mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-30 11:06:39 +07:00
86fd03d160
The lantency of the transition from suspend and hibernate is platform-dependent. Thus we should not refer the lantency in the documentation. Signed-off-by: Zhang Rui <rui.zhang@intel.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
85 lines
3.4 KiB
Plaintext
85 lines
3.4 KiB
Plaintext
|
|
System Power Management States
|
|
|
|
|
|
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
|
|
|
|
state: Freeze / Low-Power Idle
|
|
ACPI state: S0
|
|
String: "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
|
|
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
|
|
support, or it can be used in addition to Suspend-to-RAM (memory sleep)
|
|
to provide reduced resume latency.
|
|
|
|
|
|
State: Standby / Power-On Suspend
|
|
ACPI State: S1
|
|
String: "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
|
|
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.
|
|
|
|
|
|
State: Suspend-to-RAM
|
|
ACPI State: S3
|
|
String: "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.
|
|
|
|
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.
|
|
|
|
For at least ACPI, STR requires some minimal boot-strapping code to
|
|
resume the system from STR. This may be true on other platforms.
|
|
|
|
|
|
State: Suspend-to-disk
|
|
ACPI State: S4
|
|
String: "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).
|