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There are systems in which the platform doesn't support any special
sleep states, so suspend-to-idle (PM_SUSPEND_FREEZE) is the only
available system sleep state. However, some user space frameworks
only use the "mem" and (sometimes) "standby" sleep state labels, so
the users of those systems need to modify user space in order to be
able to use system suspend at all and that may be a pain in practice.
Commit 0399d4db3e (PM / sleep: Introduce command line argument for
sleep state enumeration) attempted to address this problem by adding
a command line argument to change the meaning of the "mem" string in
/sys/power/state to make it trigger suspend-to-idle (instead of
suspend-to-RAM).
However, there also are systems in which the platform does support
special sleep states, but suspend-to-idle is the preferred one anyway
(it even may save more energy than the platform-provided sleep states
in some cases) and the above commit doesn't help in those cases.
For this reason, rework the system sleep state selection interface
again (but preserve backwards compatibiliby). Namely, add a new
sysfs file, /sys/power/mem_sleep, that will control the system
suspend mode triggered by writing "mem" to /sys/power/state (in
analogy with what /sys/power/disk does for hibernation). Make it
select suspend-to-RAM ("deep" sleep) by default (if supported) and
fall back to suspend-to-idle ("s2idle") otherwise and add a new
command line argument, mem_sleep_default, allowing that default to
be overridden if need be.
At the same time, drop the relative_sleep_states command line
argument that doesn't make sense any more.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mario Limonciello <mario.limonciello@dell.com>
126 lines
6.1 KiB
Plaintext
126 lines
6.1 KiB
Plaintext
System Power Management Sleep States
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(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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The kernel supports up to four system sleep states generically, although three
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of them depend on the platform support code to implement the low-level details
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for each state.
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The states are represented by strings that can be read or written to the
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/sys/power/state file. Those strings may be "mem", "standby", "freeze" and
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"disk", where the last three always represent Power-On Suspend (if supported),
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Suspend-To-Idle and hibernation (Suspend-To-Disk), respectively.
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The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file.
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It contains strings representing the available modes of system suspend that may
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be triggered by writing "mem" to /sys/power/state. These modes are "s2idle"
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(Suspend-To-Idle), "shallow" (Power-On Suspend) and "deep" (Suspend-To-RAM).
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The "s2idle" mode is always available, while the other ones are only available
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if supported by the platform (if not supported, the strings representing them
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are not present in /sys/power/mem_sleep). The string representing the suspend
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mode to be used subsequently is enclosed in square brackets. Writing one of
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the other strings present in /sys/power/mem_sleep to it causes the suspend mode
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to be used subsequently to change to the one represented by that string.
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Consequently, there are two ways to cause the system to go into the
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Suspend-To-Idle sleep state. The first one is to write "freeze" directly to
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/sys/power/state. The second one is to write "s2idle" to /sys/power/mem_sleep
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and then to wrtie "mem" to /sys/power/state. Similarly, there are two ways
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to cause the system to go into the Power-On Suspend sleep state (the strings to
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write to the control files in that case are "standby" or "shallow" and "mem",
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respectively) if that state is supported by the platform. In turn, there is
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only one way to cause the system to go into the Suspend-To-RAM state (write
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"deep" into /sys/power/mem_sleep and "mem" into /sys/power/state).
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The default suspend mode (ie. the one to be used without writing anything into
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/sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or
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"s2idle", but it can be overridden by the value of the "mem_sleep_default"
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parameter in the kernel command line.
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The properties of all of the sleep states are described below.
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State: Suspend-To-Idle
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ACPI state: S0
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Label: "s2idle" ("freeze")
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This state is a generic, pure software, light-weight, system sleep state.
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It allows more energy to be saved relative to runtime 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 Power-On Suspend/Suspend-to-RAM
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support, or it can be used in addition to Suspend-to-RAM to provide reduced
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resume latency. It is always supported.
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State: Standby / Power-On Suspend
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ACPI State: S1
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Label: "shallow" ("standby")
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This state, if supported, offers moderate, though real, power savings, while
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providing a relatively low-latency transition back to a working system. No
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operating 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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In addition to freezing user space and putting all I/O devices into low-power
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states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline
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and all low-level system functions are suspended during transitions into this
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state. For this reason, it should allow more energy to be saved relative to
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Suspend-To-Idle, but the resume latency will generally be greater than for that
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state.
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State: Suspend-to-RAM
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ACPI State: S3
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Label: "deep"
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This state, if supported, offers significant power savings as everything in the
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system is put into a low-power state, except for memory, which should be placed
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into the self-refresh mode to retain its contents. All of the steps carried out
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when entering Power-On Suspend are also carried out during transitions to STR.
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Additional operations may take place depending on the platform capabilities. In
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particular, on ACPI systems the kernel passes control to the BIOS (platform
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firmware) as the last step during STR transitions and that usually results in
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powering down some more low-level components that aren't directly controlled by
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the kernel.
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System and device state is saved and kept in memory. All devices are suspended
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and put into low-power states. In many cases, all peripheral buses lose power
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when entering STR, so devices must be able to handle the transition back to the
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"on" state.
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For at least ACPI, STR requires some minimal boot-strapping code to resume the
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system from it. This may be the case on other platforms too.
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State: Suspend-to-disk
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ACPI State: S4
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Label: "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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