Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/suspend-2.6

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/suspend-2.6:
  PM: PM QOS update fix
  Freezer / cgroup freezer: Update stale locking comments
  PM / platform_bus: Allow runtime PM by default
  i2c: Fix bus-level power management callbacks
  PM QOS update
  PM / Hibernate: Fix block_io.c printk warning
  PM / Hibernate: Group swap ops
  PM / Hibernate: Move the first_sector out of swsusp_write
  PM / Hibernate: Separate block_io
  PM / Hibernate: Snapshot cleanup
  FS / libfs: Implement simple_write_to_buffer
  PM / Hibernate: document open(/dev/snapshot) side effects
  PM / Runtime: Add sysfs debug files
  PM: Improve device power management document
  PM: Update device power management document
  PM: Allow runtime_suspend methods to call pm_schedule_suspend()
  PM: pm_wakeup - switch to using bool

31 files changed, 1251 insertions, 949 deletions

diff --git a/Documentation/power/devices.txt b/Documentation/power/devices.txt
index c9abbd86bc18..57080cd74575 100644
--- a/Documentation/power/devices.txt
+++ b/Documentation/power/devices.txt
@@ -1,7 +1,13 @@
+Device Power Management
+
+Copyright (c) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
+Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
+
+
 Most of the code in Linux is device drivers, so most of the Linux power
-management code is also driver-specific.  Most drivers will do very little;
-others, especially for platforms with small batteries (like cell phones),
-will do a lot.
+management (PM) code is also driver-specific.  Most drivers will do very
+little; others, especially for platforms with small batteries (like cell
+phones), will do a lot.
 
 This writeup gives an overview of how drivers interact with system-wide
 power management goals, emphasizing the models and interfaces that are
@@ -15,9 +21,10 @@ Drivers will use one or both of these models to put devices into low-power
 states:
 
     System Sleep model:
-	Drivers can enter low power states as part of entering system-wide
-	low-power states like "suspend-to-ram", or (mostly for systems with
-	disks) "hibernate" (suspend-to-disk).
+	Drivers can enter low-power states as part of entering system-wide
+	low-power states like "suspend" (also known as "suspend-to-RAM"), or
+	(mostly for systems with disks) "hibernation" (also known as
+	"suspend-to-disk").
 
 	This is something that device, bus, and class drivers collaborate on
 	by implementing various role-specific suspend and resume methods to
@@ -25,33 +32,41 @@ states:
 	them without loss of data.
 
 	Some drivers can manage hardware wakeup events, which make the system
-	leave that low-power state.  This feature may be disabled using the
-	relevant /sys/devices/.../power/wakeup file; enabling it may cost some
-	power usage, but let the whole system enter low power states more often.
+	leave the low-power state.  This feature may be enabled or disabled
+	using the relevant /sys/devices/.../power/wakeup file (for Ethernet
+	drivers the ioctl interface used by ethtool may also be used for this
+	purpose); enabling it may cost some power usage, but let the whole
+	system enter low-power states more often.
 
     Runtime Power Management model:
-	Drivers may also enter low power states while the system is running,
-	independently of other power management activity.  Upstream drivers
-	will normally not know (or care) if the device is in some low power
-	state when issuing requests; the driver will auto-resume anything
-	that's needed when it gets a request.
-
-	This doesn't have, or need much infrastructure; it's just something you
-	should do when writing your drivers.  For example, clk_disable() unused
-	clocks as part of minimizing power drain for currently-unused hardware.
-	Of course, sometimes clusters of drivers will collaborate with each
-	other, which could involve task-specific power management.
-
-There's not a lot to be said about those low power states except that they
-are very system-specific, and often device-specific.  Also, that if enough
-drivers put themselves into low power states (at "runtime"), the effect may be
-the same as entering some system-wide low-power state (system sleep) ... and
-that synergies exist, so that several drivers using runtime pm might put the
-system into a state where even deeper power saving options are available.
-
-Most suspended devices will have quiesced all I/O:  no more DMA or irqs, no
-more data read or written, and requests from upstream drivers are no longer
-accepted.  A given bus or platform may have different requirements though.
+	Devices may also be put into low-power states while the system is
+	running, independently of other power management activity in principle.
+	However, devices are not generally independent of each other (for
+	example, a parent device cannot be suspended unless all of its child
+	devices have been suspended).  Moreover, depending on the bus type the
+	device is on, it may be necessary to carry out some bus-specific
+	operations on the device for this purpose.  Devices put into low power
+	states at run time may require special handling during system-wide power
+	transitions (suspend or hibernation).
+
+	For these reasons not only the device driver itself, but also the
+	appropriate subsystem (bus type, device type or device class) driver and
+	the PM core are involved in runtime power management.  As in the system
+	sleep power management case, they need to collaborate by implementing
+	various role-specific suspend and resume methods, so that the hardware
+	is cleanly powered down and reactivated without data or service loss.
+
+There's not a lot to be said about those low-power states except that they are
+very system-specific, and often device-specific.  Also, that if enough devices
+have been put into low-power states (at runtime), the effect may be very similar
+to entering some system-wide low-power state (system sleep) ... and that
+synergies exist, so that several drivers using runtime PM might put the system
+into a state where even deeper power saving options are available.
+
+Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
+for wakeup events), no more data read or written, and requests from upstream
+drivers are no longer accepted.  A given bus or platform may have different
+requirements though.
 
 Examples of hardware wakeup events include an alarm from a real time clock,
 network wake-on-LAN packets, keyboard or mouse activity, and media insertion
@@ -60,129 +75,152 @@ or removal (for PCMCIA, MMC/SD, USB, and so on).
 
 Interfaces for Entering System Sleep States
 ===========================================
-Most of the programming interfaces a device driver needs to know about
-relate to that first model:  entering a system-wide low power state,
-rather than just minimizing power consumption by one device.
-
-
-Bus Driver Methods
-------------------
-The core methods to suspend and resume devices reside in struct bus_type.
-These are mostly of interest to people writing infrastructure for busses
-like PCI or USB, or because they define the primitives that device drivers
-may need to apply in domain-specific ways to their devices:
-
-struct bus_type {
-	...
-	int  (*suspend)(struct device *dev, pm_message_t state);
-	int  (*resume)(struct device *dev);
+There are programming interfaces provided for subsystems (bus type, device type,
+device class) and device drivers to allow them to participate in the power
+management of devices they are concerned with.  These interfaces cover both
+system sleep and runtime power management.
+
+
+Device Power Management Operations
+----------------------------------
+Device power management operations, at the subsystem level as well as at the
+device driver level, are implemented by defining and populating objects of type
+struct dev_pm_ops:
+
+struct dev_pm_ops {
+	int (*prepare)(struct device *dev);
+	void (*complete)(struct device *dev);
+	int (*suspend)(struct device *dev);
+	int (*resume)(struct device *dev);
+	int (*freeze)(struct device *dev);
+	int (*thaw)(struct device *dev);
+	int (*poweroff)(struct device *dev);
+	int (*restore)(struct device *dev);
+	int (*suspend_noirq)(struct device *dev);
+	int (*resume_noirq)(struct device *dev);
+	int (*freeze_noirq)(struct device *dev);
+	int (*thaw_noirq)(struct device *dev);
+	int (*poweroff_noirq)(struct device *dev);
+	int (*restore_noirq)(struct device *dev);
+	int (*runtime_suspend)(struct device *dev);
+	int (*runtime_resume)(struct device *dev);
+	int (*runtime_idle)(struct device *dev);
 };
 
-Bus drivers implement those methods as appropriate for the hardware and
-the drivers using it; PCI works differently from USB, and so on.  Not many
-people write bus drivers; most driver code is a "device driver" that
-builds on top of bus-specific framework code.
+This structure is defined in include/linux/pm.h and the methods included in it
+are also described in that file.  Their roles will be explained in what follows.
+For now, it should be sufficient to remember that the last three methods are
+specific to runtime power management while the remaining ones are used during
+system-wide power transitions.
 
-For more information on these driver calls, see the description later;
-they are called in phases for every device, respecting the parent-child
-sequencing in the driver model tree.  Note that as this is being written,
-only the suspend() and resume() are widely available; not many bus drivers
-leverage all of those phases, or pass them down to lower driver levels.
+There also is a deprecated "old" or "legacy" interface for power management
+operations available at least for some subsystems.  This approach does not use
+struct dev_pm_ops objects and it is suitable only for implementing system sleep
+power management methods.  Therefore it is not described in this document, so
+please refer directly to the source code for more information about it.
 
 
-/sys/devices/.../power/wakeup files
------------------------------------
-All devices in the driver model have two flags to control handling of
-wakeup events, which are hardware signals that can force the device and/or
-system out of a low power state.  These are initialized by bus or device
-driver code using device_init_wakeup(dev,can_wakeup).
+Subsystem-Level Methods
+-----------------------
+The core methods to suspend and resume devices reside in struct dev_pm_ops
+pointed to by the pm member of struct bus_type, struct device_type and
+struct class.  They are mostly of interest to the people writing infrastructure
+for buses, like PCI or USB, or device type and device class drivers.
 
-The "can_wakeup" flag just records whether the device (and its driver) can
-physically support wakeup events.  When that flag is clear, the sysfs
-"wakeup" file is empty, and device_may_wakeup() returns false.
+Bus drivers implement these methods as appropriate for the hardware and the
+drivers using it; PCI works differently from USB, and so on.  Not many people
+write subsystem-level drivers; most driver code is a "device driver" that builds
+on top of bus-specific framework code.
 
-For devices that can issue wakeup events, a separate flag controls whether
-that device should try to use its wakeup mechanism.  The initial value of
-device_may_wakeup() will be true, so that the device's "wakeup" file holds
-the value "enabled".  Userspace can change that to "disabled" so that
-device_may_wakeup() returns false; or change it back to "enabled" (so that
-it returns true again).
+For more information on these driver calls, see the description later;
+they are called in phases for every device, respecting the parent-child
+sequencing in the driver model tree.
 
 
-EXAMPLE:  PCI Device Driver Methods
+/sys/devices/.../power/wakeup files
 -----------------------------------
-PCI framework software calls these methods when the PCI device driver bound
-to a device device has provided them:
-
-struct pci_driver {
-	...
-	int  (*suspend)(struct pci_device *pdev, pm_message_t state);
-	int  (*suspend_late)(struct pci_device *pdev, pm_message_t state);
+All devices in the driver model have two flags to control handling of wakeup
+events (hardware signals that can force the device and/or system out of a low
+power state).  These flags are initialized by bus or device driver code using
+device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
+include/linux/pm_wakeup.h.
 
-	int  (*resume_early)(struct pci_device *pdev);
-	int  (*resume)(struct pci_device *pdev);
-};
-
-Drivers will implement those methods, and call PCI-specific procedures
-like pci_set_power_state(), pci_enable_wake(), pci_save_state(), and
-pci_restore_state() to manage PCI-specific mechanisms.  (PCI config space
-could be saved during driver probe, if it weren't for the fact that some
-systems rely on userspace tweaking using setpci.)  Devices are suspended
-before their bridges enter low power states, and likewise bridges resume
-before their devices.
-
-
-Upper Layers of Driver Stacks
------------------------------
-Device drivers generally have at least two interfaces, and the methods
-sketched above are the ones which apply to the lower level (nearer PCI, USB,
-or other bus hardware).  The network and block layers are examples of upper
-level interfaces, as is a character device talking to userspace.
-
-Power management requests normally need to flow through those upper levels,
-which often use domain-oriented requests like "blank that screen".  In
-some cases those upper levels will have power management intelligence that
-relates to end-user activity, or other devices that work in cooperation.
-
-When those interfaces are structured using class interfaces, there is a
-standard way to have the upper layer stop issuing requests to a given
-class device (and restart later):
-
-struct class {
-	...
-	int  (*suspend)(struct device *dev, pm_message_t state);
-	int  (*resume)(struct device *dev);
-};
-
-Those calls are issued in specific phases of the process by which the
-system enters a low power "suspend" state, or resumes from it.
-
-
-Calling Drivers to Enter System Sleep States
-============================================
-When the system enters a low power state, each device's driver is asked
-to suspend the device by putting it into state compatible with the target
+The "can_wakeup" flag just records whether the device (and its driver) can
+physically support wakeup events.  The device_set_wakeup_capable() routine
+affects this flag.  The "should_wakeup" flag controls whether the device should
+try to use its wakeup mechanism.  device_set_wakeup_enable() affects this flag;
+for the most part drivers should not change its value.  The initial value of
+should_wakeup is supposed to be false for the majority of devices; the major
+exceptions are power buttons, keyboards, and Ethernet adapters whose WoL
+(wake-on-LAN) feature has been set up with ethtool.
+
+Whether or not a device is capable of issuing wakeup events is a hardware
+matter, and the kernel is responsible for keeping track of it.  By contrast,
+whether or not a wakeup-capable device should issue wakeup events is a policy
+decision, and it is managed by user space through a sysfs attribute: the
+power/wakeup file.  User space can write the strings "enabled" or "disabled" to
+set or clear the should_wakeup flag, respectively.  Reads from the file will
+return the corresponding string if can_wakeup is true, but if can_wakeup is
+false then reads will return an empty string, to indicate that the device
+doesn't support wakeup events.  (But even though the file appears empty, writes
+will still affect the should_wakeup flag.)
+
+The device_may_wakeup() routine returns true only if both flags are set.
+Drivers should check this routine when putting devices in a low-power state
+during a system sleep transition, to see whether or not to enable the devices'
+wakeup mechanisms.  However for runtime power management, wakeup events should
+be enabled whenever the device and driver both support them, regardless of the
+should_wakeup flag.
+
+
+/sys/devices/.../power/control files
+------------------------------------
+Each device in the driver model has a flag to control whether it is subject to
+runtime power management.  This flag, called runtime_auto, is initialized by the
+bus type (or generally subsystem) code using pm_runtime_allow() or
+pm_runtime_forbid(); the default is to allow runtime power management.
+
+The setting can be adjusted by user space by writing either "on" or "auto" to
+the device's power/control sysfs file.  Writing "auto" calls pm_runtime_allow(),
+setting the flag and allowing the device to be runtime power-managed by its
+driver.  Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
+the device to full power if it was in a low-power state, and preventing the
+device from being runtime power-managed.  User space can check the current value
+of the runtime_auto flag by reading the file.
+
+The device's runtime_auto flag has no effect on the handling of system-wide
+power transitions.  In particular, the device can (and in the majority of cases
+should and will) be put into a low-power state during a system-wide transition
+to a sleep state even though its runtime_auto flag is clear.
+
+For more information about the runtime power management framework, refer to
+Documentation/power/runtime_pm.txt.
+
+
+Calling Drivers to Enter and Leave System Sleep States
+======================================================
+When the system goes into a sleep state, each device's driver is asked to
+suspend the device by putting it into a state compatible with the target
 system state.  That's usually some version of "off", but the details are
 system-specific.  Also, wakeup-enabled devices will usually stay partly
 functional in order to wake the system.
 
-When the system leaves that low power state, the device's driver is asked
-to resume it.  The suspend and resume operations always go together, and
-both are multi-phase operations.
+When the system leaves that low-power state, the device's driver is asked to
+resume it by returning it to full power.  The suspend and resume operations
+always go together, and both are multi-phase operations.
 
-For simple drivers, suspend might quiesce the device using the class code
-and then turn its hardware as "off" as possible with late_suspend.  The
+For simple drivers, suspend might quiesce the device using class code
+and then turn its hardware as "off" as possible during suspend_noirq.  The
 matching resume calls would then completely reinitialize the hardware
 before reactivating its class I/O queues.
 
-More power-aware drivers drivers will use more than one device low power
-state, either at runtime or during system sleep states, and might trigger
-system wakeup events.
+More power-aware drivers might prepare the devices for triggering system wakeup
+events.
 
 
 Call Sequence Guarantees
 ------------------------
-To ensure that bridges and similar links needed to talk to a device are
+To ensure that bridges and similar links needing to talk to a device are
 available when the device is suspended or resumed, the device tree is
 walked in a bottom-up order to suspend devices.  A top-down order is
 used to resume those devices.
@@ -194,67 +232,310 @@ its parent; and can't be removed or suspended after that parent.
 The policy is that the device tree should match hardware bus topology.
 (Or at least the control bus, for devices which use multiple busses.)
 In particular, this means that a device registration may fail if the parent of
-the device is suspending (ie. has been chosen by the PM core as the next
+the device is suspending (i.e. has been chosen by the PM core as the next
 device to suspend) or has already suspended, as well as after all of the other
 devices have been suspended.  Device drivers must be prepared to cope with such
 situations.
 
 
-Suspending Devices
-------------------
-Suspending a given device is done in several phases.  Suspending the
-system always includes every phase, executing calls for every device
-before the next phase begins.  Not all busses or classes support all
-these callbacks; and not all drivers use all the callbacks.
+System Power Management Phases
+------------------------------
+Suspending or resuming the system is done in several phases.  Different phases
+are used for standby or memory sleep states ("suspend-to-RAM") and the
+hibernation state ("suspend-to-disk").  Each phase involves executing callbacks
+for every device before the next phase begins.  Not all busses or classes
+support all these callbacks and not all drivers use all the callbacks.  The
+various phases always run after tasks have been frozen and before they are
+unfrozen.  Furthermore, the *_noirq phases run at a time when IRQ handlers have
+been disabled (except for those marked with the IRQ_WAKEUP flag).
 
-The phases are seen by driver notifications issued in this order:
+Most phases use bus, type, and class callbacks (that is, methods defined in
+dev->bus->pm, dev->type->pm, and dev->class->pm).  The prepare and complete
+phases are exceptions; they use only bus callbacks.  When multiple callbacks
+are used in a phase, they are invoked in the order: <class, type, bus> during
+power-down transitions and in the opposite order during power-up transitions.
+For example, during the suspend phase the PM core invokes
 
-   1	class.suspend(dev, message) is called after tasks are frozen, for
-	devices associated with a class that has such a method.  This
-	method may sleep.
+	dev->class->pm.suspend(dev);
+	dev->type->pm.suspend(dev);
+	dev->bus->pm.suspend(dev);
 
-	Since I/O activity usually comes from such higher layers, this is
-	a good place to quiesce all drivers of a given type (and keep such
-	code out of those drivers).
+before moving on to the next device, whereas during the resume phase the core
+invokes
 
-   2	bus.suspend(dev, message) is called next.  This method may sleep,
-	and is often morphed into a device driver call with bus-specific
-	parameters and/or rules.
+	dev->bus->pm.resume(dev);
+	dev->type->pm.resume(dev);
+	dev->class->pm.resume(dev);
 
-	This call should handle parts of device suspend logic that require
-	sleeping.  It probably does work to quiesce the device which hasn't
-	been abstracted into class.suspend().
+These callbacks may in turn invoke device- or driver-specific methods stored in
+dev->driver->pm, but they don't have to.
 
-The pm_message_t parameter is currently used to refine those semantics
-(described later).
 
-At the end of those phases, drivers should normally have stopped all I/O
-transactions (DMA, IRQs), saved enough state that they can re-initialize
-or restore previous state (as needed by the hardware), and placed the
-device into a low-power state.  On many platforms they will also use
-clk_disable() to gate off one or more clock sources; sometimes they will
-also switch off power supplies, or reduce voltages.  Drivers which have
-runtime PM support may already have performed some or all of the steps
-needed to prepare for the upcoming system sleep state.
+Entering System Suspend
+-----------------------
+When the system goes into the standby or memory sleep state, the phases are:
+
+		prepare, suspend, suspend_noirq.
+
+    1.	The prepare phase is meant to prevent races by preventing new devices
+	from being registered; the PM core would never know that all the
+	children of a device had been suspended if new children could be
+	registered at will.  (By contrast, devices may be unregistered at any
+	time.)  Unlike the other suspend-related phases, during the prepare
+	phase the device tree is traversed top-down.
+
+	The prepare phase uses only a bus callback.  After the callback method
+	returns, no new children may be registered below the device.  The method
+	may also prepare the device or driver in some way for the upcoming
+	system power transition, but it should not put the device into a
+	low-power state.
+
+    2.	The suspend methods should quiesce the device to stop it from performing
+	I/O.  They also may save the device registers and put it into the
+	appropriate low-power state, depending on the bus type the device is on,
+	and they may enable wakeup events.
+
+    3.	The suspend_noirq phase occurs after IRQ handlers have been disabled,
+	which means that the driver's interrupt handler will not be called while
+	the callback method is running.  The methods should save the values of
+	the device's registers that weren't saved previously and finally put the
+	device into the appropriate low-power state.
+
+	The majority of subsystems and device drivers need not implement this
+	callback.  However, bus types allowing devices to share interrupt
+	vectors, like PCI, generally need it; otherwise a driver might encounter
+	an error during the suspend phase by fielding a shared interrupt
+	generated by some other device after its own device had been set to low
+	power.
+
+At the end of these phases, drivers should have stopped all I/O transactions
+(DMA, IRQs), saved enough state that they can re-initialize or restore previous
+state (as needed by the hardware), and placed the device into a low-power state.
+On many platforms they will gate off one or more clock sources; sometimes they
+will also switch off power supplies or reduce voltages.  (Drivers supporting
+runtime PM may already have performed some or all of these steps.)
+
+If device_may_wakeup(dev) returns true, the device should be prepared for
+generating hardware wakeup signals to trigger a system wakeup event when the
+system is in the sleep state.  For example, enable_irq_wake() might identify
+GPIO signals hooked up to a switch or other external hardware, and
+pci_enable_wake() does something similar for the PCI PME signal.
+
+If any of these callbacks returns an error, the system won't enter the desired
+low-power state.  Instead the PM core will unwind its actions by resuming all
+the devices that were suspended.
+
+
+Leaving System Suspend
+----------------------
+When resuming from standby or memory sleep, the phases are:
+
+		resume_noirq, resume, complete.
+
+    1.	The resume_noirq callback methods should perform any actions needed
+	before the driver's interrupt handlers are invoked.  This generally
+	means undoing the actions of the suspend_noirq phase.  If the bus type
+	permits devices to share interrupt vectors, like PCI, the method should
+	bring the device and its driver into a state in which the driver can
+	recognize if the device is the source of incoming interrupts, if any,
+	and handle them correctly.
+
+	For example, the PCI bus type's ->pm.resume_noirq() puts the device into
+	the full-power state (D0 in the PCI terminology) and restores the
+	standard configuration registers of the device.  Then it calls the
+	device driver's ->pm.resume_noirq() method to perform device-specific
+	actions.
+
+    2.	The resume methods should bring the the device back to its operating
+	state, so that it can perform normal I/O.  This generally involves
+	undoing the actions of the suspend phase.
+
+    3.	The complete phase uses only a bus callback.  The method should undo the
+	actions of the prepare phase.  Note, however, that new children may be
+	registered below the device as soon as the resume callbacks occur; it's
+	not necessary to wait until the complete phase.
+
+At the end of these phases, drivers should be as functional as they were before
+suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
+gated on.  Even if the device was in a low-power state before the system sleep
+because of runtime power management, afterwards it should be back in its
+full-power state.  There are multiple reasons why it's best to do this; they are
+discussed in more detail in Documentation/power/runtime_pm.txt.
 
-When any driver sees that its device_can_wakeup(dev), it should make sure
-to use the relevant hardware signals to trigger a system wakeup event.
-For example, enable_irq_wake() might identify GPIO signals hooked up to
-a switch or other external hardware, and pci_enable_wake() does something
-similar for PCI's PME# signal.
+However, the details here may again be platform-specific.  For example,
+some systems support multiple "run" states, and the mode in effect at
+the end of resume might not be the one which preceded suspension.
+That means availability of certain clocks or power supplies changed,
+which could easily affect how a driver works.
+
+Drivers need to be able to handle hardware which has been reset since the
+suspend methods were called, for example by complete reinitialization.
+This may be the hardest part, and the one most protected by NDA'd documents
+and chip errata.  It's simplest if the hardware state hasn't changed since
+the suspend was carried out, but that can't be guaranteed (in fact, it ususally
+is not the case).
+
+Drivers must also be prepared to notice that the device has been removed
+while the system was powered down, whenever that's physically possible.
+PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
+where common Linux platforms will see such removal.  Details of how drivers
+will notice and handle such removals are currently bus-specific, and often
+involve a separate thread.
+
+These callbacks may return an error value, but the PM core will ignore such
+errors since there's nothing it can do about them other than printing them in
+the system log.
+
+
+Entering Hibernation
+--------------------
+Hibernating the system is more complicated than putting it into the standby or
+memory sleep state, because it involves creating and saving a system image.
+Therefore there are more phases for hibernation, with a different set of
+callbacks.  These phases always run after tasks have been frozen and memory has
+been freed.
+
+The general procedure for hibernation is to quiesce all devices (freeze), create
+an image of the system memory while everything is stable, reactivate all
+devices (thaw), write the image to permanent storage, and finally shut down the
+system (poweroff).  The phases used to accomplish this are:
+
+	prepare, freeze, freeze_noirq, thaw_noirq, thaw, complete,
+	prepare, poweroff, poweroff_noirq
+
+    1.	The prepare phase is discussed in the "Entering System Suspend" section
+	above.
+
+    2.	The freeze methods should quiesce the device so that it doesn't generate
+	IRQs or DMA, and they may need to save the values of device registers.
+	However the device does not have to be put in a low-power state, and to
+	save time it's best not to do so.  Also, the device should not be
+	prepared to generate wakeup events.
+
+    3.	The freeze_noirq phase is analogous to the suspend_noirq phase discussed
+	above, except again that the device should not be put in a low-power
+	state and should not be allowed to generate wakeup events.
+
+At this point the system image is created.  All devices should be inactive and
+the contents of memory should remain undisturbed while this happens, so that the
+image forms an atomic snapshot of the system state.
+
+    4.	The thaw_noirq phase is analogous to the resume_noirq phase discussed
+	above.  The main difference is that its methods can assume the device is
+	in the same state as at the end of the freeze_noirq phase.
+
+    5.	The thaw phase is analogous to the resume phase discussed above.  Its
+	methods should bring the device back to an operating state, so that it
+	can be used for saving the image if necessary.
+
+    6.	The complete phase is discussed in the "Leaving System Suspend" section
+	above.
+
+At this point the system image is saved, and the devices then need to be
+prepared for the upcoming system shutdown.  This is much like suspending them
+before putting the system into the standby or memory sleep state, and the phases
+are similar.
+
+    7.	The prepare phase is discussed above.
+
+    8.	The poweroff phase is analogous to the suspend phase.
+
+    9.	The poweroff_noirq phase is analogous to the suspend_noirq phase.
+
+The poweroff and poweroff_noirq callbacks should do essentially the same things
+as the suspend and suspend_noirq callbacks.  The only notable difference is that
+they need not store the device register values, because the registers should
+already have been stored during the freeze or freeze_noirq phases.<