[PATCH 1/2] PM: docs: Describe high-level PM strategies and sleep states

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

Reorganize the power management part of admin-guide by adding a
description of major power management strategies supported by the
kernel (system-wide and working-state power management) to it and
dividing the rest of the material into the system-wide PM and
working-state PM chapters.

On top of that, add a description of system sleep states to the
system-wide PM chapter.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---
 Documentation/admin-guide/pm/index.rst         |    5 
 Documentation/admin-guide/pm/sleep-states.rst  |  234 +++++++++++++++++++++++++
 Documentation/admin-guide/pm/strategies.rst    |   52 +++++
 Documentation/admin-guide/pm/system-wide.rst   |   15 +
 Documentation/admin-guide/pm/working-state.rst |   16 +
 5 files changed, 320 insertions(+), 2 deletions(-)

Index: linux-pm/Documentation/admin-guide/pm/index.rst
===================================================================
--- linux-pm.orig/Documentation/admin-guide/pm/index.rst
+++ linux-pm/Documentation/admin-guide/pm/index.rst
@@ -5,8 +5,9 @@ Power Management
 .. toctree::
    :maxdepth: 2
 
-   cpufreq
-   intel_pstate
+   strategies
+   system-wide
+   working-state
 
 .. only::  subproject and html
 
Index: linux-pm/Documentation/admin-guide/pm/sleep-states.rst
===================================================================
--- /dev/null
+++ linux-pm/Documentation/admin-guide/pm/sleep-states.rst
@@ -0,0 +1,234 @@
+===================
+System Sleep States
+===================
+
+::
+
+ Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
+
+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, includig
+hibernation and up to three variants of system suspend.  The sleep states that
+can be supported by the kernel are listed below.
+
+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 `Standby`_ or
+`Suspend-to-RAM`_, 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
+-------
+
+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 `Suspend-to-Idle`_ 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 `Suspend-to-Idle`_, 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 `Suspend-to-Idle`_ 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.
+
+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 `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 `Suspend-to-Idle`_ and `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
+-----------
+
+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
+	`Hibernation`_, `Suspend-to-Idle`_ and `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 `Suspend-to-Idle`_
+	and, by convention, "shallow" and "deep" represent `Standby`_ and
+	`Suspend-to-RAM`_, 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
+`Suspend-to-Idle`_ 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 `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 `Suspend-to-RAM`_ 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 `Suspend-to-RAM`_) 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 `Suspend-to-RAM`_ is supported.
Index: linux-pm/Documentation/admin-guide/pm/system-wide.rst
===================================================================
--- /dev/null
+++ linux-pm/Documentation/admin-guide/pm/system-wide.rst
@@ -0,0 +1,15 @@
+============================
+System-Wide Power Management
+============================
+
+.. toctree::
+   :maxdepth: 2
+
+   sleep-states
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
Index: linux-pm/Documentation/admin-guide/pm/working-state.rst
===================================================================
--- /dev/null
+++ linux-pm/Documentation/admin-guide/pm/working-state.rst
@@ -0,0 +1,16 @@
+==============================
+Working-State Power Management
+==============================
+
+.. toctree::
+   :maxdepth: 2
+
+   cpufreq
+   intel_pstate
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
Index: linux-pm/Documentation/admin-guide/pm/strategies.rst
===================================================================
--- /dev/null
+++ linux-pm/Documentation/admin-guide/pm/strategies.rst
@@ -0,0 +1,52 @@
+===========================
+Power Management Strategies
+===========================
+
+::
+
+ Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
+
+The Linux kernel supports two major high-level power management strategies.
+
+One of them is based on using global low-power states of the whole system in
+which user space code cannot be executed and the overall system activity is
+significantly reduced, referred to as :doc:`sleep states <sleep-states>`.  The
+kernel puts the system into one of these states when requested by user space
+and the system stays in it until a special signal is received from one of
+designated devices, triggering a transition to the ``working state`` in which
+user space code can run.  Because sleep states are global and the whole system
+is affected by the state changes, this strategy is referred to as the
+:doc:`system-wide power management <system-wide>`.
+
+The other strategy, referred to as the
+:doc:`working-state power management <working-state>`, is based on adjusting the
+power states of individual hardware components of the system, as needed, in the
+working state.  In consequence, if this strategy is in use, the working state
+of the system usually does not correspond to any particular physical
+configuration of it, but can be treated as a metastate covering a range of
+different power states of the system in which the individual components of it
+can be either ``active`` (in use) or ``inactive`` (idle).  If they are active,
+they have to be in power states allowing them to process data and to be accessed
+by software.  In turn, if they are inactive, they are expected to be in
+low-power states in which they may not be accessible.
+
+If all of the system components are active, the system as a whole is regarded as
+``runtime active`` and that situation typically corresponds to the maximum power
+draw (or maximum energy usage) of it.  If all of them are inactive, the system
+as a whole is regarded as ``runtime idle`` which may be very close to a sleep
+state from the physical system configuration and power draw perspective, but
+then it takes much less time and effort to start executing user space code than
+for the same system in a sleep state.  However, transitions from sleep states
+back to the working state can only be started by a limited set of devices, so
+typically the system can spend much more time in a sleep state than it can be
+runtime idle in one go.  For this reason, systems usually use less energy in
+sleep states than when they are runtime idle most of the time.
+
+Moreover, the two power management strategies address different usage scenarios.
+Namely, if the user indicates that the system will not be in use going forward,
+for example by closing its lid (if the system is a laptop), it probably should
+go into a sleep state at that point.  On the other hand, if the user simply goes
+away from the laptop keyboard, it probably should stay in the working state and
+use the working-state power management in case it becomes idle, because the user
+may come back to it at any time and then may want the system to be immediately
+accessible.