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diff --git a/Documentation/power/states.txt b/Documentation/power/states.txt new file mode 100644 index 000000000..50f3ef917 --- /dev/null +++ b/Documentation/power/states.txt @@ -0,0 +1,109 @@ +System Power Management Sleep States + +(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> + +The kernel supports up to four system sleep states generically, although three +of them depend on the platform support code to implement the low-level details +for each state. + +The states are represented by strings that can be read or written to the +/sys/power/state file. Those strings may be "mem", "standby", "freeze" and +"disk", where the last one always represents hibernation (Suspend-To-Disk) and +the meaning of the remaining ones depends on the relative_sleep_states command +line argument. + +For relative_sleep_states=1, the strings "mem", "standby" and "freeze" label the +available non-hibernation sleep states from the deepest to the shallowest, +respectively. In that case, "mem" is always present in /sys/power/state, +because there is at least one non-hibernation sleep state in every system. If +the given system supports two non-hibernation sleep states, "standby" is present +in /sys/power/state in addition to "mem". If the system supports three +non-hibernation sleep states, "freeze" will be present in /sys/power/state in +addition to "mem" and "standby". + +For relative_sleep_states=0, which is the default, the following descriptions +apply. + +state: Suspend-To-Idle +ACPI state: S0 +Label: "freeze" + +This state is a generic, pure software, light-weight, system sleep state. +It allows more energy to be saved relative to runtime idle by freezing user +space and putting all I/O devices into low-power states (possibly +lower-power than available at run time), such that the processors can +spend more time in their idle states. + +This state can be used for platforms without Power-On Suspend/Suspend-to-RAM +support, or it can be used in addition to Suspend-to-RAM (memory sleep) +to provide reduced resume latency. It is always supported. + + +State: Standby / Power-On Suspend +ACPI State: S1 +Label: "standby" + +This state, if supported, offers moderate, though real, power savings, while +providing a relatively 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. + +In addition to freezing user space and putting all I/O devices into low-power +states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline +and all low-level system functions are suspended during transitions into this +state. For this reason, it should allow more energy to be saved relative to +Suspend-To-Idle, but the resume latency will generally be greater than for that +state. + + +State: Suspend-to-RAM +ACPI State: S3 +Label: "mem" + +This state, if supported, offers significant power savings as everything in the +system is put into a low-power state, except for memory, which should be placed +into the self-refresh mode to retain its contents. All of the steps carried out +when entering Power-On Suspend are also carried out during transitions to STR. +Additional operations may take place depending on the platform capabilities. In +particular, on ACPI systems the kernel passes control to the BIOS (platform +firmware) as the last step during STR transitions and that usually results in +powering down some more low-level components that aren't directly controlled by +the kernel. + +System and device state is saved and kept in memory. All devices are suspended +and put into low-power states. 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 it. This may be the case on other platforms too. + + +State: Suspend-to-disk +ACPI State: S4 +Label: "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. + +For suspend-to-disk, 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. +Alternatively, userspace can do most of the actual suspend to disk work, +see userland-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). |