[RFC,01/13] Documentation: PM: Add documentation for S0ix Standby States

diff --git a/Documentation/admin-guide/pm/standby-states.rst b/Documentation/admin-guide/pm/standby-states.rst
new file mode 100644
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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+=====================
+S0ix Standby States
+=====================
+
+:Copyright: |copy| 2024 Antheas Kapenekakis
+
+:Author: Antheas Kapenekakis <lkml@antheas.dev>
+
+With the advent of modern mobile devices, users have become accustomed to instant
+wake-up times and always-on connectivity. To meet these expectations, modern
+standby was created, which is a standard that allows the platform to seamlessly
+transition between an S3-like low-power idle state and a set of low power active
+states, where connectivity is maintained, and the system is responsive to user
+input. Current x86 hardware supports 5 different standby states, which are:
+"Deepest run-time idle platform state" or "DRIPS" (S3-like), "Sleep", "Resume",
+"Screen Off", and "Active".
+
+The system begins in the "Active" state. Either due to user inactivity or
+user action (e.g., pressing the power button), it transitions to the "Screen Off"
+state. Afterwards, it is free to transition between the "Sleep", "DRIPS", and
+"Screen Off" states until user action is received. Once that happens, the system
+begins to transition to the "Active" state. From "DRIPS" or "Sleep", it
+transitions to "Resume", where the Power Limit (PLx) is restored to its normal
+level, to speed up finishing "Sleep". Then, it transitions to "Screen Off".
+If on "Screen Off" or after the transition, the display is prepared to turn on
+and the system transitions to "Active" alongside turning it on.
+
+To maintain battery life, in the Windows implementation, the system is allocated
+a maximum percentage of battery and time it can use while staying in idle states.
+By default, this is 5% of battery or up to 2 days, where the system designer/OEM
+is able to tweak these values. If the system exceeds either the battery
+percentage or time limit, it enters Hibernation (S4), through a concept
+called "Adaptive Hibernate".
+
+
+S0ix Standby States
+==================================
+The following idle states are supported::
+
+                 ↓→  <Hibernate (S4)>
+    <DRIPS> ↔ <Sleep> ↔ <Screen Off> ↔ <Active>
+        →       →  <Resume>  ↑
+
+.. _s2idle_drips:
+
+DRIPS
+-----
+
+The "Deepest run-time idle platform state" or "DRIPS" is the lowest power idle
+state that the system can enter. It is similar to the S3 state, with the
+difference that the system may wake up faster than S3 and due to a larger number
+of interrupts (e.g., fingerprint sensor, touchpad, touchscreen). This state
+is entered when the system is told to suspend to idle, through conventional
+means (see :doc:`sleep states <sleep-states>`). The system can only transition
+to "DRIPS" while it is in the "Sleep" state. If it is not, the kernel will
+automatically transition to the "Sleep" state before beginning the suspend
+sequence and restore the previous state afterwards. After the kernel has
+suspended, the notifications LSP0 Entry and Exit are used.
+
+.. _s2idle_sleep:
+
+Sleep
+-----
+
+The "Sleep" state is a low power idle state where the kernel is fully active.
+However, userspace has been partially frozen, particularly desktop applications,
+and only essential "value adding" activities are allowed to run. This is not
+enforced by the kernel and is the responsibility of userspace (e.g., systemd).
+Hardware wise, the Sleep Entry and Exit firmware notifications are fired, which
+may lower the Power Limit (PLx), pulse the suspend light, turn off the keyboard
+lighting or disable a handheld device's gamepad. This state is associated with
+the firmware notifications "Sleep Entry" and "Sleep Exit".
+
+.. _s2idle_resume:
+
+Resume
+------
+
+The "Resume" state is a faux "Sleep" state that is used to fire the Turn On
+Display firmware notification when the system is in the "Sleep" state but
+intends to turn on the display. It solves the problem of system designers
+limiting the Power Limit (PLx) while the system is in the "Sleep" state causing
+the system to wake up slower than desired. This firmware notification is used
+to restore the normal Power Limit of the system, while having it stay in the
+"Sleep" state.  As such, the system can only transition to the "Resume" state
+while in the "Sleep" state and cannot re-transition to the "Sleep" state
+afterwards.
+
+.. _s2idle_screen_off:
+
+Screen Off
+----------
+
+The "Screen Off" state is the state the system enters when all its displays
+(virtual or real) turn off. It is used to signify the user is not actively
+using the system. The associated firmware notifications of "Display On" and
+"Display Off" are used by manufacturers to turn off certain hardware
+components that are associated with the display being on, e.g., a handheld
+device's controller and RGB. Windows implements a 5-second grace period
+before firing this callback when the screen turns off due to inactivity.
+
+.. _s2idle_active:
+
+Active
+------
+
+Finally, the "Active" state is the default state of the system and the one it
+has when it is turned on. It is the state where the system is fully operational,
+the displays of the device are on, and the user is actively interacting with
+the system.
+
+Basic ``sysfs`` Interface for S0ix Standby transitions
+=============================================================
+
+The file :file:`/sys/power/standby` can be used to transition the system between
+the different standby states. The file accepts the following values: ``active``,
+``screen_off``, ``sleep``, and ``resume``. File writes will block until the
+transition completes. It will return ``-EINVAL`` when asking for an unsupported
+state or, e.g., requesting ``resume`` when not in the ``sleep`` state. If there
+is an error during the transition, the transition will pause on the last
+error-free state and return an error. The file can be read to retrieve the
+current state (and potential ones) using the following format:
+``[active] screen_off sleep resume``. The state "DRIPS" is omitted, as it is
+entered through the conventional suspend to idle path and userspace will never
+be able to see its value due to being suspended.
+
+Before entering the "Screen Off" state or suspending, it is recommended that
+userspace marks all CRTCs as inactive (DPMS). Otherwise, there will be a split
+second where the display of the device is on, but the presentation of the system
+is inactive (e.g., the power button pulses), which is undesirable.
\ No newline at end of file
diff --git a/Documentation/admin-guide/pm/system-wide.rst b/Documentation/admin-guide/pm/system-wide.rst
index 1a1924d71006..411775fae4ac 100644
--- a/Documentation/admin-guide/pm/system-wide.rst
+++ b/Documentation/admin-guide/pm/system-wide.rst
@@ -8,4 +8,5 @@  System-Wide Power Management
    :maxdepth: 2
 
    sleep-states
+   standby-states
    suspend-flows