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Documentation / power / pm_qos_interface.txt




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Based on kernel version 3.19. Page generated on 2015-02-13 21:22 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.  Two device PM QoS flags are defined currently: PM_QOS_FLAG_NO_POWER_OFF
102	and PM_QOS_FLAG_REMOTE_WAKEUP.
103	
104	Note: The aggregated target values are implemented in such a way that reading
105	the aggregated value does not require any locking mechanism.
106	
107	
108	From kernel mode the use of this interface is the following:
109	
110	int dev_pm_qos_add_request(device, handle, type, value):
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	
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	
139	int dev_pm_qos_add_ancestor_request(dev, handle, type, value)
140	Add a PM QoS request for the first direct ancestor of the given device whose
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).
144	
145	int dev_pm_qos_expose_latency_limit(device, value)
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.
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
152	PM QoS list of resume latency constraints and remove sysfs attribute
153	pm_qos_resume_latency_us from the device's power directory.
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.
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
172	is changed (for resume latency device PM QoS only).
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.
180	The callback is called when the aggregated value for any device is changed
181	(for resume latency device PM QoS only).
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.
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.
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