About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / power / pm_qos_interface.txt


Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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.  One device PM QoS flag is defined currently: PM_QOS_FLAG_NO_POWER_OFF.
102	
103	Note: The aggregated target values are implemented in such a way that reading
104	the aggregated value does not require any locking mechanism.
105	
106	
107	From kernel mode the use of this interface is the following:
108	
109	int dev_pm_qos_add_request(device, handle, type, value):
110	Will insert an element into the list for that identified device with the
111	target value.  Upon change to this list the new target is recomputed and any
112	registered notifiers are called only if the target value is now different.
113	Clients of dev_pm_qos need to save the handle for future use in other
114	dev_pm_qos API functions.
115	
116	int dev_pm_qos_update_request(handle, new_value):
117	Will update the list element pointed to by the handle with the new target value
118	and recompute the new aggregated target, calling the notification trees if the
119	target is changed.
120	
121	int dev_pm_qos_remove_request(handle):
122	Will remove the element.  After removal it will update the aggregate target and
123	call the notification trees if the target was changed as a result of removing
124	the request.
125	
126	s32 dev_pm_qos_read_value(device):
127	Returns the aggregated value for a given device's constraints list.
128	
129	enum pm_qos_flags_status dev_pm_qos_flags(device, mask)
130	Check PM QoS flags of the given device against the given mask of flags.
131	The meaning of the return values is as follows:
132		PM_QOS_FLAGS_ALL: All flags from the mask are set
133		PM_QOS_FLAGS_SOME: Some flags from the mask are set
134		PM_QOS_FLAGS_NONE: No flags from the mask are set
135		PM_QOS_FLAGS_UNDEFINED: The device's PM QoS structure has not been
136				initialized or the list of requests is empty.
137	
138	int dev_pm_qos_add_ancestor_request(dev, handle, type, value)
139	Add a PM QoS request for the first direct ancestor of the given device whose
140	power.ignore_children flag is unset (for DEV_PM_QOS_RESUME_LATENCY requests)
141	or whose power.set_latency_tolerance callback pointer is not NULL (for
142	DEV_PM_QOS_LATENCY_TOLERANCE requests).
143	
144	int dev_pm_qos_expose_latency_limit(device, value)
145	Add a request to the device's PM QoS list of resume latency constraints and
146	create a sysfs attribute pm_qos_resume_latency_us under the device's power
147	directory allowing user space to manipulate that request.
148	
149	void dev_pm_qos_hide_latency_limit(device)
150	Drop the request added by dev_pm_qos_expose_latency_limit() from the device's
151	PM QoS list of resume latency constraints and remove sysfs attribute
152	pm_qos_resume_latency_us from the device's power directory.
153	
154	int dev_pm_qos_expose_flags(device, value)
155	Add a request to the device's PM QoS list of flags and create sysfs attribute
156	pm_qos_no_power_off under the device's power directory allowing user space to
157	change the value of the PM_QOS_FLAG_NO_POWER_OFF flag.
158	
159	void dev_pm_qos_hide_flags(device)
160	Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS list
161	of flags and remove sysfs attribute pm_qos_no_power_off from the device's power
162	directory.
163	
164	Notification mechanisms:
165	The per-device PM QoS framework has a per-device notification tree.
166	
167	int dev_pm_qos_add_notifier(device, notifier):
168	Adds a notification callback function for the device.
169	The callback is called when the aggregated value of the device constraints list
170	is changed (for resume latency device PM QoS only).
171	
172	int dev_pm_qos_remove_notifier(device, notifier):
173	Removes the notification callback function for the device.
174	
175	
176	Active state latency tolerance
177	
178	This device PM QoS type is used to support systems in which hardware may switch
179	to energy-saving operation modes on the fly.  In those systems, if the operation
180	mode chosen by the hardware attempts to save energy in an overly aggressive way,
181	it may cause excess latencies to be visible to software, causing it to miss
182	certain protocol requirements or target frame or sample rates etc.
183	
184	If there is a latency tolerance control mechanism for a given device available
185	to software, the .set_latency_tolerance callback in that device's dev_pm_info
186	structure should be populated.  The routine pointed to by it is should implement
187	whatever is necessary to transfer the effective requirement value to the
188	hardware.
189	
190	Whenever the effective latency tolerance changes for the device, its
191	.set_latency_tolerance() callback will be executed and the effective value will
192	be passed to it.  If that value is negative, which means that the list of
193	latency tolerance requirements for the device is empty, the callback is expected
194	to switch the underlying hardware latency tolerance control mechanism to an
195	autonomous mode if available.  If that value is PM_QOS_LATENCY_ANY, in turn, and
196	the hardware supports a special "no requirement" setting, the callback is
197	expected to use it.  That allows software to prevent the hardware from
198	automatically updating the device's latency tolerance in response to its power
199	state changes (e.g. during transitions from D3cold to D0), which generally may
200	be done in the autonomous latency tolerance control mode.
201	
202	If .set_latency_tolerance() is present for the device, sysfs attribute
203	pm_qos_latency_tolerance_us will be present in the devivce's power directory.
204	Then, user space can use that attribute to specify its latency tolerance
205	requirement for the device, if any.  Writing "any" to it means "no requirement,
206	but do not let the hardware control latency tolerance" and writing "auto" to it
207	allows the hardware to be switched to the autonomous mode if there are no other
208	requirements from the kernel side in the device's list.
209	
210	Kernel code can use the functions described above along with the
211	DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update
212	latency tolerance requirements for devices.
Hide Line Numbers


About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License.