ia64/linux-2.6.18-xen.hg

view Documentation/power/states.txt @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
children
line source
2 System Power Management States
5 The kernel supports three power management states generically, though
6 each is dependent on platform support code to implement the low-level
7 details for each state. This file describes each state, what they are
8 commonly called, what ACPI state they map to, and what string to write
9 to /sys/power/state to enter that state
12 State: Standby / Power-On Suspend
13 ACPI State: S1
14 String: "standby"
16 This state offers minimal, though real, power savings, while providing
17 a very low-latency transition back to a working system. No operating
18 state is lost (the CPU retains power), so the system easily starts up
19 again where it left off.
21 We try to put devices in a low-power state equivalent to D1, which
22 also offers low power savings, but low resume latency. Not all devices
23 support D1, and those that don't are left on.
25 A transition from Standby to the On state should take about 1-2
26 seconds.
29 State: Suspend-to-RAM
30 ACPI State: S3
31 String: "mem"
33 This state offers significant power savings as everything in the
34 system is put into a low-power state, except for memory, which is
35 placed in self-refresh mode to retain its contents.
37 System and device state is saved and kept in memory. All devices are
38 suspended and put into D3. In many cases, all peripheral buses lose
39 power when entering STR, so devices must be able to handle the
40 transition back to the On state.
42 For at least ACPI, STR requires some minimal boot-strapping code to
43 resume the system from STR. This may be true on other platforms.
45 A transition from Suspend-to-RAM to the On state should take about
46 3-5 seconds.
49 State: Suspend-to-disk
50 ACPI State: S4
51 String: "disk"
53 This state offers the greatest power savings, and can be used even in
54 the absence of low-level platform support for power management. This
55 state operates similarly to Suspend-to-RAM, but includes a final step
56 of writing memory contents to disk. On resume, this is read and memory
57 is restored to its pre-suspend state.
59 STD can be handled by the firmware or the kernel. If it is handled by
60 the firmware, it usually requires a dedicated partition that must be
61 setup via another operating system for it to use. Despite the
62 inconvenience, this method requires minimal work by the kernel, since
63 the firmware will also handle restoring memory contents on resume.
65 If the kernel is responsible for persistantly saving state, a mechanism
66 called 'swsusp' (Swap Suspend) is used to write memory contents to
67 free swap space. swsusp has some restrictive requirements, but should
68 work in most cases. Some, albeit outdated, documentation can be found
69 in Documentation/power/swsusp.txt.
71 Once memory state is written to disk, the system may either enter a
72 low-power state (like ACPI S4), or it may simply power down. Powering
73 down offers greater savings, and allows this mechanism to work on any
74 system. However, entering a real low-power state allows the user to
75 trigger wake up events (e.g. pressing a key or opening a laptop lid).
77 A transition from Suspend-to-Disk to the On state should take about 30
78 seconds, though it's typically a bit more with the current
79 implementation.