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Documentation / power / pm_qos_interface.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:12 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.
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, type, 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.
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