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

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+====================
+PCI Power Management
+====================
+
+Copyright (c) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
+
+An overview of concepts and the Linux kernel's interfaces related to PCI power
+management.  Based on previous work by Patrick Mochel <mochel@transmeta.com>
+(and others).
+
+This document only covers the aspects of power management specific to PCI
+devices.  For general description of the kernel's interfaces related to device
+power management refer to Documentation/driver-api/pm/devices.rst and
+Documentation/power/runtime_pm.rst.
+
+.. contents:
+
+   1. Hardware and Platform Support for PCI Power Management
+   2. PCI Subsystem and Device Power Management
+   3. PCI Device Drivers and Power Management
+   4. Resources
+
+
+1. Hardware and Platform Support for PCI Power Management
+=========================================================
+
+1.1. Native and Platform-Based Power Management
+-----------------------------------------------
+
+In general, power management is a feature allowing one to save energy by putting
+devices into states in which they draw less power (low-power states) at the
+price of reduced functionality or performance.
+
+Usually, a device is put into a low-power state when it is underutilized or
+completely inactive.  However, when it is necessary to use the device once
+again, it has to be put back into the "fully functional" state (full-power
+state).  This may happen when there are some data for the device to handle or
+as a result of an external event requiring the device to be active, which may
+be signaled by the device itself.
+
+PCI devices may be put into low-power states in two ways, by using the device
+capabilities introduced by the PCI Bus Power Management Interface Specification,
+or with the help of platform firmware, such as an ACPI BIOS.  In the first
+approach, that is referred to as the native PCI power management (native PCI PM)
+in what follows, the device power state is changed as a result of writing a
+specific value into one of its standard configuration registers.  The second
+approach requires the platform firmware to provide special methods that may be
+used by the kernel to change the device's power state.
+
+Devices supporting the native PCI PM usually can generate wakeup signals called
+Power Management Events (PMEs) to let the kernel know about external events
+requiring the device to be active.  After receiving a PME the kernel is supposed
+to put the device that sent it into the full-power state.  However, the PCI Bus
+Power Management Interface Specification doesn't define any standard method of
+delivering the PME from the device to the CPU and the operating system kernel.
+It is assumed that the platform firmware will perform this task and therefore,
+even though a PCI device is set up to generate PMEs, it also may be necessary to
+prepare the platform firmware for notifying the CPU of the PMEs coming from the
+device (e.g. by generating interrupts).
+
+In turn, if the methods provided by the platform firmware are used for changing
+the power state of a device, usually the platform also provides a method for
+preparing the device to generate wakeup signals.  In that case, however, it
+often also is necessary to prepare the device for generating PMEs using the
+native PCI PM mechanism, because the method provided by the platform depends on
+that.
+
+Thus in many situations both the native and the platform-based power management
+mechanisms have to be used simultaneously to obtain the desired result.
+
+1.2. Native PCI Power Management
+--------------------------------
+
+The PCI Bus Power Management Interface Specification (PCI PM Spec) was
+introduced between the PCI 2.1 and PCI 2.2 Specifications.  It defined a
+standard interface for performing various operations related to power
+management.
+
+The implementation of the PCI PM Spec is optional for conventional PCI devices,
+but it is mandatory for PCI Express devices.  If a device supports the PCI PM
+Spec, it has an 8 byte power management capability field in its PCI
+configuration space.  This field is used to describe and control the standard
+features related to the native PCI power management.
+
+The PCI PM Spec defines 4 operating states for devices (D0-D3) and for buses
+(B0-B3).  The higher the number, the less power is drawn by the device or bus
+in that state.  However, the higher the number, the longer the latency for
+the device or bus to return to the full-power state (D0 or B0, respectively).
+
+There are two variants of the D3 state defined by the specification.  The first
+one is D3hot, referred to as the software accessible D3, because devices can be
+programmed to go into it.  The second one, D3cold, is the state that PCI devices
+are in when the supply voltage (Vcc) is removed from them.  It is not possible
+to program a PCI device to go into D3cold, although there may be a programmable
+interface for putting the bus the device is on into a state in which Vcc is
+removed from all devices on the bus.
+
+PCI bus power management, however, is not supported by the Linux kernel at the
+time of this writing and therefore it is not covered by this document.
+
+Note that every PCI device can be in the full-power state (D0) or in D3cold,
+regardless of whether or not it implements the PCI PM Spec.  In addition to
+that, if the PCI PM Spec is implemented by the device, it must support D3hot
+as well as D0.  The support for the D1 and D2 power states is optional.
+
+PCI devices supporting the PCI PM Spec can be programmed to go to any of the
+supported low-power states (except for D3cold).  While in D1-D3hot the
+standard configuration registers of the device must be accessible to software
+(i.e. the device is required to respond to PCI configuration accesses), although
+its I/O and memory spaces are then disabled.  This allows the device to be
+programmatically put into D0.  Thus the kernel can switch the device back and
+forth between D0 and the supported low-power states (except for D3cold) and the
+possible power state transitions the device can undergo are the following:
+
++----------------------------+
+| Current State | New State  |
++----------------------------+
+| D0            | D1, D2, D3 |
++----------------------------+
+| D1            | D2, D3     |
++----------------------------+
+| D2            | D3         |
++----------------------------+
+| D1, D2, D3    | D0         |
++----------------------------+
+
+The transition from D3cold to D0 occurs when the supply voltage is provided to
+the device (i.e. power is restored).  In that case the device returns to D0 with
+a full power-on reset sequence and the power-on defaults are restored to the
+device by hardware just as at initial power up.
+
+PCI devices supporting the PCI PM Spec can be programmed to generate PMEs
+while in any power state (D0-D3), but they are not required to be capable
+of generating PMEs from all supported power states.  In particular, the
+capability of generating PMEs from D3cold is optional and depends on the
+presence of additional voltage (3.3Vaux) allowing the device to remain
+sufficiently active to generate a wakeup signal.
+
+1.3. ACPI Device Power Management
+---------------------------------
+
+The platform firmware support for the power management of PCI devices is
+system-specific.  However, if the system in question is compliant with the
+Advanced Configuration and Power Interface (ACPI) Specification, like the
+majority of x86-based systems, it is supposed to implement device power
+management interfaces defined by the ACPI standard.
+
+For this purpose the ACPI BIOS provides special functions called "control
+methods" that may be executed by the kernel to perform specific tasks, such as
+putting a device into a low-power state.  These control methods are encoded
+using special byte-code language called the ACPI Machine Language (AML) and
+stored in the machine's BIOS.  The kernel loads them from the BIOS and executes
+them as needed using an AML interpreter that translates the AML byte code into
+computations and memory or I/O space accesses.  This way, in theory, a BIOS
+writer can provide the kernel with a means to perform actions depending
+on the system design in a system-specific fashion.
+
+ACPI control methods may be divided into global control methods, that are not
+associated with any particular devices, and device control methods, that have
+to be defined separately for each device supposed to be handled with the help of
+the platform.  This means, in particular, that ACPI device control methods can
+only be used to handle devices that the BIOS writer knew about in advance.  The
+ACPI methods used for device power management fall into that category.
+
+The ACPI specification assumes that devices can be in one of four power states
+labeled as D0, D1, D2, and D3 that roughly correspond to the native PCI PM
+D0-D3 states (although the difference between D3hot and D3cold is not taken
+into account by ACPI).  Moreover, for each power state of a device there is a
+set of power resources that have to be enabled for the device to be put into
+that state.  These power resources are controlled (i.e. enabled or disabled)
+with the help of their own control methods, _ON and _OFF, that have to be
+defined individually for each of them.
+
+To put a device into the ACPI power state Dx (where x is a number between 0 and
+3 inclusive) the kernel is supposed to (1) enable the power resources required
+by the device in this state using their _ON control methods and (2) execute the
+_PSx control method defined for the device.  In addition to that, if the device
+is going to be put into a low-power state (D1-D3) and is supposed to generate
+wakeup signals from that state, the _DSW (or _PSW, replaced with _DSW by ACPI
+3.0) control method defined for it has to be executed before _PSx.  Power
+resources that are not required by the device in the target power state and are
+not required any more by any other device should be disabled (by executing their
+_OFF control methods).  If the current power state of the device is D3, it can
+only be put into D0 this way.
+
+However, quite often the power states of devices are changed during a
+system-wide transition into a sleep state or back into the working state.  ACPI
+defines four system sleep states, S1, S2, S3, and S4, and denotes the system
+working state as S0.  In general, the target system sleep (or working) state
+determines the highest power (lowest number) state the device can be put
+into and the kernel is supposed to obtain this information by executing the
+device's _SxD control method (where x is a number between 0 and 4 inclusive).
+If the device is required to wake up the system from the target sleep state, the
+lowest power (highest number) state it can be put into is also determined by the
+target state of the system.  The kernel is then supposed to use the device's
+_SxW control method to obtain the number of that state.  It also is supposed to
+use the device's _PRW control method to learn which power resources need to be
+enabled for the device to be able to generate wakeup signals.
+
+1.4. Wakeup Signaling
+---------------------
+
+Wakeup signals generated by PCI devices, either as native PCI PMEs, or as
+a result of the execution of the _DSW (or _PSW) ACPI control method before
+putting the device into a low-power state, have to be caught and handled as
+appropriate.  If they are sent while the system is in the working state
+(ACPI S0), they should be translated into interrupts so that the kernel can
+put the devices generating them into the full-power state and take care of the
+events that triggered them.  In turn, if they are sent while the system is
+sleeping, they should cause the system's core logic to trigger wakeup.
+
+On ACPI-based systems wakeup signals sent by conventional PCI devices are
+converted into ACPI General-Purpose Events (GPEs) which are hardware signals
+from the system core logic generated in response to various events that need to
+be acted upon.  Every GPE is associated with one or more sources of potentially
+interesting events.  In particular, a GPE may be associated with a PCI device
+capable of signaling wakeup.  The information on the connections between GPEs
+and event sources is recorded in the system's ACPI BIOS from where it can be
+read by the kernel.
+
+If a PCI device known to the system's ACPI BIOS signals wakeup, the GPE
+associated with it (if there is one) is triggered.  The GPEs associated with PCI
+bridges may also be triggered in response to a wakeup signal from one of the
+devices below the bridge (this also is the case for root bridges) and, for
+example, native PCI PMEs from devices unknown to the system's ACPI BIOS may be
+handled this way.
+
+A GPE may be triggered when the system is sleeping (i.e. when it is in one of
+the ACPI S1-S4 states), in which case system wakeup is started by its core logic
+(the device that was the source of the signal causing the system wakeup to occur
+may be identified later).  The GPEs used in such situations are referred to as
+wakeup GPEs.
+
+Usually, however, GPEs are also triggered when the system is in the working
+state (ACPI S0) and in that case the system's core logic generates a System
+Control Interrupt (SCI) to notify the kernel of the event.  Then, the SCI
+handler identifies the GPE that caused the interrupt to be generated which,
+in turn, allows the kernel to identify the source of the event (that may be
+a PCI device signaling wakeup).  The GPEs used for notifying the kernel of
+events occurring while the system is in the working state are referred to as
+runtime GPEs.
+
+Unfortunately, there is no standard way of handling wakeup signals sent by
+conventional PCI devices on systems that are not ACPI-based, but there is one
+for PCI Express devices.  Namely, the PCI Express Base Specification introduced
+a native mechanism for converting native PCI PMEs into interrupts generated by
+root ports.  For conventional PCI devices native PMEs are out-of-band, so they
+are routed separately and they need not pass through bridges (in principle they
+may be routed directly to the system's core logic), but for PCI Express devices
+they are in-band messages that have to pass through the PCI Express hierarchy,
+including the root port on the path from the device to the Root Complex.  Thus
+it was possible to introduce a mechanism by which a root port generates an
+interrupt whenever it receives a PME message from one of the devices below it.
+The PCI Express Requester ID of the device that sent the PME message is then
+recorded in one of the root port's configuration registers from where it may be
+read by the interrupt handler allowing the device to be identified.  [PME
+messages sent by PCI Express endpoints integrated with the Root Complex don't
+pass through root ports, but instead they cause a Root Complex Event Collector
+(if there is one) to generate interrupts.]
+
+In principle the native PCI Express PME signaling may also be used on ACPI-based
+systems along with the GPEs, but to use it the kernel has to ask the system's
+ACPI BIOS to release control of root port configuration registers.  The ACPI
+BIOS, however, is not required to allow the kernel to control these registers
+and if it doesn't do that, the kernel must not modify their contents.  Of course
+the native PCI Express PME signaling cannot be used by the kernel in that case.
+
+
+2. PCI Subsystem and Device Power Management
+============================================
+
+2.1. Device Power Management Callbacks
+--------------------------------------
+
+The PCI Subsystem participates in the power management of PCI devices in a
+number of ways.  First of all, it provides an intermediate code layer between
+the device power management core (PM core) and PCI device drivers.
+Specifically, the pm field of the PCI subsystem's struct bus_type object,
+pci_bus_type, points to a struct dev_pm_ops object, pci_dev_pm_ops, containing
+pointers to several device power management callbacks::
+
+  const struct dev_pm_ops pci_dev_pm_ops = {
+	.prepare = pci_pm_prepare,
+	.complete = pci_pm_complete,
+	.suspend = pci_pm_suspend,
+	.resume = pci_pm_resume,
+	.freeze = pci_pm_freeze,
+	.thaw = pci_pm_thaw,
+	.poweroff = pci_pm_poweroff,
+	.restore = pci_pm_restore,
+	.suspend_noirq = pci_pm_suspend_noirq,
+	.resume_noirq = pci_pm_resume_noirq,
+	.freeze_noirq = pci_pm_freeze_noirq,
+	.thaw_noirq = pci_pm_thaw_noirq,
+	.poweroff_noirq = pci_pm_poweroff_noirq,
+	.restore_noirq = pci_pm_restore_noirq,
+	.runtime_suspend = pci_pm_runtime_suspend,
+	.runtime_resume = pci_pm_runtime_resume,
+	.runtime_idle = pci_pm_runtime_idle,
+  };
+
+These callbacks are executed by the PM core in various situations related to
+device power management and they, in turn, execute power management callbacks
+provided by PCI device drivers.  They also perform power management operations
+involving some standard configuration registers of PCI devices that device
+drivers need not know or care about.
+
+The structure representing a PCI device, struct pci_dev, contains several fields
+that these callbacks operate on::
+
+  struct pci_dev {
+	...
+	pci_power_t     current_state;  /* Current operating state. */
+	int		pm_cap;		/* PM capability offset in the
+					   configuration space */
+	unsigned int	pme_support:5;	/* Bitmask of states from which PME#
+					   can be generated */
+	unsigned int	pme_interrupt:1;/* Is native PCIe PME signaling used? */
+	unsigned int	d1_support:1;	/* Low power state D1 is supported */
+	unsigned int	d2_support:1;	/* Low power state D2 is supported */
+	unsigned int	no_d1d2:1;	/* D1 and D2 are forbidden */
+	unsigned int	wakeup_prepared:1;  /* Device prepared for wake up */
+	unsigned int	d3hot_delay;	/* D3hot->D0 transition time in ms */
+	...
+  };
+
+They also indirectly use some fields of the struct device that is embedded in
+struct pci_dev.
+
+2.2. Device Initialization
+--------------------------
+
+The PCI subsystem's first task related to device power management is to
+prepare the device for power management and initialize the fields of struct
+pci_dev used for this purpose.  This happens in two functions defined in
+drivers/pci/pci.c, pci_pm_init() and platform_pci_wakeup_init().
+
+The first of these functions checks if the device supports native PCI PM
+and if that's the case the offset of its power management capability structure
+in the configuration space is stored in the pm_cap field of the device's struct
+pci_dev object.  Next, the function checks which PCI low-power states are
+supported by the device and from which low-power states the device can generate
+native PCI PMEs.  The power management fields of the device's struct pci_dev and
+the struct device embedded in it are updated accordingly and the generation of
+PMEs by the device is disabled.
+
+The second function checks if the device can be prepared to signal wakeup with
+the help of the platform firmware, such as the ACPI BIOS.  If that is the case,
+the function updates the wakeup fields in struct device embedded in the
+device's struct pci_dev and uses the firmware-provided method to prevent the
+device from signaling wakeup.
+
+At this point the device is ready for power management.  For driverless devices,
+however, this functionality is limited to a few basic operations carried out
+during system-wide transitions to a sleep state and back to the working state.
+
+2.3. Runtime Device Power Management
+------------------------------------
+
+The PCI subsystem plays a vital role in the runtime power management of PCI
+devices.  For this purpose it uses the general runtime power management
+(runtime PM) framework described in Documentation/power/runtime_pm.rst.
+Namely, it provides subsystem-level callbacks::
+
+	pci_pm_runtime_suspend()
+	pci_pm_runtime_resume()
+	pci_pm_runtime_idle()
+
+that are executed by the core runtime PM routines.  It also implements the
+entire mechanics necessary for handling runtime wakeup signals from PCI devices
+in low-power states, which at the time of this writing works for both the native
+PCI Express PME signaling and the ACPI GPE-based wakeup signaling described in
+Section 1.
+
+First, a PCI device is put into a low-power state, or suspended, with the help
+of pm_schedule_suspend() or pm_runtime_suspend() which for PCI devices call
+pci_pm_runtime_suspend() to do the actual job.  For this to work, the device's
+driver has to provide a pm->runtime_suspend() callback (see below), which is
+run by pci_pm_runtime_suspend() as the first action.  If the driver's callback
+returns successfully, the device's standard configuration registers are saved,
+the device is prepared to generate wakeup signals and, finally, it is put into
+the target low-power state.
+
+The low-power state to put the device into is the lowest-power (highest number)
+state from which it can signal wakeup.  The exact method of signaling wakeup is
+system-dependent and is determined by the PCI subsystem on the basis of the
+reported capabilities of the device and the platform firmware.  To prepare the
+device for signaling wakeup and put it into the selected low-power state, the
+PCI subsystem can use the platform firmware as well as the device's native PCI
+PM capabilities, if supported.
+
+It is expected that the device driver's pm->runtime_suspend() callback will
+not attempt to prepare the device for signaling wakeup or to put it into a
+low-power state.  The driver ought to leave these tasks to the PCI subsystem
+that has all of the information necessary to perform them.
+
+A suspended device is brought back into the "active" state, or resumed,
+with the help of pm_request_resume() or pm_runtime_resume() which both call
+pci_pm_runtime_resume() for PCI devices.  Again, this only works if the device's
+driver provides a pm->runtime_resume() callback (see below).  However, before
+the driver's callback is executed, pci_pm_runtime_resume() brings the device
+back into the full-power state, prevents it from signaling wakeup while in that
+state and restores its standard configuration registers.  Thus the driver's
+callback need not worry about the PCI-specific aspects of the device resume.
+
+Note that generally pci_pm_runtime_resume() may be called in two different
+situations.  First, it may be called at the request of the device's driver, for
+example if there are some data for it to process.  Second, it may be called
+as a result of a wakeup signal from the device itself (this sometimes is
+referred to as "remote wakeup").  Of course, for this purpose the wakeup signal
+is handled in one of the ways described in Section 1 and finally converted into
+a notification for the PCI subsystem after the source device has been
+identified.
+
+The pci_pm_runtime_idle() function, called for PCI devices by pm_runtime_idle()
+and pm_request_idle(), executes the device driver's pm->runtime_idle()
+callback, if defined, and if that callback doesn't return error code (or is not
+present at all), suspends the device with the help of pm_runtime_suspend().
+Sometimes pci_pm_runtime_idle() is called automatically by the PM core (for
+example, it is called right after the device has just been resumed), in which
+cases it is expected to suspend the device if that makes sense.  Usually,
+however, the PCI subsystem doesn't really know if the device really can be
+suspended, so it lets the device's driver decide by running its
+pm->runtime_idle() callback.
+
+2.4. System-Wide Power Transitions
+----------------------------------
+There are a few different types of system-wide power transitions, described in
+Documentation/driver-api/pm/devices.rst.  Each of them requires devices to be
+handled in a specific way and the PM core executes subsystem-level power
+management callbacks for this purpose.  They are executed in phases such that
+each phase involves executing the same subsystem-level callback for every device
+belonging to the given subsystem before the next phase begins.  These phases
+always run after tasks have been frozen.
+
+2.4.1. System Suspend
+^^^^^^^^^^^^^^^^^^^^^
+
+When the system is going into a sleep state in which the contents of memory will
+be preserved, such as one of the ACPI sleep states S1-S3, the phases are:
+
+	prepare, suspend, suspend_noirq.
+
+The following PCI bus type's callbacks, respectively, are used in these phases::
+
+	pci_pm_prepare()
+	pci_pm_suspend()
+	pci_pm_suspend_noirq()
+
+The pci_pm_prepare() routine first puts the device into the "fully functional"
+state with the help of pm_runtime_resume().  Then, it executes the device
+driver's pm->prepare() callback if defined (i.e. if the driver's struct
+dev_pm_ops object is present and the prepare pointer in that object is valid).
+
+The pci_pm_suspend() routine first checks if the device's driver implements
+legacy PCI suspend routines (see Section 3), in which case the driver's legacy
+suspend callback is executed, if present, and its result is returned.  Next, if
+the device's driver doesn't provide a struct dev_pm_ops object (containing
+pointers to the driver's callbacks), pci_pm_default_suspend() is called, which
+simply turns off the device's bus master capability and runs
+pcibios_disable_device() to disable it, unless the device is a bridge (PCI
+bridges are ignored by this routine).  Next, the device driver's pm->suspend()
+callback is executed, if defined, and its result is returned if it fails.
+Finally, pci_fixup_device() is called to apply hardware suspend quirks related
+to the device if necessary.
+
+Note that the suspend phase is carried out asynchronously for PCI devices, so
+the pci_pm_suspend() callback may be executed in parallel for any pair of PCI
+devices that don't depend on each other in a known way (i.e. none of the paths
+in the device tree from the root bridge to a leaf device contains both of them).
+
+The pci_pm_suspend_noirq() routine is executed after suspend_device_irqs() has
+been called, which means that the device driver's interrupt handler won't be
+invoked while this routine is running.  It first checks if the device's driver
+implements legacy PCI suspends routines (Section 3), in which case the legacy
+late suspend routine is called and its result is returned (the standard
+configuration registers of the device are saved if the driver's callback hasn't
+done that).  Second, if the device driver's struct dev_pm_ops object is not
+present, the device's standard configuration registers are saved and the routine
+returns success.  Otherwise the device driver's pm->suspend_noirq() callback is
+executed, if present, and its result is returned if it fails.  Next, if the
+device's standard configuration registers haven't been saved yet (one of the
+device driver's callbacks executed before might do that), pci_pm_suspend_noirq()
+saves them, prepares the device to signal wakeup (if necessary) and puts it into
+a low-power state.
+
+The low-power state to put the device into is the lowest-power (highest number)
+state from which it can signal wakeup while the system is in the target sleep
+state.  Just like in the runtime PM case described above, the mechanism of
+signaling wakeup is system-dependent and determined by the PCI subsystem, which
+is also responsible for preparing the device to signal wakeup from the system's
+target sleep state as appropriate.
+
+PCI device drivers (that don't implement legacy power management callbacks) are
+generally not expected to prepare devices for signaling wakeup or to put them
+into low-power states.  However, if one of the driver's suspend callbacks
+(pm->suspend() or pm->suspend_noirq()) saves the device's standard configuration
+registers, pci_pm_suspend_noirq() will assume that the device has been prepared
+to signal wakeup and put into a low-power state by the driver (the driver is
+then assumed to have used the helper functions provided by the PCI subsystem for
+this purpose).  PCI device drivers are not encouraged to do that, but in some
+rare cases doing that in the driver may be the optimum approach.
+
+2.4.2. System Resume
+^^^^^^^^^^^^^^^^^^^^
+
+When the system is undergoing a transition from a sleep state in which the
+contents of memory have been preserved, such as one of the ACPI sleep states
+S1-S3, into the working state (ACPI S0), the phases are:
+
+	resume_noirq, resume, complete.
+
+The following PCI bus type's callbacks, respectively, are executed in these
+phases::
+
+	pci_pm_resume_noirq()
+	pci_pm_resume()
+	pci_pm_complete()
+
+The pci_pm_resume_noirq() routine first puts the device into the full-power
+state, restores its standard configuration registers and applies early resume
+hardware quirks related to the device, if necessary.  This is done
+unconditionally, regardless of whether or not the device's driver implements
+legacy PCI power management callbacks (this way all PCI devices are in the
+full-power state and their standard configuration registers have been restored
+when their interrupt handlers are invoked for the first time during resume,
+which allows the kernel to avoid problems with the handling of shared interrupts
+by drivers whose devices are still suspended).  If legacy PCI power management
+callbacks (see Section 3) are implemented by the device's driver, the legacy
+early resume callback is executed and its result is returned.  Otherwise, the
+device driver's pm->resume_noirq() callback is executed, if defined, and its
+result is returned.
+
+The pci_pm_resume() routine first checks if the device's standard configuration
+registers have been restored and restores them if that's not the case (this
+only is necessary in the error path during a failing suspend).  Next, resume
+hardware quirks related to the device are applied, if necessary, and if the
+device's driver implements legacy PCI power management callbacks (see
+Section 3), the driver's legacy resume callback is executed and its result is
+returned.  Otherwise, the device's wakeup signaling mechanisms are blocked and
+its driver's pm->resume() callback is executed, if defined (the callback's
+result is then returned).
+
+The resume phase is carried out asynchronously for PCI devices, like the
+suspend phase described above, which means that if two PCI devices don't depend
+on each other in a known way, the pci_pm_resume() routine may be executed for
+the both of them in parallel.
+
+The pci_pm_complete() routine only executes the device driver's pm->complete()
+callback, if defined.
+
+2.4.3. System Hibernation
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+System hibernation is more complicated than system suspend, because it requires
+a system image to be created and written into a persistent storage medium.  The
+image is created atomically and all devices are quiesced, or frozen, before that
+happens.
+
+The freezing of devices is carried out after enough memory has been freed (at
+the time of this writing the image creation requires at least 50% of system RAM
+to be free) in the following three phases:
+
+	prepare, freeze, freeze_noirq
+
+that correspond to the PCI bus type's callbacks::
+
+	pci_pm_prepare()
+	pci_pm_freeze()
+	pci_pm_freeze_noirq()
+
+This means that the prepare phase is exactly the same as for system suspend.
+The other two phases, however, are different.
+
+The pci_pm_freeze() routine is quite similar to pci_pm_suspend(), but it runs
+the device driver's pm->freeze() callback, if defined, instead of pm->suspend(),
+and it doesn't apply the suspend-related hardware quirks.  It is executed
+asynchronously for different PCI devices that don't depend on each other in a
+known way.
+
+The pci_pm_freeze_noirq() routine, in turn, is similar to
+pci_pm_suspend_noirq(), but it calls the device driver's pm->freeze_noirq()
+routine instead of pm->suspend_noirq().  It also doesn't attempt to prepare the
+device for signaling wakeup and put it into a low-power state.  Still, it saves
+the device's standard configuration registers if they haven't been saved by one
+of the driver's callbacks.
+
+Once the image has been created, it has to be saved.  However, at this point all
+devices are frozen and they cannot handle I/O, while their ability to handle
+I/O is obviously necessary for the image saving.  Thus they have to be brought
+back to the fully functional state and this is done in the following phases:
+
+	thaw_noirq, thaw, complete
+
+using the following PCI bus type's callbacks::
+
+	pci_pm_thaw_noirq()
+	pci_pm_thaw()
+	pci_pm_complete()
+
+respectively.
+
+The first of them, pci_pm_thaw_noirq(), is analogous to pci_pm_resume_noirq().
+It puts the device into the full power state and restores its standard
+configuration registers.  It also executes the device driver's pm->thaw_noirq()
+callback, if defined, instead of pm->resume_noirq().
+
+The pci_pm_thaw() routine is similar to pci_pm_resume(), but it runs the device
+driver's pm->thaw() callback instead of pm->resume().  It is executed
+asynchronously for different PCI devices that don't depend on each other in a
+known way.
+
+The complete phase is the same as for system resume.
+
+After saving the image, devices need to be powered down before the system can
+enter the target sleep state (ACPI S4 for ACPI-based systems).  This is done in
+three phases:
+
+	prepare, poweroff, poweroff_noirq
+
+where the prepare phase is exactly the same as for system suspend.  The other
+two phases are analogous to the suspend and suspend_noirq phases, respectively.
+The PCI subsystem-level callbacks they correspond to::
+
+	pci_pm_poweroff()
+	pci_pm_poweroff_noirq()
+
+work in analogy with pci_pm_suspend() and pci_pm_poweroff_noirq(), respectively,
+although they don't attempt to save the device's standard configuration
+registers.
+
+2.4.4. System Restore
+^^^^^^^^^^^^^^^^^^^^^
+
+System restore requires a hibernation image to be loaded into memory and the
+pre-hibernation memory contents to be restored before the pre-hibernation system
+activity can be resumed.
+
+As described in Documentation/driver-api/pm/devices.rst, the hibernation image
+is loaded into memory by a fresh instance of the kernel, called the boot kernel,
+which in turn is loaded and run by a boot loader in the usual way.  After the
+boot kernel has loaded the image, it needs to replace its own code and data with
+the code and data of the "hibernated" kernel stored within the image, called the
+image kernel.  For this purpose all devices are frozen just like before creating
+the image during hibernation, in the
+
+	prepare, freeze, freeze_noirq
+
+phases described above.  However, the devices affected by these phases are only
+those having drivers in the boot kernel; other devices will still be in whatever
+state the boot loader left them.
+
+Should the restoration of the pre-hibernation memory contents fail, the boot
+kernel would go through the "thawing" procedure described above, using the
+thaw_noirq, thaw, and complete phases (that will only affect the devices having
+drivers in the boot kernel), and then continue running normally.
+
+If the pre-hibernation memory contents are restored successfully, which is the
+usual situation, control is passed to the image kernel, which then becomes
+responsible for bringing the system back to the working state.  To achieve this,
+it must restore the devices' pre-hibernation functionality, which is done much
+like waking up from the memory sleep state, although it involves different
+phases:
+
+	restore_noirq, restore, complete
+
+The first two of these are analogous to the resume_noirq and resume phases
+described above, respectively, and correspond to the following PCI subsystem
+callbacks::
+
+	pci_pm_restore_noirq()
+	pci_pm_restore()
+
+These callbacks work in analogy with pci_pm_resume_noirq() and pci_pm_resume(),
+respectively, but they execute the device driver's pm->restore_noirq() and
+pm->restore() callbacks, if available.
+
+The complete phase is carried out in exactly the same way as during system
+resume.
+
+
+3. PCI Device Drivers and Power Management
+==========================================
+
+3.1. Power Management Callbacks
+-------------------------------
+
+PCI device drivers participate in power management by providing callbacks to be
+executed by the PCI subsystem's power management routines described above and by
+controlling the runtime power management of their devices.
+
+At the time of this writing there are two ways to define power management
+callbacks for a PCI device driver, the recommended one, based on using a
+dev_pm_ops structure described in Documentation/driver-api/pm/devices.rst, and
+the "legacy" one, in which the .suspend() and .resume() callbacks from struct
+pci_driver are used.  The legacy approach, however, doesn't allow one to define
+runtime power management callbacks and is not really suitable for any new
+drivers.  Therefore it is not covered by this document (refer to the source code
+to learn more about it).
+
+It is recommended that all PCI device drivers define a struct dev_pm_ops object
+containing pointers to power management (PM) callbacks that will be executed by
+the PCI subsystem's PM routines in various circumstances.  A pointer to the
+driver's struct dev_pm_ops object has to be assigned to the driver.pm field in
+its struct pci_driver object.  Once that has happened, the "legacy" PM callbacks
+in struct pci_driver are ignored (even if they are not NULL).
+
+The PM callbacks in struct dev_pm_ops are not mandatory and if they are not
+defined (i.e. the respective fields of struct dev_pm_ops are unset) the PCI
+subsystem will handle the device in a simplified default manner.  If they are
+defined, though, they are expected to behave as described in the following
+subsections.
+
+3.1.1. prepare()
+^^^^^^^^^^^^^^^^
+
+The prepare() callback is executed during system suspend, during hibernation
+(when a hibernation image is about to be created), during power-off after
+saving a hibernation image and during system restore, when a hibernation image
+has just been loaded into memory.
+
+This callback is only necessary if the driver's device has children that in
+general may be registered at any time.  In that case the role of the prepare()
+callback is to prevent new children of the device from being registered until
+one of the resume_noirq(), thaw_noirq(), or restore_noirq() callbacks is run.
+
+In addition to that the prepare() callback may carry out some operations
+preparing the device to be suspended, although it should not allocate memory
+(if additional memory is required to suspend the device, it has to be
+preallocated earlier, for example in a suspend/hibernate notifier as described
+in Documentation/driver-api/pm/notifiers.rst).
+
+3.1.2. suspend()
+^^^^^^^^^^^^^^^^
+
+The suspend() callback is only executed during system suspend, after prepare()
+callbacks have been executed for all devices in the system.
+
+This callback is expected to quiesce the device and prepare it to be put into a
+low-power state by the PCI subsystem.  It is not required (in fact it even is
+not recommended) that a PCI driver's suspend() callback save the standard
+configuration registers of the device, prepare it for waking up the system, or
+put it into a low-power state.  All of these operations can very well be taken
+care of by the PCI subsystem, without the driver's participation.
+
+However, in some rare case it is convenient to carry out these operations in
+a PCI driver.  Then, pci_save_state(), pci_prepare_to_sleep(), and
+pci_set_power_state() should be used to save the device's standard configuration
+registers, to prepare it for system wakeup (if necessary), and to put it into a
+low-power state, respectively.  Moreover, if the driver calls pci_save_state(),
+the PCI subsystem will not execute either pci_prepare_to_sleep(), or
+pci_set_power_state() for its device, so the driver is then responsible for
+handling the device as appropriate.
+
+While the suspend() callback is being executed, the driver's interrupt handler
+can be invoked to handle an interrupt from the device, so all suspend-related
+operations relying on the driver's ability to handle interrupts should be
+carried out in this callback.
+
+3.1.3. suspend_noirq()
+^^^^^^^^^^^^^^^^^^^^^^
+
+The suspend_noirq() callback is only executed during system suspend, after
+suspend() callbacks have been executed for all devices in the system and
+after device interrupts have been disabled by the PM core.
+
+The difference between suspend_noirq() and suspend() is that the driver's
+interrupt handler will not be invoked while suspend_noirq() is running.  Thus
+suspend_noirq() can carry out operations that would cause race conditions to
+arise if they were performed in suspend().
+
+3.1.4. freeze()
+^^^^^^^^^^^^^^^
+
+The freeze() callback is hibernation-specific and is executed in two situations,
+during hibernation, after prepare() callbacks have been executed for all devices
+in preparation for the creation of a system image, and during restore,
+after a system image has been loaded into memory from persistent storage and the
+prepare() callbacks have been executed for all devices.
+
+The role of this callback is analogous to the role of the suspend() callback
+described above.  In fact, they only need to be different in the rare cases when
+the driver takes the responsibility for putting the device into a low-power
+state.
+
+In that cases the freeze() callback should not prepare the device system wakeup
+or put it into a low-power state.  Still, either it or freeze_noirq() should
+save the device's standard configuration registers using pci_save_state().
+
+3.1.5. freeze_noirq()
+^^^^^^^^^^^^^^^^^^^^^
+
+The freeze_noirq() callback is hibernation-specific.  It is executed during
+hibernation, after prepare() and freeze() callbacks have been executed for all
+devices in preparation for the creation of a system image, and during restore,
+after a system image has been loaded into memory and after prepare() and
+freeze() callbacks have been executed for all devices.  It is always executed
+after device interrupts have been disabled by the PM core.
+
+The role of this callback is analogous to the role of the suspend_noirq()
+callback described above and it very rarely is necessary to define
+freeze_noirq().
+
+The difference between freeze_noirq() and freeze() is analogous to the
+difference between suspend_noirq() and suspend().
+
+3.1.6. poweroff()
+^^^^^^^^^^^^^^^^^
+
+The poweroff() callback is hibernation-specific.  It is executed when the system
+is about to be powered off after saving a hibernation image to a persistent
+storage.  prepare() callbacks are executed for all devices before poweroff() is
+called.
+
+The role of this callback is analogous to the role of the suspend() and freeze()
+callbacks described above, although it does not need to save the contents of
+the device's registers.  In particular, if the driver wants to put the device
+into a low-power state itself instead of allowing the PCI subsystem to do that,
+the poweroff() callback should use pci_prepare_to_sleep() and
+pci_set_power_state() to prepare the device for system wakeup and to put it
+into a low-power state, respectively, but it need not save the device's standard
+configuration registers.
+
+3.1.7. poweroff_noirq()
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The poweroff_noirq() callback is hibernation-specific.  It is executed after
+poweroff() callbacks have been executed for all devices in the system.
+
+The role of this callback is analogous to the role of the suspend_noirq() and
+freeze_noirq() callbacks described above, but it does not need to save the
+contents of the device's registers.
+
+The difference between poweroff_noirq() and poweroff() is analogous to the
+difference between suspend_noirq() and suspend().
+
+3.1.8. resume_noirq()
+^^^^^^^^^^^^^^^^^^^^^
+
+The resume_noirq() callback is only executed during system resume, after the
+PM core has enabled the non-boot CPUs.  The driver's interrupt handler will not
+be invoked while resume_noirq() is running, so this callback can carry out
+operations that might race with the interrupt handler.
+
+Since the PCI subsystem unconditionally puts all devices into the full power
+state in the resume_noirq phase of system resume and restores their standard
+configuration registers, resume_noirq() is usually not necessary.  In general
+it should only be used for performing operations that would lead to race
+conditions if carried out by resume().
+
+3.1.9. resume()
+^^^^^^^^^^^^^^^
+
+The resume() callback is only executed during system resume, after
+resume_noirq() callbacks have been executed for all devices in the system and
+device interrupts have been enabled by the PM core.
+
+This callback is responsible for restoring the pre-suspend configuration of the
+device and bringing it back to the fully functional state.  The device should be
+able to process I/O in a usual way after resume() has returned.
+
+3.1.10. thaw_noirq()
+^^^^^^^^^^^^^^^^^^^^
+
+The thaw_noirq() callback is hibernation-specific.  It is executed after a
+system image has been created and the non-boot CPUs have been enabled by the PM
+core, in the thaw_noirq phase of hibernation.  It also may be executed if the
+loading of a hibernation image fails during system restore (it is then executed
+after enabling the non-boot CPUs).  The driver's interrupt handler will not be
+invoked while thaw_noirq() is running.
+
+The role of this callback is analogous to the role of resume_noirq().  The
+difference between these two callbacks is that thaw_noirq() is executed after
+freeze() and freeze_noirq(), so in general it does not need to modify the
+contents of the device's registers.
+
+3.1.11. thaw()
+^^^^^^^^^^^^^^
+
+The thaw() callback is hibernation-specific.  It is executed after thaw_noirq()
+callbacks have been executed for all devices in the system and after device
+interrupts have been enabled by the PM core.
+
+This callback is responsible for restoring the pre-freeze configuration of
+the device, so that it will work in a usual way after thaw() has returned.
+
+3.1.12. restore_noirq()
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The restore_noirq() callback is hibernation-specific.  It is executed in the
+restore_noirq phase of hibernation, when the boot kernel has passed control to
+the image kernel and the non-boot CPUs have been enabled by the image kernel's
+PM core.
+
+This callback is analogous to resume_noirq() with the exception that it cannot
+make any assumption on the previous state of the device, even if the BIOS (or
+generally the platform firmware) is known to preserve that state over a
+suspend-resume cycle.
+
+For the vast majority of PCI device drivers there is no difference between
+resume_noirq() and restore_noirq().
+
+3.1.13. restore()
+^^^^^^^^^^^^^^^^^
+
+The restore() callback is hibernation-specific.  It is executed after
+restore_noirq() callbacks have been executed for all devices in the system and
+after the PM core has enabled device drivers' interrupt handlers to be invoked.
+
+This callback is analogous to resume(), just like restore_noirq() is analogous
+to resume_noirq().  Consequently, the difference between restore_noirq() and
+restore() is analogous to the difference between resume_noirq() and resume().
+
+For the vast majority of PCI device drivers there is no difference between
+resume() and restore().
+
+3.1.14. complete()
+^^^^^^^^^^^^^^^^^^
+
+The complete() callback is executed in the following situations:
+
+  - during system resume, after resume() callbacks have been executed for all
+    devices,
+  - during hibernation, before saving the system image, after thaw() callbacks
+    have been executed for all devices,
+  - during system restore, when the system is going back to its pre-hibernation
+    state, after restore() callbacks have been executed for all devices.
+
+It also may be executed if the loading of a hibernation image into memory fails
+(in that case it is run after thaw() callbacks have been executed for all
+devices that have drivers in the boot kernel).
+
+This callback is entirely optional, although it may be necessary if the
+prepare() callback performs operations that need to be reversed.
+
+3.1.15. runtime_suspend()
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The runtime_suspend() callback is specific to device runtime power management
+(runtime PM).  It is executed by the PM core's runtime PM framework when the
+device is about to be suspended (i.e. quiesced and put into a low-power state)
+at run time.
+
+This callback is responsible for freezing the device and preparing it to be
+put into a low-power state, but it must allow the PCI subsystem to perform all
+of the PCI-specific actions necessary for suspending the device.
+
+3.1.16. runtime_resume()
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+The runtime_resume() callback is specific to device runtime PM.  It is executed
+by the PM core's runtime PM framework when the device is about to be resumed
+(i.e. put into the full-power state and programmed to process I/O normally) at
+run time.
+
+This callback is responsible for restoring the normal functionality of the
+device after it has been put into the full-power state by the PCI subsystem.
+The device is expected to be able to process I/O in the usual way after
+runtime_resume() has returned.
+
+3.1.17. runtime_idle()
+^^^^^^^^^^^^^^^^^^^^^^
+
+The runtime_idle() callback is specific to device runtime PM.  It is executed
+by the PM core's runtime PM framework whenever it may be desirable to suspend
+the device according to the PM core's information.  In particular, it is
+automatically executed right after runtime_resume() has returned in case the
+resume of the device has happened as a result of a spurious event.
+
+This callback is optional, but if it is not implemented or if it returns 0, the
+PCI subsystem will call pm_runtime_suspend() for the device, which in turn will
+cause the driver's runtime_suspend() callback to be executed.
+
+3.1.18. Pointing Multiple Callback Pointers to One Routine
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Although in principle each of the callbacks described in the previous
+subsections can be defined as a separate function, it often is convenient to
+point two or more members of struct dev_pm_ops to the same routine.  There are
+a few convenience macros that can be used for this purpose.
+
+The SIMPLE_DEV_PM_OPS macro declares a struct dev_pm_ops object with one
+suspend routine pointed to by the .suspend(), .freeze(), and .poweroff()
+members and one resume routine pointed to by the .resume(), .thaw(), and
+.restore() members.  The other function pointers in this struct dev_pm_ops are
+unset.
+
+The UNIVERSAL_DEV_PM_OPS macro is similar to SIMPLE_DEV_PM_OPS, but it
+additionally sets the .runtime_resume() pointer to the same value as
+.resume() (and .thaw(), and .restore()) and the .runtime_suspend() pointer to
+the same value as .suspend() (and .freeze() and .poweroff()).
+
+The SET_SYSTEM_SLEEP_PM_OPS can be used inside of a declaration of struct
+dev_pm_ops to indicate that one suspend routine is to be pointed to by the
+.suspend(), .freeze(), and .poweroff() members and one resume routine is to
+be pointed to by the .resume(), .thaw(), and .restore() members.
+
+3.1.19. Driver Flags for Power Management
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The PM core allows device drivers to set flags that influence the handling of
+power management for the devices by the core itself and by middle layer code
+including the PCI bus type.  The flags should be set once at the driver probe
+time with the help of the dev_pm_set_driver_flags() function and they should not
+be updated directly afterwards.
+
+The DPM_FLAG_NO_DIRECT_COMPLETE flag prevents the PM core from using the
+direct-complete mechanism allowing device suspend/resume callbacks to be skipped
+if the device is in runtime suspend when the system suspend starts.  That also
+affects all of the ancestors of the device, so this flag should only be used if
+absolutely necessary.
+
+The DPM_FLAG_SMART_PREPARE flag causes the PCI bus type to return a positive
+value from pci_pm_prepare() only if the ->prepare callback provided by the
+driver of the device returns a positive value.  That allows the driver to opt
+out from using the direct-complete mechanism dynamically (whereas setting
+DPM_FLAG_NO_DIRECT_COMPLETE means permanent opt-out).
+
+The DPM_FLAG_SMART_SUSPEND flag tells the PCI bus type that from the driver's
+perspective the device can be safely left in runtime suspend during system
+suspend.  That causes pci_pm_suspend(), pci_pm_freeze() and pci_pm_poweroff()
+to avoid resuming the device from runtime suspend unless there are PCI-specific
+reasons for doing that.  Also, it causes pci_pm_suspend_late/noirq() and
+pci_pm_poweroff_late/noirq() to return early if the device remains in runtime
+suspend during the "late" phase of the system-wide transition under way.
+Moreover, if the device is in runtime suspend in pci_pm_resume_noirq() or
+pci_pm_restore_noirq(), its runtime PM status will be changed to "active" (as it
+is going to be put into D0 going forward).
+
+Setting the DPM_FLAG_MAY_SKIP_RESUME flag means that the driver allows its
+"noirq" and "early" resume callbacks to be skipped if the device can be left
+in suspend after a system-wide transition into the working state.  This flag is
+taken into consideration by the PM core along with the power.may_skip_resume
+status bit of the device which is set by pci_pm_suspend_noirq() in certain
+situations.  If the PM core determines that the driver's "noirq" and "early"
+resume callbacks should be skipped, the dev_pm_skip_resume() helper function
+will return "true" and that will cause pci_pm_resume_noirq() and
+pci_pm_resume_early() to return upfront without touching the device and
+executing the driver callbacks.
+
+3.2. Device Runtime Power Management
+------------------------------------
+
+In addition to providing device power management callbacks PCI device drivers
+are responsible for controlling the runtime power management (runtime PM) of
+their devices.
+
+The PCI device runtime PM is optional, but it is recommended that PCI device
+drivers implement it at least in the cases where there is a reliable way of
+verifying that the device is not used (like when the network cable is detached
+from an Ethernet adapter or there are no devices attached to a USB controller).
+
+To support the PCI runtime PM the driver first needs to implement the
+runtime_suspend() and runtime_resume() callbacks.  It also may need to implement
+the runtime_idle() callback to prevent the device from being suspended again
+every time right after the runtime_resume() callback has returned
+(alternatively, the runtime_suspend() callback will have to check if the
+device should really be suspended and return -EAGAIN if that is not the case).
+
+The runtime PM of PCI devices is enabled by default by the PCI core.  PCI
+device drivers do not need to enable it and should not attempt to do so.
+However, it is blocked by pci_pm_init() that runs the pm_runtime_forbid()
+helper function.  In addition to that, the runtime PM usage counter of
+each PCI device is incremented by local_pci_probe() before executing the
+probe callback provided by the device's driver.
+
+If a PCI driver implements the runtime PM callbacks and intends to use the
+runtime PM framework provided by the PM core and the PCI subsystem, it needs
+to decrement the device's runtime PM usage counter in its probe callback
+function.  If it doesn't do that, the counter will always be different from
+zero for the device and it will never be runtime-suspended.  The simplest
+way to do that is by calling pm_runtime_put_noidle(), but if the driver
+wants to schedule an autosuspend right away, for example, it may call
+pm_runtime_put_autosuspend() instead for this purpose.  Generally, it
+just needs to call a function that decrements the devices usage counter
+from its probe routine to make runtime PM work for the device.
+
+It is important to remember that the driver's runtime_suspend() callback
+may be executed right after the usage counter has been decremented, because
+user space may already have caused the pm_runtime_allow() helper function
+unblocking the runtime PM of the device to run via sysfs, so the driver must
+be prepared to cope with that.
+
+The driver itself should not call pm_runtime_allow(), though.  Instead, it
+should let user space or some platform-specific code do that (user space can
+do it via sysfs as stated above), but it must be prepared to handle the
+runtime PM of the device correctly as soon as pm_runtime_allow() is called
+(which may happen at any time, even before the driver is loaded).
+
+When the driver's remove callback runs, it has to balance the decrementation
+of the device's runtime PM usage counter at the probe time.  For this reason,
+if it has decremented the counter in its probe callback, it must run
+pm_runtime_get_noresume() in its remove callback.  [Since the core carries
+out a runtime resume of the device and bumps up the device's usage counter
+before running the driver's remove callback, the runtime PM of the device
+is effectively disabled for the duration of the remove execution and all
+runtime PM helper functions incrementing the device's usage counter are
+then effectively equivalent to pm_runtime_get_noresume().]
+
+The runtime PM framework works by processing requests to suspend or resume
+devices, or to check if they are idle (in which cases it is reasonable to
+subsequently request that they be suspended).  These requests are represented
+by work items put into the power management workqueue, pm_wq.  Although there
+are a few situations in which power management requests are automatically
+queued by the PM core (for example, after processing a request to resume a
+device the PM core automatically queues a request to check if the device is
+idle), device drivers are generally responsible for queuing power management
+requests for their devices.  For this purpose they should use the runtime PM
+helper functions provided by the PM core, discussed in
+Documentation/power/runtime_pm.rst.
+
+Devices can also be suspended and resumed synchronously, without placing a
+request into pm_wq.  In the majority of cases this also is done by their
+drivers that use helper functions provided by the PM core for this purpose.
+
+For more information on the runtime PM of devices refer to
+Documentation/power/runtime_pm.rst.
+
+
+4. Resources
+============
+
+PCI Local Bus Specification, Rev. 3.0
+
+PCI Bus Power Management Interface Specification, Rev. 1.2
+
+Advanced Configuration and Power Interface (ACPI) Specification, Rev. 3.0b
+
+PCI Express Base Specification, Rev. 2.0
+
+Documentation/driver-api/pm/devices.rst
+
+Documentation/power/runtime_pm.rst