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