_request(handle, param_class, target_value): 40 Will insert an element into the list for that identified PM QoS class with the 41 target value. Upon change to this list the new target is recomputed and any 42 registered notifiers are called only if the target value is now different. 43 Clients of pm_qos need to save the returned handle for future use in other 44 pm_qos API functions. 45 46 void pm_qos_update_request(handle, new_target_value): 47 Will update the list element pointed to by the handle with the new target value 48 and recompute the new aggregated target, calling the notification tree if the 49 target is changed. 50 51 void pm_qos_remove_request(handle): 52 Will remove the element. After removal it will update the aggregate target and 53 call the notification tree if the target was changed as a result of removing 54 the request. 55 56 int pm_qos_request(param_class): 57 Returns the aggregated value for a given PM QoS class. 58 59 int pm_qos_request_active(handle): 60 Returns if the request is still active, i.e. it has not been removed from a 61 PM QoS class constraints list. 62 63 int pm_qos_add_notifier(param_class, notifier): 64 Adds a notification callback function to the PM QoS class. The callback is 65 called when the aggregated value for the PM QoS class is changed. 66 67 int pm_qos_remove_notifier(int param_class, notifier): 68 Removes the notification callback function for the PM QoS class. 69 70 71 From user mode: 72 Only processes can register a pm_qos request. To provide for automatic 73 cleanup of a process, the interface requires the process to register its 74 parameter requests in the following way: 75 76 To register the default pm_qos target for the specific parameter, the process 77 must open one of /dev/[cpu_dma_latency, network_latency, network_throughput] 78 79 As long as the device node is held open that process has a registered 80 request on the parameter. 81 82 To change the requested target value the process needs to write an s32 value to 83 the open device node. Alternatively the user mode program could write a hex 84 string for the value using 10 char long format e.g. "0x12345678". This 85 translates to a pm_qos_update_request call. 86 87 To remove the user mode request for a target value simply close the device 88 node. 89 90 91 2. PM QoS per-device latency and flags framework 92 93 For each device, there are three lists of PM QoS requests. Two of them are 94 maintained along with the aggregated targets of resume latency and active 95 state latency tolerance (in microseconds) and the third one is for PM QoS flags. 96 Values are updated in response to changes of the request list. 97 98 The target values of resume latency and active state latency tolerance are 99 simply the minimum of the request values held in the parameter list elements. 100 The PM QoS flags aggregate value is a gather (bitwise OR) of all list elements' 101 values. One device PM QoS flag is defined currently: PM_QOS_FLAG_NO_POWER_OFF. 102 103 Note: The aggregated target values are implemented in such a way that reading 104 the aggregated value does not require any locking mechanism. 105 106 107 From kernel mode the use of this interface is the following: 108 109 int dev_pm_qos_add_request(device, handle, type, value): 110 Will insert an element into the list for that identified device with the 111 target value. Upon change to this list the new target is recomputed and any 112 registered notifiers are called only if the target value is now different. 113 Clients of dev_pm_qos need to save the handle for future use in other 114 dev_pm_qos API functions. 115 116 int dev_pm_qos_update_request(handle, new_value): 117 Will update the list element pointed to by the handle with the new target value 118 and recompute the new aggregated target, calling the notification trees if the 119 target is changed. 120 121 int dev_pm_qos_remove_request(handle): 122 Will remove the element. After removal it will update the aggregate target and 123 call the notification trees if the target was changed as a result of removing 124 the request. 125 126 s32 dev_pm_qos_read_value(device): 127 Returns the aggregated value for a given device's constraints list. 128 129 enum pm_qos_flags_status dev_pm_qos_flags(device, mask) 130 Check PM QoS flags of the given device against the given mask of flags. 131 The meaning of the return values is as follows: 132 PM_QOS_FLAGS_ALL: All flags from the mask are set 133 PM_QOS_FLAGS_SOME: Some flags from the mask are set 134 PM_QOS_FLAGS_NONE: No flags from the mask are set 135 PM_QOS_FLAGS_UNDEFINED: The device's PM QoS structure has not been 136 initialized or the list of requests is empty. 137 138 int dev_pm_qos_add_ancestor_request(dev, handle, type, value) 139 Add a PM QoS request for the first direct ancestor of the given device whose 140 power.ignore_children flag is unset (for DEV_PM_QOS_RESUME_LATENCY requests) 141 or whose power.set_latency_tolerance callback pointer is not NULL (for 142 DEV_PM_QOS_LATENCY_TOLERANCE requests). 143 144 int dev_pm_qos_expose_latency_limit(device, value) 145 Add a request to the device's PM QoS list of resume latency constraints and 146 create a sysfs attribute pm_qos_resume_latency_us under the device's power 147 directory allowing user space to manipulate that request. 148 149 void dev_pm_qos_hide_latency_limit(device) 150 Drop the request added by dev_pm_qos_expose_latency_limit() from the device's 151 PM QoS list of resume latency constraints and remove sysfs attribute 152 pm_qos_resume_latency_us from the device's power directory. 153 154 int dev_pm_qos_expose_flags(device, value) 155 Add a request to the device's PM QoS list of flags and create sysfs attribute 156 pm_qos_no_power_off under the device's power directory allowing user space to 157 change the value of the PM_QOS_FLAG_NO_POWER_OFF flag. 158 159 void dev_pm_qos_hide_flags(device) 160 Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS list 161 of flags and remove sysfs attribute pm_qos_no_power_off from the device's power 162 directory. 163 164 Notification mechanisms: 165 The per-device PM QoS framework has a per-device notification tree. 166 167 int dev_pm_qos_add_notifier(device, notifier): 168 Adds a notification callback function for the device. 169 The callback is called when the aggregated value of the device constraints list 170 is changed (for resume latency device PM QoS only). 171 172 int dev_pm_qos_remove_notifier(device, notifier): 173 Removes the notification callback function for the device. 174 175 176 Active state latency tolerance 177 178 This device PM QoS type is used to support systems in which hardware may switch 179 to energy-saving operation modes on the fly. In those systems, if the operation 180 mode chosen by the hardware attempts to save energy in an overly aggressive way, 181 it may cause excess latencies to be visible to software, causing it to miss 182 certain protocol requirements or target frame or sample rates etc. 183 184 If there is a latency tolerance control mechanism for a given device available 185 to software, the .set_latency_tolerance callback in that device's dev_pm_info 186 structure should be populated. The routine pointed to by it is should implement 187 whatever is necessary to transfer the effective requirement value to the 188 hardware. 189 190 Whenever the effective latency tolerance changes for the device, its 191 .set_latency_tolerance() callback will be executed and the effective value will 192 be passed to it. If that value is negative, which means that the list of 193 latency tolerance requirements for the device is empty, the callback is expected 194 to switch the underlying hardware latency tolerance control mechanism to an 195 autonomous mode if available. If that value is PM_QOS_LATENCY_ANY, in turn, and 196 the hardware supports a special "no requirement" setting, the callback is 197 expected to use it. That allows software to prevent the hardware from 198 automatically updating the device's latency tolerance in response to its power 199 state changes (e.g. during transitions from D3cold to D0), which generally may 200 be done in the autonomous latency tolerance control mode. 201 202 If .set_latency_tolerance() is present for the device, sysfs attribute 203 pm_qos_latency_tolerance_us will be present in the devivce's power directory. 204 Then, user space can use that attribute to specify its latency tolerance 205 requirement for the device, if any. Writing "any" to it means "no requirement, 206 but do not let the hardware control latency tolerance" and writing "auto" to it 207 allows the hardware to be switched to the autonomous mode if there are no other 208 requirements from the kernel side in the device's list. 209 210 Kernel code can use the functions described above along with the 211 DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update 212 latency tolerance requirements for devices.
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