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Diffstat (limited to 'Documentation/power/states.txt')
| -rw-r--r-- | Documentation/power/states.txt | 87 |
1 files changed, 56 insertions, 31 deletions
diff --git a/Documentation/power/states.txt b/Documentation/power/states.txt index 442d43df9b2..50f3ef9177c 100644 --- a/Documentation/power/states.txt +++ b/Documentation/power/states.txt @@ -1,62 +1,87 @@ +System Power Management Sleep States -System Power Management 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 kernel supports four power management states generically, though -one is generic and the other three are 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 +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. -state: Freeze / Low-Power Idle +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 -String: "freeze" +Label: "freeze" -This state is a generic, pure software, light-weight, low-power state. -It allows more energy to be saved relative to idle by freezing user +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 Standby/Suspend-to-RAM + +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. +to provide reduced resume latency. It is always supported. State: Standby / Power-On Suspend ACPI State: S1 -String: "standby" +Label: "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 +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. -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. +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 -String: "mem" +Label: "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. +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 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. +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 STR. This may be true on other platforms. +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 -String: "disk" +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 |