Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+===================
+System Sleep States
+===================
+
+::
+
+ Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+
+Sleep states are global low-power states of the entire system in which user
+space code cannot be executed and the overall system activity is significantly
+reduced.
+
+
+Sleep States That Can Be Supported
+==================================
+
+Depending on its configuration and the capabilities of the platform it runs on,
+the Linux kernel can support up to four system sleep states, including
+hibernation and up to three variants of system suspend.  The sleep states that
+can be supported by the kernel are listed below.
+
+.. _s2idle:
+
+Suspend-to-Idle
+---------------
+
+This is a generic, pure software, light-weight variant of system suspend (also
+referred to as S2I or S2Idle).  It allows more energy to be saved relative to
+runtime idle by freezing user space, suspending the timekeeping and putting all
+I/O devices into low-power states (possibly lower-power than available in the
+working state), such that the processors can spend time in their deepest idle
+states while the system is suspended.
+
+The system is woken up from this state by in-band interrupts, so theoretically
+any devices that can cause interrupts to be generated in the working state can
+also be set up as wakeup devices for S2Idle.
+
+This state can be used on platforms without support for :ref:`standby <standby>`
+or :ref:`suspend-to-RAM <s2ram>`, or it can be used in addition to any of the
+deeper system suspend variants to provide reduced resume latency.  It is always
+supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option is set.
+
+.. _standby:
+
+Standby
+-------
+
+This state, if supported, offers moderate, but real, energy savings, while
+providing a relatively straightforward transition back to the working state.  No
+operating state is lost (the system core logic retains power), so the system can
+go back to where it left off easily enough.
+
+In addition to freezing user space, suspending the timekeeping and putting all
+I/O devices into low-power states, which is done for :ref:`suspend-to-idle
+<s2idle>` 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 :ref:`suspend-to-idle <s2idle>`, but
+the resume latency will generally be greater than for that state.
+
+The set of devices that can wake up the system from this state usually is
+reduced relative to :ref:`suspend-to-idle <s2idle>` and it may be necessary to
+rely on the platform for setting up the wakeup functionality as appropriate.
+
+This state is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration
+option is set and the support for it is registered by the platform with the
+core system suspend subsystem.  On ACPI-based systems this state is mapped to
+the S1 system state defined by ACPI.
+
+.. _s2ram:
+
+Suspend-to-RAM
+--------------
+
+This state (also referred to as STR or S2RAM), if supported, offers significant
+energy 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 :ref:`standby <standby>`
+are also carried out during transitions to S2RAM.  Additional operations may
+take place depending on the platform capabilities.  In particular, on ACPI-based
+systems the kernel passes control to the platform firmware (BIOS) as the last
+step during S2RAM transitions and that usually results in powering down some
+more low-level components that are not directly controlled by the kernel.
+
+The state of devices and CPUs is saved and held in memory.  All devices are
+suspended and put into low-power states.  In many cases, all peripheral buses
+lose power when entering S2RAM, so devices must be able to handle the transition
+back to the "on" state.
+
+On ACPI-based systems S2RAM requires some minimal boot-strapping code in the
+platform firmware to resume the system from it.  This may be the case on other
+platforms too.
+
+The set of devices that can wake up the system from S2RAM usually is reduced
+relative to :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it
+may be necessary to rely on the platform for setting up the wakeup functionality
+as appropriate.
+
+S2RAM is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option
+is set and the support for it is registered by the platform with the core system
+suspend subsystem.  On ACPI-based systems it is mapped to the S3 system state
+defined by ACPI.
+
+.. _hibernation:
+
+Hibernation
+-----------
+
+This state (also referred to as Suspend-to-Disk or STD) offers the greatest
+energy savings and can be used even in the absence of low-level platform support
+for system suspend.  However, it requires some low-level code for resuming the
+system to be present for the underlying CPU architecture.
+
+Hibernation is significantly different from any of the system suspend variants.
+It takes three system state changes to put it into hibernation and two system
+state changes to resume it.
+
+First, when hibernation is triggered, the kernel stops all system activity and
+creates a snapshot image of memory to be written into persistent storage.  Next,
+the system goes into a state in which the snapshot image can be saved, the image
+is written out and finally the system goes into the target low-power state in
+which power is cut from almost all of its hardware components, including memory,
+except for a limited set of wakeup devices.
+
+Once the snapshot image has been written out, the system may either enter a
+special low-power state (like ACPI S4), or it may simply power down itself.
+Powering down means minimum power draw and it allows this mechanism to work on
+any system.  However, entering a special low-power state may allow additional
+means of system wakeup to be used  (e.g. pressing a key on the keyboard or
+opening a laptop lid).
+
+After wakeup, control goes to the platform firmware that runs a boot loader
+which boots a fresh instance of the kernel (control may also go directly to
+the boot loader, depending on the system configuration, but anyway it causes
+a fresh instance of the kernel to be booted).  That new instance of the kernel
+(referred to as the ``restore kernel``) looks for a hibernation image in
+persistent storage and if one is found, it is loaded into memory.  Next, all
+activity in the system is stopped and the restore kernel overwrites itself with
+the image contents and jumps into a special trampoline area in the original
+kernel stored in the image (referred to as the ``image kernel``), which is where
+the special architecture-specific low-level code is needed.  Finally, the
+image kernel restores the system to the pre-hibernation state and allows user
+space to run again.
+
+Hibernation is supported if the :c:macro:`CONFIG_HIBERNATION` kernel
+configuration option is set.  However, this option can only be set if support
+for the given CPU architecture includes the low-level code for system resume.
+
+
+Basic ``sysfs`` Interfaces for System Suspend and Hibernation
+=============================================================
+
+The following files located in the :file:`/sys/power/` directory can be used by
+user space for sleep states control.
+
+``state``
+	This file contains a list of strings representing sleep states supported
+	by the kernel.  Writing one of these strings into it causes the kernel
+	to start a transition of the system into the sleep state represented by
+	that string.
+
+	In particular, the strings "disk", "freeze" and "standby" represent the
+	:ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
+	:ref:`standby <standby>` sleep states, respectively.  The string "mem"
+	is interpreted in accordance with the contents of the ``mem_sleep`` file
+	described below.
+
+	If the kernel does not support any system sleep states, this file is
+	not present.
+
+``mem_sleep``
+	This file contains a list of strings representing supported system
+	suspend	variants and allows user space to select the variant to be
+	associated with the "mem" string in the ``state`` file described above.
+
+	The strings that may be present in this file are "s2idle", "shallow"
+	and "deep".  The string "s2idle" always represents :ref:`suspend-to-idle
+	<s2idle>` and, by convention, "shallow" and "deep" represent
+	:ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
+	respectively.
+
+	Writing one of the listed strings into this file causes the system
+	suspend variant represented by it to be associated with the "mem" string
+	in the ``state`` file.  The string representing the suspend variant
+	currently associated with the "mem" string in the ``state`` file
+	is listed in square brackets.
+
+	If the kernel does not support system suspend, this file is not present.
+
+``disk``
+	This file contains a list of strings representing different operations
+	that can be carried out after the hibernation image has been saved.  The
+	possible options are as follows:
+
+	``platform``
+		Put the system into a special low-power state (e.g. ACPI S4) to
+		make additional wakeup options available and possibly allow the
+		platform firmware to take a simplified initialization path after
+		wakeup.
+
+	``shutdown``
+		Power off the system.
+
+	``reboot``
+		Reboot the system (useful for diagnostics mostly).
+
+	``suspend``
+		Hybrid system suspend.  Put the system into the suspend sleep
+		state selected through the ``mem_sleep`` file described above.
+		If the system is successfully woken up from that state, discard
+		the hibernation image and continue.  Otherwise, use the image
+		to restore the previous state of the system.
+
+	``test_resume``
+		Diagnostic operation.  Load the image as though the system had
+		just woken up from hibernation and the currently running kernel
+		instance was a restore kernel and follow up with full system
+		resume.
+
+	Writing one of the listed strings into this file causes the option
+	represented by it to be selected.
+
+	The currently selected option is shown in square brackets which means
+	that the operation represented by it will be carried out after creating
+	and saving the image next time hibernation is triggered by writing
+	``disk`` to :file:`/sys/power/state`.
+
+	If the kernel does not support hibernation, this file is not present.
+
+According to the above, there are two ways to make the system go into the
+:ref:`suspend-to-idle <s2idle>` state.  The first one is to write "freeze"
+directly to :file:`/sys/power/state`.  The second one is to write "s2idle" to
+:file:`/sys/power/mem_sleep` and then to write "mem" to
+:file:`/sys/power/state`.  Likewise, there are two ways to make the system go
+into the :ref:`standby <standby>` state (the strings to write to the control
+files in that case are "standby" or "shallow" and "mem", respectively) if that
+state is supported by the platform.  However, there is only one way to make the
+system go into the :ref:`suspend-to-RAM <s2ram>` state (write "deep" into
+:file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`).
+
+The default suspend variant (ie. the one to be used without writing anything
+into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
+supporting :ref:`suspend-to-RAM <s2ram>`) or "s2idle", but it can be overridden
+by the value of the "mem_sleep_default" parameter in the kernel command line.
+On some ACPI-based systems, depending on the information in the ACPI tables, the
+default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported.