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Documentation / power / pm_qos_interface.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 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.
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