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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 deleted file mode 100644 index bc4548245a24..000000000000 --- a/Documentation/power/states.txt +++ /dev/null @@ -1,125 +0,0 @@ -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 three always represent Power-On Suspend (if supported), -Suspend-To-Idle and hibernation (Suspend-To-Disk), respectively. - -The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file. -It contains strings representing the available modes of system suspend that may -be triggered by writing "mem" to /sys/power/state. These modes are "s2idle" -(Suspend-To-Idle), "shallow" (Power-On Suspend) and "deep" (Suspend-To-RAM). -The "s2idle" mode is always available, while the other ones are only available -if supported by the platform (if not supported, the strings representing them -are not present in /sys/power/mem_sleep). The string representing the suspend -mode to be used subsequently is enclosed in square brackets. Writing one of -the other strings present in /sys/power/mem_sleep to it causes the suspend mode -to be used subsequently to change to the one represented by that string. - -Consequently, there are two ways to cause the system to go into the -Suspend-To-Idle sleep state. The first one is to write "freeze" directly to -/sys/power/state. The second one is to write "s2idle" to /sys/power/mem_sleep -and then to write "mem" to /sys/power/state. Similarly, there are two ways -to cause the system to go into the Power-On Suspend sleep state (the strings to -write to the control files in that case are "standby" or "shallow" and "mem", -respectively) if that state is supported by the platform. In turn, there is -only one way to cause the system to go into the Suspend-To-RAM state (write -"deep" into /sys/power/mem_sleep and "mem" into /sys/power/state). - -The default suspend mode (ie. the one to be used without writing anything into -/sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or -"s2idle", but it can be overridden by the value of the "mem_sleep_default" -parameter in the kernel command line. - -The properties of all of the sleep states are described below. - - -State: Suspend-To-Idle -ACPI state: S0 -Label: "s2idle" ("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 to provide reduced -resume latency. It is always supported. - - -State: Standby / Power-On Suspend -ACPI State: S1 -Label: "shallow" ("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: "deep" - -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). |
