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Diffstat (limited to 'Documentation/power/states.txt')
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diff --git a/Documentation/power/states.txt b/Documentation/power/states.txt new file mode 100644 index 000000000000..3e5e5d3ff419 --- /dev/null +++ b/Documentation/power/states.txt @@ -0,0 +1,79 @@ + +System Power Management States + + +The kernel supports three power management states generically, though +each is dependent on platform support code to implement the low-level +details for each state. This file describes each state, what they are +commonly called, what ACPI state they map to, and what string to write +to /sys/power/state to enter that state + + +State: Standby / Power-On Suspend +ACPI State: S1 +String: "standby" + +This state offers minimal, though real, power savings, while providing +a very low-latency transition back to a working system. No operating +state is lost (the CPU retains power), so the system easily starts up +again where it left off. + +We try to put devices in a low-power state equivalent to D1, which +also offers low power savings, but low resume latency. Not all devices +support D1, and those that don't are left on. + +A transition from Standby to the On state should take about 1-2 +seconds. + + +State: Suspend-to-RAM +ACPI State: S3 +String: "mem" + +This state offers significant power savings as everything in the +system is put into a low-power state, except for memory, which is +placed in self-refresh mode to retain its contents. + +System and device state is saved and kept in memory. All devices are +suspended and put into D3. In many cases, all peripheral buses lose +power when entering STR, so devices must be able to handle the +transition back to the On state. + +For at least ACPI, STR requires some minimal boot-strapping code to +resume the system from STR. This may be true on other platforms. + +A transition from Suspend-to-RAM to the On state should take about +3-5 seconds. + + +State: Suspend-to-disk +ACPI State: S4 +String: "disk" + +This state offers the greatest power savings, and can be used even in +the absence of low-level platform support for power management. This +state operates similarly to Suspend-to-RAM, but includes a final step +of writing memory contents to disk. On resume, this is read and memory +is restored to its pre-suspend state. + +STD can be handled by the firmware or the kernel. If it is handled by +the firmware, it usually requires a dedicated partition that must be +setup via another operating system for it to use. Despite the +inconvenience, this method requires minimal work by the kernel, since +the firmware will also handle restoring memory contents on resume. + +If the kernel is responsible for persistantly saving state, a mechanism +called 'swsusp' (Swap Suspend) is used to write memory contents to +free swap space. swsusp has some restrictive requirements, but should +work in most cases. Some, albeit outdated, documentation can be found +in Documentation/power/swsusp.txt. + +Once memory state is written to disk, the system may either enter a +low-power state (like ACPI S4), or it may simply power down. Powering +down offers greater savings, and allows this mechanism to work on any +system. However, entering a real low-power state allows the user to +trigger wake up events (e.g. pressing a key or opening a laptop lid). + +A transition from Suspend-to-Disk to the On state should take about 30 +seconds, though it's typically a bit more with the current +implementation. |