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/power/devices.txt and
  Documentation/power/runtime_pm.txt.
  
  ---------------------------------------------------------------------------
  
  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 a low-power state (D1-D3), but they are not required to be capable
  of generating PMEs from all supported low-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	d3_delay;	/* D3->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.txt.
  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/power/devices.txt.  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(),
  but it doesn't put the device into the full power state and doesn't attempt to
  restore 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 it 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/power/devices.txt, 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/power/devices.txt, and the
  "legacy" one, in which the .suspend(), .suspend_late(), .resume_early(), 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/power/notifiers.txt).
  
  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.
  
  3.2. 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 disabled by default.  It is also blocked by
  pci_pm_init() that runs the pm_runtime_forbid() helper function.  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 should enable this
  feature by executing the pm_runtime_enable() helper function.  However, the
  driver should not call the pm_runtime_allow() helper function unblocking
  the runtime PM of the device.  Instead, it should allow user space or some
  platform-specific code to do that (user space can do it via sysfs), although
  once it has called pm_runtime_enable(), 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).  [It also is possible that user space causes
  pm_runtime_allow() to be called via sysfs before the driver is loaded, so in
  fact the driver has to be prepared to handle the runtime PM of the device as
  soon as it calls pm_runtime_enable().]
  
  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.txt.
  
  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.txt.
  
  
  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/power/devices.txt
  Documentation/power/runtime_pm.txt