Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.
1 2 System Power Management States 3 4 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 9 commonly called, what ACPI state they map to, and what string to write 10 to /sys/power/state to enter that state 11 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 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 40 State: Suspend-to-RAM 41 ACPI State: S3 42 String: "mem" 43 44 This state offers significant power savings as everything in the 45 system is put into a low-power state, except for memory, which is 46 placed in self-refresh mode to retain its contents. 47 48 System and device state is saved and kept in memory. All devices are 49 suspended and put into D3. In many cases, all peripheral buses lose 50 power when entering STR, so devices must be able to handle the 51 transition back to the On state. 52 53 For at least ACPI, STR requires some minimal boot-strapping code to 54 resume the system from STR. This may be true on other platforms. 55 56 57 State: Suspend-to-disk 58 ACPI State: S4 59 String: "disk" 60 61 This state offers the greatest power savings, and can be used even in 62 the absence of low-level platform support for power management. This 63 state operates similarly to Suspend-to-RAM, but includes a final step 64 of writing memory contents to disk. On resume, this is read and memory 65 is restored to its pre-suspend state. 66 67 STD can be handled by the firmware or the kernel. If it is handled by 68 the firmware, it usually requires a dedicated partition that must be 69 setup via another operating system for it to use. Despite the 70 inconvenience, this method requires minimal work by the kernel, since 71 the firmware will also handle restoring memory contents on resume. 72 73 For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used 74 to write memory contents to free swap space. swsusp has some restrictive 75 requirements, but should work in most cases. Some, albeit outdated, 76 documentation can be found in Documentation/power/swsusp.txt. 77 Alternatively, userspace can do most of the actual suspend to disk work, 78 see userland-swsusp.txt. 79 80 Once memory state is written to disk, the system may either enter a 81 low-power state (like ACPI S4), or it may simply power down. Powering 82 down offers greater savings, and allows this mechanism to work on any 83 system. However, entering a real low-power state allows the user to 84 trigger wake up events (e.g. pressing a key or opening a laptop lid).