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1 | |
2 | System Power Management States | |
3 | ||
4 | ||
dc5aeae4 ZR |
5 | The kernel supports four power management states generically, though |
6 | one is generic and the other three are dependent on platform support | |
7 | code to implement the low-level details for each state. | |
8 | This file describes each state, what they are | |
1da177e4 LT |
9 | commonly called, what ACPI state they map to, and what string to write |
10 | to /sys/power/state to enter that state | |
11 | ||
dc5aeae4 ZR |
12 | state: Freeze / Low-Power Idle |
13 | ACPI state: S0 | |
14 | String: "freeze" | |
15 | ||
16 | This state is a generic, pure software, light-weight, low-power state. | |
17 | It allows more energy to be saved relative to idle by freezing user | |
18 | space and putting all I/O devices into low-power states (possibly | |
19 | lower-power than available at run time), such that the processors can | |
20 | spend more time in their idle states. | |
21 | This state can be used for platforms without Standby/Suspend-to-RAM | |
22 | support, or it can be used in addition to Suspend-to-RAM (memory sleep) | |
23 | to provide reduced resume latency. | |
24 | ||
1da177e4 LT |
25 | |
26 | State: Standby / Power-On Suspend | |
27 | ACPI State: S1 | |
28 | String: "standby" | |
29 | ||
30 | This state offers minimal, though real, power savings, while providing | |
31 | a very low-latency transition back to a working system. No operating | |
32 | state is lost (the CPU retains power), so the system easily starts up | |
33 | again where it left off. | |
34 | ||
35 | We try to put devices in a low-power state equivalent to D1, which | |
36 | also offers low power savings, but low resume latency. Not all devices | |
37 | support D1, and those that don't are left on. | |
38 | ||
39 | A transition from Standby to the On state should take about 1-2 | |
40 | seconds. | |
41 | ||
42 | ||
43 | State: Suspend-to-RAM | |
44 | ACPI State: S3 | |
45 | String: "mem" | |
46 | ||
47 | This state offers significant power savings as everything in the | |
48 | system is put into a low-power state, except for memory, which is | |
49 | placed in self-refresh mode to retain its contents. | |
50 | ||
51 | System and device state is saved and kept in memory. All devices are | |
52 | suspended and put into D3. In many cases, all peripheral buses lose | |
53 | power when entering STR, so devices must be able to handle the | |
54 | transition back to the On state. | |
55 | ||
56 | For at least ACPI, STR requires some minimal boot-strapping code to | |
57 | resume the system from STR. This may be true on other platforms. | |
58 | ||
59 | A transition from Suspend-to-RAM to the On state should take about | |
60 | 3-5 seconds. | |
61 | ||
62 | ||
63 | State: Suspend-to-disk | |
64 | ACPI State: S4 | |
65 | String: "disk" | |
66 | ||
67 | This state offers the greatest power savings, and can be used even in | |
68 | the absence of low-level platform support for power management. This | |
69 | state operates similarly to Suspend-to-RAM, but includes a final step | |
70 | of writing memory contents to disk. On resume, this is read and memory | |
71 | is restored to its pre-suspend state. | |
72 | ||
73 | STD can be handled by the firmware or the kernel. If it is handled by | |
74 | the firmware, it usually requires a dedicated partition that must be | |
75 | setup via another operating system for it to use. Despite the | |
76 | inconvenience, this method requires minimal work by the kernel, since | |
77 | the firmware will also handle restoring memory contents on resume. | |
78 | ||
a8b7228c AC |
79 | For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used |
80 | to write memory contents to free swap space. swsusp has some restrictive | |
81 | requirements, but should work in most cases. Some, albeit outdated, | |
82 | documentation can be found in Documentation/power/swsusp.txt. | |
83 | Alternatively, userspace can do most of the actual suspend to disk work, | |
84 | see userland-swsusp.txt. | |
1da177e4 LT |
85 | |
86 | Once memory state is written to disk, the system may either enter a | |
87 | low-power state (like ACPI S4), or it may simply power down. Powering | |
88 | down offers greater savings, and allows this mechanism to work on any | |
89 | system. However, entering a real low-power state allows the user to | |
11d77d0c | 90 | trigger wake up events (e.g. pressing a key or opening a laptop lid). |
1da177e4 LT |
91 | |
92 | A transition from Suspend-to-Disk to the On state should take about 30 | |
93 | seconds, though it's typically a bit more with the current | |
94 | implementation. |