85 lines
3.4 KiB
Plaintext
85 lines
3.4 KiB
Plaintext
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System Power Management States
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The kernel supports four power management states generically, though
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one is generic and the other three are dependent on platform support
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code to implement the low-level details for each state.
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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: Freeze / Low-Power Idle
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ACPI state: S0
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String: "freeze"
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This state is a generic, pure software, light-weight, low-power state.
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It allows more energy to be saved relative to idle by freezing user
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space and putting all I/O devices into low-power states (possibly
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lower-power than available at run time), such that the processors can
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spend more time in their idle states.
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This state can be used for platforms without Standby/Suspend-to-RAM
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support, or it can be used in addition to Suspend-to-RAM (memory sleep)
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to provide reduced resume latency.
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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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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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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' (Swap Suspend) is used
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to write memory contents to free swap space. swsusp has some restrictive
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requirements, but should work in most cases. Some, albeit outdated,
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documentation can be found in Documentation/power/swsusp.txt.
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Alternatively, userspace can do most of the actual suspend to disk work,
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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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