Based on kernel version 3.9. Page generated on 2013-05-02 23:12 EST.
1 2 System Power Management States 3 4 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 10 11 12 State: Standby / Power-On Suspend 13 ACPI State: S1 14 String: "standby" 15 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. 20 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. 24 25 A transition from Standby to the On state should take about 1-2 26 seconds. 27 28 29 State: Suspend-to-RAM 30 ACPI State: S3 31 String: "mem" 32 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. 36 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. 41 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. 44 45 A transition from Suspend-to-RAM to the On state should take about 46 3-5 seconds. 47 48 49 State: Suspend-to-disk 50 ACPI State: S4 51 String: "disk" 52 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. 58 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. 64 65 For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used 66 to write memory contents to free swap space. swsusp has some restrictive 67 requirements, but should work in most cases. Some, albeit outdated, 68 documentation can be found in Documentation/power/swsusp.txt. 69 Alternatively, userspace can do most of the actual suspend to disk work, 70 see userland-swsusp.txt. 71 72 Once memory state is written to disk, the system may either enter a 73 low-power state (like ACPI S4), or it may simply power down. Powering 74 down offers greater savings, and allows this mechanism to work on any 75 system. However, entering a real low-power state allows the user to 76 trigger wake up events (e.g. pressing a key or opening a laptop lid). 77 78 A transition from Suspend-to-Disk to the On state should take about 30 79 seconds, though it's typically a bit more with the current 80 implementation.