1=============================== 2PM Quality Of Service Interface 3=============================== 4 5This interface provides a kernel and user mode interface for registering 6performance expectations by drivers, subsystems and user space applications on 7one of the parameters. 8 9Two different PM QoS frameworks are available: 10 * CPU latency QoS. 11 * The per-device PM QoS framework provides the API to manage the 12 per-device latency constraints and PM QoS flags. 13 14The latency unit used in the PM QoS framework is the microsecond (usec). 15 16 171. PM QoS framework 18=================== 19 20A global list of CPU latency QoS requests is maintained along with an aggregated 21(effective) target value. The aggregated target value is updated with changes 22to the request list or elements of the list. For CPU latency QoS, the 23aggregated target value is simply the min of the request values held in the list 24elements. 25 26Note: the aggregated target value is implemented as an atomic variable so that 27reading the aggregated value does not require any locking mechanism. 28 29From kernel space the use of this interface is simple: 30 31void cpu_latency_qos_add_request(handle, target_value): 32 Will insert an element into the CPU latency QoS list with the target value. 33 Upon change to this list the new target is recomputed and any registered 34 notifiers are called only if the target value is now different. 35 Clients of PM QoS need to save the returned handle for future use in other 36 PM QoS API functions. 37 38void cpu_latency_qos_update_request(handle, new_target_value): 39 Will update the list element pointed to by the handle with the new target 40 value and recompute the new aggregated target, calling the notification tree 41 if the target is changed. 42 43void cpu_latency_qos_remove_request(handle): 44 Will remove the element. After removal it will update the aggregate target 45 and call the notification tree if the target was changed as a result of 46 removing the request. 47 48int cpu_latency_qos_limit(): 49 Returns the aggregated value for the CPU latency QoS. 50 51int cpu_latency_qos_request_active(handle): 52 Returns if the request is still active, i.e. it has not been removed from the 53 CPU latency QoS list. 54 55int cpu_latency_qos_add_notifier(notifier): 56 Adds a notification callback function to the CPU latency QoS. The callback is 57 called when the aggregated value for the CPU latency QoS is changed. 58 59int cpu_latency_qos_remove_notifier(notifier): 60 Removes the notification callback function from the CPU latency QoS. 61 62 63From user space: 64 65The infrastructure exposes one device node, /dev/cpu_dma_latency, for the CPU 66latency QoS. 67 68Only processes can register a PM QoS request. To provide for automatic 69cleanup of a process, the interface requires the process to register its 70parameter requests as follows. 71 72To register the default PM QoS target for the CPU latency QoS, the process must 73open /dev/cpu_dma_latency. 74 75As long as the device node is held open that process has a registered 76request on the parameter. 77 78To change the requested target value, the process needs to write an s32 value to 79the open device node. Alternatively, it can write a hex string for the value 80using the 10 char long format e.g. "0x12345678". This translates to a 81cpu_latency_qos_update_request() call. 82 83To remove the user mode request for a target value simply close the device 84node. 85 86 872. PM QoS per-device latency and flags framework 88================================================ 89 90For each device, there are three lists of PM QoS requests. Two of them are 91maintained along with the aggregated targets of resume latency and active 92state latency tolerance (in microseconds) and the third one is for PM QoS flags. 93Values are updated in response to changes of the request list. 94 95The target values of resume latency and active state latency tolerance are 96simply the minimum of the request values held in the parameter list elements. 97The PM QoS flags aggregate value is a gather (bitwise OR) of all list elements' 98values. One device PM QoS flag is defined currently: PM_QOS_FLAG_NO_POWER_OFF. 99 100Note: The aggregated target values are implemented in such a way that reading 101the 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): 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 113int dev_pm_qos_update_request(handle, new_value): 114 Will update the list element pointed to by the handle with the new target 115 value and recompute the new aggregated target, calling the notification 116 trees if the target is changed. 117 118int dev_pm_qos_remove_request(handle): 119 Will remove the element. After removal it will update the aggregate target 120 and call the notification trees if the target was changed as a result of 121 removing the request. 122 123s32 dev_pm_qos_read_value(device, type): 124 Returns the aggregated value for a given device's constraints list. 125 126enum 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: 129 130 PM_QOS_FLAGS_ALL: 131 All flags from the mask are set 132 PM_QOS_FLAGS_SOME: 133 Some flags from the mask are set 134 PM_QOS_FLAGS_NONE: 135 No flags from the mask are set 136 PM_QOS_FLAGS_UNDEFINED: 137 The device's PM QoS structure has not been initialized 138 or the list of requests is empty. 139 140int dev_pm_qos_add_ancestor_request(dev, handle, type, value) 141 Add a PM QoS request for the first direct ancestor of the given device whose 142 power.ignore_children flag is unset (for DEV_PM_QOS_RESUME_LATENCY requests) 143 or whose power.set_latency_tolerance callback pointer is not NULL (for 144 DEV_PM_QOS_LATENCY_TOLERANCE requests). 145 146int dev_pm_qos_expose_latency_limit(device, value) 147 Add a request to the device's PM QoS list of resume latency constraints and 148 create a sysfs attribute pm_qos_resume_latency_us under the device's power 149 directory allowing user space to manipulate that request. 150 151void dev_pm_qos_hide_latency_limit(device) 152 Drop the request added by dev_pm_qos_expose_latency_limit() from the device's 153 PM QoS list of resume latency constraints and remove sysfs attribute 154 pm_qos_resume_latency_us from the device's power directory. 155 156int dev_pm_qos_expose_flags(device, value) 157 Add a request to the device's PM QoS list of flags and create sysfs attribute 158 pm_qos_no_power_off under the device's power directory allowing user space to 159 change the value of the PM_QOS_FLAG_NO_POWER_OFF flag. 160 161void dev_pm_qos_hide_flags(device) 162 Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS 163 list of flags and remove sysfs attribute pm_qos_no_power_off from the device's 164 power directory. 165 166Notification mechanisms: 167 168The per-device PM QoS framework has a per-device notification tree. 169 170int dev_pm_qos_add_notifier(device, notifier, type): 171 Adds a notification callback function for the device for a particular request 172 type. 173 174 The callback is called when the aggregated value of the device constraints 175 list is changed. 176 177int dev_pm_qos_remove_notifier(device, notifier, type): 178 Removes the notification callback function for the device. 179 180 181Active state latency tolerance 182^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 183 184This device PM QoS type is used to support systems in which hardware may switch 185to energy-saving operation modes on the fly. In those systems, if the operation 186mode chosen by the hardware attempts to save energy in an overly aggressive way, 187it may cause excess latencies to be visible to software, causing it to miss 188certain protocol requirements or target frame or sample rates etc. 189 190If there is a latency tolerance control mechanism for a given device available 191to software, the .set_latency_tolerance callback in that device's dev_pm_info 192structure should be populated. The routine pointed to by it is should implement 193whatever is necessary to transfer the effective requirement value to the 194hardware. 195 196Whenever the effective latency tolerance changes for the device, its 197.set_latency_tolerance() callback will be executed and the effective value will 198be passed to it. If that value is negative, which means that the list of 199latency tolerance requirements for the device is empty, the callback is expected 200to switch the underlying hardware latency tolerance control mechanism to an 201autonomous mode if available. If that value is PM_QOS_LATENCY_ANY, in turn, and 202the hardware supports a special "no requirement" setting, the callback is 203expected to use it. That allows software to prevent the hardware from 204automatically updating the device's latency tolerance in response to its power 205state changes (e.g. during transitions from D3cold to D0), which generally may 206be done in the autonomous latency tolerance control mode. 207 208If .set_latency_tolerance() is present for the device, sysfs attribute 209pm_qos_latency_tolerance_us will be present in the devivce's power directory. 210Then, user space can use that attribute to specify its latency tolerance 211requirement for the device, if any. Writing "any" to it means "no requirement, 212but do not let the hardware control latency tolerance" and writing "auto" to it 213allows the hardware to be switched to the autonomous mode if there are no other 214requirements from the kernel side in the device's list. 215 216Kernel code can use the functions described above along with the 217DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update 218latency tolerance requirements for devices. 219