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bf1db69f | 1 | PM Quality Of Service Interface. |
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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 | ||
e3cba324 | 7 | Two different PM QoS frameworks are available: |
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8 | 1. PM QoS classes for cpu_dma_latency, network_latency, network_throughput, |
9 | memory_bandwidth. | |
e3cba324 | 10 | 2. the per-device PM QoS framework provides the API to manage the per-device latency |
d30b82a4 | 11 | constraints and PM QoS flags. |
d82b3518 | 12 | |
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13 | Each parameters have defined units: |
14 | * latency: usec | |
15 | * timeout: usec | |
16 | * throughput: kbs (kilo bit / sec) | |
7990da71 | 17 | * memory bandwidth: mbs (mega bit / sec) |
bf1db69f | 18 | |
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19 | |
20 | 1. PM QoS framework | |
21 | ||
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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 | ||
ed77134b | 28 | For each parameter a list of performance requests is maintained along with |
d82b3518 | 29 | an aggregated target value. The aggregated target value is updated with |
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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 | |
d82b3518 | 32 | in the parameter list elements. |
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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 | ||
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36 | |
37 | From kernel mode the use of this interface is simple: | |
d82b3518 | 38 | |
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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 | |
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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. | |
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43 | Clients of pm_qos need to save the returned handle for future use in other |
44 | pm_qos API functions. | |
d82b3518 | 45 | |
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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. | |
d82b3518 | 55 | |
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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 | ||
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70 | |
71 | From user mode: | |
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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: | |
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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 | |
ed77134b | 80 | request on the parameter. |
d82b3518 | 81 | |
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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. | |
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86 | |
87 | To remove the user mode request for a target value simply close the device | |
88 | node. | |
89 | ||
90 | ||
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91 | 2. PM QoS per-device latency and flags framework |
92 | ||
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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. | |
d30b82a4 | 97 | |
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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. Two device PM QoS flags are defined currently: PM_QOS_FLAG_NO_POWER_OFF | |
102 | and PM_QOS_FLAG_REMOTE_WAKEUP. | |
e3cba324 | 103 | |
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104 | Note: The aggregated target values are implemented in such a way that reading |
105 | the aggregated value does not require any locking mechanism. | |
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106 | |
107 | ||
108 | From kernel mode the use of this interface is the following: | |
109 | ||
ae0fb4b7 | 110 | int dev_pm_qos_add_request(device, handle, type, value): |
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111 | Will insert an element into the list for that identified device with the |
112 | target value. Upon change to this list the new target is recomputed and any | |
113 | registered notifiers are called only if the target value is now different. | |
114 | Clients of dev_pm_qos need to save the handle for future use in other | |
115 | dev_pm_qos API functions. | |
116 | ||
117 | int dev_pm_qos_update_request(handle, new_value): | |
118 | Will update the list element pointed to by the handle with the new target value | |
119 | and recompute the new aggregated target, calling the notification trees if the | |
120 | target is changed. | |
121 | ||
122 | int dev_pm_qos_remove_request(handle): | |
123 | Will remove the element. After removal it will update the aggregate target and | |
124 | call the notification trees if the target was changed as a result of removing | |
125 | the request. | |
126 | ||
127 | s32 dev_pm_qos_read_value(device): | |
128 | Returns the aggregated value for a given device's constraints list. | |
129 | ||
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130 | enum pm_qos_flags_status dev_pm_qos_flags(device, mask) |
131 | Check PM QoS flags of the given device against the given mask of flags. | |
132 | The meaning of the return values is as follows: | |
133 | PM_QOS_FLAGS_ALL: All flags from the mask are set | |
134 | PM_QOS_FLAGS_SOME: Some flags from the mask are set | |
135 | PM_QOS_FLAGS_NONE: No flags from the mask are set | |
136 | PM_QOS_FLAGS_UNDEFINED: The device's PM QoS structure has not been | |
137 | initialized or the list of requests is empty. | |
138 | ||
71d821fd | 139 | int dev_pm_qos_add_ancestor_request(dev, handle, type, value) |
d30b82a4 | 140 | Add a PM QoS request for the first direct ancestor of the given device whose |
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141 | power.ignore_children flag is unset (for DEV_PM_QOS_RESUME_LATENCY requests) |
142 | or whose power.set_latency_tolerance callback pointer is not NULL (for | |
143 | DEV_PM_QOS_LATENCY_TOLERANCE requests). | |
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144 | |
145 | int dev_pm_qos_expose_latency_limit(device, value) | |
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146 | Add a request to the device's PM QoS list of resume latency constraints and |
147 | create a sysfs attribute pm_qos_resume_latency_us under the device's power | |
148 | directory allowing user space to manipulate that request. | |
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149 | |
150 | void dev_pm_qos_hide_latency_limit(device) | |
151 | Drop the request added by dev_pm_qos_expose_latency_limit() from the device's | |
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152 | PM QoS list of resume latency constraints and remove sysfs attribute |
153 | pm_qos_resume_latency_us from the device's power directory. | |
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154 | |
155 | int dev_pm_qos_expose_flags(device, value) | |
156 | Add a request to the device's PM QoS list of flags and create sysfs attributes | |
157 | pm_qos_no_power_off and pm_qos_remote_wakeup under the device's power directory | |
158 | allowing user space to change these flags' value. | |
159 | ||
160 | void dev_pm_qos_hide_flags(device) | |
161 | Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS list | |
162 | of flags and remove sysfs attributes pm_qos_no_power_off and pm_qos_remote_wakeup | |
163 | under the device's power directory. | |
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164 | |
165 | Notification mechanisms: | |
166 | The per-device PM QoS framework has 2 different and distinct notification trees: | |
167 | a per-device notification tree and a global notification tree. | |
168 | ||
169 | int dev_pm_qos_add_notifier(device, notifier): | |
170 | Adds a notification callback function for the device. | |
171 | The callback is called when the aggregated value of the device constraints list | |
b02f6695 | 172 | is changed (for resume latency device PM QoS only). |
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173 | |
174 | int dev_pm_qos_remove_notifier(device, notifier): | |
175 | Removes the notification callback function for the device. | |
176 | ||
177 | int dev_pm_qos_add_global_notifier(notifier): | |
178 | Adds a notification callback function in the global notification tree of the | |
179 | framework. | |
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180 | The callback is called when the aggregated value for any device is changed |
181 | (for resume latency device PM QoS only). | |
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182 | |
183 | int dev_pm_qos_remove_global_notifier(notifier): | |
184 | Removes the notification callback function from the global notification tree | |
185 | of the framework. | |
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186 | |
187 | ||
188 | Active state latency tolerance | |
189 | ||
190 | This device PM QoS type is used to support systems in which hardware may switch | |
191 | to energy-saving operation modes on the fly. In those systems, if the operation | |
192 | mode chosen by the hardware attempts to save energy in an overly aggressive way, | |
193 | it may cause excess latencies to be visible to software, causing it to miss | |
194 | certain protocol requirements or target frame or sample rates etc. | |
195 | ||
196 | If there is a latency tolerance control mechanism for a given device available | |
197 | to software, the .set_latency_tolerance callback in that device's dev_pm_info | |
198 | structure should be populated. The routine pointed to by it is should implement | |
199 | whatever is necessary to transfer the effective requirement value to the | |
200 | hardware. | |
201 | ||
202 | Whenever the effective latency tolerance changes for the device, its | |
203 | .set_latency_tolerance() callback will be executed and the effective value will | |
204 | be passed to it. If that value is negative, which means that the list of | |
205 | latency tolerance requirements for the device is empty, the callback is expected | |
206 | to switch the underlying hardware latency tolerance control mechanism to an | |
207 | autonomous mode if available. If that value is PM_QOS_LATENCY_ANY, in turn, and | |
208 | the hardware supports a special "no requirement" setting, the callback is | |
209 | expected to use it. That allows software to prevent the hardware from | |
210 | automatically updating the device's latency tolerance in response to its power | |
211 | state changes (e.g. during transitions from D3cold to D0), which generally may | |
212 | be done in the autonomous latency tolerance control mode. | |
213 | ||
214 | If .set_latency_tolerance() is present for the device, sysfs attribute | |
215 | pm_qos_latency_tolerance_us will be present in the devivce's power directory. | |
216 | Then, user space can use that attribute to specify its latency tolerance | |
217 | requirement for the device, if any. Writing "any" to it means "no requirement, | |
218 | but do not let the hardware control latency tolerance" and writing "auto" to it | |
219 | allows the hardware to be switched to the autonomous mode if there are no other | |
220 | requirements from the kernel side in the device's list. | |
221 | ||
222 | Kernel code can use the functions described above along with the | |
223 | DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update | |
224 | latency tolerance requirements for devices. |