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1 Linux Power Management Support
2
3This document briefly describes how to use power management with your
4Linux system and how to add power management support to Linux drivers.
5
6APM or ACPI?
7------------
8If you have a relatively recent x86 mobile, desktop, or server system,
9odds are it supports either Advanced Power Management (APM) or
10Advanced Configuration and Power Interface (ACPI). ACPI is the newer
11of the two technologies and puts power management in the hands of the
12operating system, allowing for more intelligent power management than
13is possible with BIOS controlled APM.
14
15The best way to determine which, if either, your system supports is to
16build a kernel with both ACPI and APM enabled (as of 2.3.x ACPI is
17enabled by default). If a working ACPI implementation is found, the
18ACPI driver will override and disable APM, otherwise the APM driver
19will be used.
20
21No, sorry, you cannot have both ACPI and APM enabled and running at
22once. Some people with broken ACPI or broken APM implementations
23would like to use both to get a full set of working features, but you
24simply cannot mix and match the two. Only one power management
25interface can be in control of the machine at once. Think about it..
26
27User-space Daemons
28------------------
29Both APM and ACPI rely on user-space daemons, apmd and acpid
30respectively, to be completely functional. Obtain both of these
31daemons from your Linux distribution or from the Internet (see below)
32and be sure that they are started sometime in the system boot process.
33Go ahead and start both. If ACPI or APM is not available on your
34system the associated daemon will exit gracefully.
35
36 apmd: http://worldvisions.ca/~apenwarr/apmd/
37 acpid: http://acpid.sf.net/
38
39Driver Interface -- OBSOLETE, DO NOT USE!
40----------------*************************
41
42Note: pm_register(), pm_access(), pm_dev_idle() and friends are
43obsolete. Please do not use them. Instead you should properly hook
44your driver into the driver model, and use its suspend()/resume()
45callbacks to do this kind of stuff.
46
47If you are writing a new driver or maintaining an old driver, it
48should include power management support. Without power management
49support, a single driver may prevent a system with power management
50capabilities from ever being able to suspend (safely).
51
52Overview:
531) Register each instance of a device with "pm_register"
542) Call "pm_access" before accessing the hardware.
55 (this will ensure that the hardware is awake and ready)
563) Your "pm_callback" is called before going into a
57 suspend state (ACPI D1-D3) or after resuming (ACPI D0)
58 from a suspend.
594) Call "pm_dev_idle" when the device is not being used
60 (optional but will improve device idle detection)
615) When unloaded, unregister the device with "pm_unregister"
62
63/*
64 * Description: Register a device with the power-management subsystem
65 *
66 * Parameters:
67 * type - device type (PCI device, system device, ...)
68 * id - instance number or unique identifier
69 * cback - request handler callback (suspend, resume, ...)
70 *
71 * Returns: Registered PM device or NULL on error
72 *
73 * Examples:
74 * dev = pm_register(PM_SYS_DEV, PM_SYS_VGA, vga_callback);
75 *
76 * struct pci_dev *pci_dev = pci_find_dev(...);
77 * dev = pm_register(PM_PCI_DEV, PM_PCI_ID(pci_dev), callback);
78 */
79struct pm_dev *pm_register(pm_dev_t type, unsigned long id, pm_callback cback);
80
81/*
82 * Description: Unregister a device with the power management subsystem
83 *
84 * Parameters:
85 * dev - PM device previously returned from pm_register
86 */
87void pm_unregister(struct pm_dev *dev);
88
89/*
90 * Description: Unregister all devices with a matching callback function
91 *
92 * Parameters:
93 * cback - previously registered request callback
94 *
95 * Notes: Provided for easier porting from old APM interface
96 */
97void pm_unregister_all(pm_callback cback);
98
99/*
100 * Power management request callback
101 *
102 * Parameters:
103 * dev - PM device previously returned from pm_register
104 * rqst - request type
105 * data - data, if any, associated with the request
106 *
107 * Returns: 0 if the request is successful
108 * EINVAL if the request is not supported
109 * EBUSY if the device is now busy and cannot handle the request
110 * ENOMEM if the device was unable to handle the request due to memory
111 *
112 * Details: The device request callback will be called before the
113 * device/system enters a suspend state (ACPI D1-D3) or
114 * or after the device/system resumes from suspend (ACPI D0).
115 * For PM_SUSPEND, the ACPI D-state being entered is passed
116 * as the "data" argument to the callback. The device
117 * driver should save (PM_SUSPEND) or restore (PM_RESUME)
118 * device context when the request callback is called.
119 *
120 * Once a driver returns 0 (success) from a suspend
121 * request, it should not process any further requests or
122 * access the device hardware until a call to "pm_access" is made.
123 */
124typedef int (*pm_callback)(struct pm_dev *dev, pm_request_t rqst, void *data);
125
126Driver Details
127--------------
128This is just a quick Q&A as a stopgap until a real driver writers'
129power management guide is available.
130
131Q: When is a device suspended?
132
133Devices can be suspended based on direct user request (eg. laptop lid
134closes), system power policy (eg. sleep after 30 minutes of console
135inactivity), or device power policy (eg. power down device after 5
136minutes of inactivity)
137
138Q: Must a driver honor a suspend request?
139
140No, a driver can return -EBUSY from a suspend request and this
141will stop the system from suspending. When a suspend request
142fails, all suspended devices are resumed and the system continues
143to run. Suspend can be retried at a later time.
144
145Q: Can the driver block suspend/resume requests?
146
147Yes, a driver can delay its return from a suspend or resume
148request until the device is ready to handle requests. It
149is advantageous to return as quickly as possible from a
150request as suspend/resume are done serially.
151
152Q: What context is a suspend/resume initiated from?
153
154A suspend or resume is initiated from a kernel thread context.
155It is safe to block, allocate memory, initiate requests
156or anything else you can do within the kernel.
157
158Q: Will requests continue to arrive after a suspend?
159
160Possibly. It is the driver's responsibility to queue(*),
161fail, or drop any requests that arrive after returning
162success to a suspend request. It is important that the
163driver not access its device until after it receives
164a resume request as the device's bus may no longer
165be active.
166
167(*) If a driver queues requests for processing after
168 resume be aware that the device, network, etc.
169 might be in a different state than at suspend time.
170 It's probably better to drop requests unless
171 the driver is a storage device.
172
173Q: Do I have to manage bus-specific power management registers
174
175No. It is the responsibility of the bus driver to manage
176PCI, USB, etc. power management registers. The bus driver
177or the power management subsystem will also enable any
178wake-on functionality that the device has.
179
180Q: So, really, what do I need to do to support suspend/resume?
181
182You need to save any device context that would
183be lost if the device was powered off and then restore
184it at resume time. When ACPI is active, there are
185three levels of device suspend states; D1, D2, and D3.
186(The suspend state is passed as the "data" argument
187to the device callback.) With D3, the device is powered
188off and loses all context, D1 and D2 are shallower power
189states and require less device context to be saved. To
190play it safe, just save everything at suspend and restore
191everything at resume.
192
193Q: Where do I store device context for suspend?
194
195Anywhere in memory, kmalloc a buffer or store it
196in the device descriptor. You are guaranteed that the
197contents of memory will be restored and accessible
198before resume, even when the system suspends to disk.
199
200Q: What do I need to do for ACPI vs. APM vs. etc?
201
202Drivers need not be aware of the specific power management
203technology that is active. They just need to be aware
204of when the overlying power management system requests
205that they suspend or resume.
206
207Q: What about device dependencies?
208
209When a driver registers a device, the power management
210subsystem uses the information provided to build a
211tree of device dependencies (eg. USB device X is on
212USB controller Y which is on PCI bus Z) When power
213management wants to suspend a device, it first sends
214a suspend request to its driver, then the bus driver,
215and so on up to the system bus. Device resumes
216proceed in the opposite direction.
217
218Q: Who do I contact for additional information about
219 enabling power management for my specific driver/device?
220
221ACPI Development mailing list: linux-acpi@vger.kernel.org
222
223System Interface -- OBSOLETE, DO NOT USE!
224----------------*************************
225If you are providing new power management support to Linux (ie.
226adding support for something like APM or ACPI), you should
227communicate with drivers through the existing generic power
228management interface.
229
230/*
231 * Send a request to all devices
232 *
233 * Parameters:
234 * rqst - request type
235 * data - data, if any, associated with the request
236 *
237 * Returns: 0 if the request is successful
238 * See "pm_callback" return for errors
239 *
240 * Details: Walk list of registered devices and call pm_send
241 * for each until complete or an error is encountered.
242 * If an error is encountered for a suspend request,
243 * return all devices to the state they were in before
244 * the suspend request.
245 */
246int pm_send_all(pm_request_t rqst, void *data);
247
248/*
249 * Find a matching device
250 *
251 * Parameters:
252 * type - device type (PCI device, system device, or 0 to match all devices)
253 * from - previous match or NULL to start from the beginning
254 *
255 * Returns: Matching device or NULL if none found
256 */
257struct pm_dev *pm_find(pm_dev_t type, struct pm_dev *from);