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947d2c2c RW |
1 | Power Management Interface for System Sleep |
2 | ||
3 | Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> | |
4 | ||
5 | The power management subsystem provides userspace with a unified sysfs interface | |
6 | for system sleep regardless of the underlying system architecture or platform. | |
7 | The interface is located in the /sys/power/ directory (assuming that sysfs is | |
8 | mounted at /sys). | |
9 | ||
10 | /sys/power/state is the system sleep state control file. | |
11 | ||
12 | Reading from it returns a list of supported sleep states, encoded as: | |
13 | ||
14 | 'freeze' (Suspend-to-Idle) | |
15 | 'standby' (Power-On Suspend) | |
16 | 'mem' (Suspend-to-RAM) | |
17 | 'disk' (Suspend-to-Disk) | |
18 | ||
19 | Suspend-to-Idle is always supported. Suspend-to-Disk is always supported | |
20 | too as long the kernel has been configured to support hibernation at all | |
21 | (ie. CONFIG_HIBERNATION is set in the kernel configuration file). Support | |
22 | for Suspend-to-RAM and Power-On Suspend depends on the capabilities of the | |
23 | platform. | |
24 | ||
25 | If one of the strings listed in /sys/power/state is written to it, the system | |
26 | will attempt to transition into the corresponding sleep state. Refer to | |
27 | Documentation/power/states.txt for a description of each of those states. | |
28 | ||
29 | /sys/power/disk controls the operating mode of hibernation (Suspend-to-Disk). | |
30 | Specifically, it tells the kernel what to do after creating a hibernation image. | |
31 | ||
32 | Reading from it returns a list of supported options encoded as: | |
33 | ||
34 | 'platform' (put the system into sleep using a platform-provided method) | |
35 | 'shutdown' (shut the system down) | |
36 | 'reboot' (reboot the system) | |
37 | 'suspend' (trigger a Suspend-to-RAM transition) | |
38 | 'test_resume' (resume-after-hibernation test mode) | |
39 | ||
40 | The currently selected option is printed in square brackets. | |
41 | ||
42 | The 'platform' option is only available if the platform provides a special | |
43 | mechanism to put the system to sleep after creating a hibernation image (ACPI | |
44 | does that, for example). The 'suspend' option is available if Suspend-to-RAM | |
45 | is supported. Refer to Documentation/power/basic_pm_debugging.txt for the | |
46 | description of the 'test_resume' option. | |
47 | ||
48 | To select an option, write the string representing it to /sys/power/disk. | |
49 | ||
50 | /sys/power/image_size controls the size of hibernation images. | |
51 | ||
52 | It can be written a string representing a non-negative integer that will be | |
53 | used as a best-effort upper limit of the image size, in bytes. The hibernation | |
54 | core will do its best to ensure that the image size will not exceed that number. | |
55 | However, if that turns out to be impossible to achieve, a hibernation image will | |
56 | still be created and its size will be as small as possible. In particular, | |
57 | writing '0' to this file will enforce hibernation images to be as small as | |
58 | possible. | |
59 | ||
60 | Reading from this file returns the current image size limit, which is set to | |
61 | around 2/5 of available RAM by default. | |
62 | ||
63 | /sys/power/pm_trace controls the PM trace mechanism saving the last suspend | |
64 | or resume event point in the RTC across reboots. | |
65 | ||
66 | It helps to debug hard lockups or reboots due to device driver failures that | |
67 | occur during system suspend or resume (which is more common) more effectively. | |
68 | ||
69 | If /sys/power/pm_trace contains '1', the fingerprint of each suspend/resume | |
70 | event point in turn will be stored in the RTC memory (overwriting the actual | |
71 | RTC information), so it will survive a system crash if one occurs right after | |
72 | storing it and it can be used later to identify the driver that caused the crash | |
73 | to happen (see Documentation/power/s2ram.txt for more information). | |
74 | ||
75 | Initially it contains '0' which may be changed to '1' by writing a string | |
76 | representing a nonzero integer into it. |