+/**
+ * struct pm_ops - device PM callbacks
+ *
+ * Several driver power state transitions are externally visible, affecting
+ * the state of pending I/O queues and (for drivers that touch hardware)
+ * interrupts, wakeups, DMA, and other hardware state. There may also be
+ * internal transitions to various low power modes, which are transparent
+ * to the rest of the driver stack (such as a driver that's ON gating off
+ * clocks which are not in active use).
+ *
+ * The externally visible transitions are handled with the help of the following
+ * callbacks included in this structure:
+ *
+ * @prepare: Prepare the device for the upcoming transition, but do NOT change
+ * its hardware state. Prevent new children of the device from being
+ * registered after @prepare() returns (the driver's subsystem and
+ * generally the rest of the kernel is supposed to prevent new calls to the
+ * probe method from being made too once @prepare() has succeeded). If
+ * @prepare() detects a situation it cannot handle (e.g. registration of a
+ * child already in progress), it may return -EAGAIN, so that the PM core
+ * can execute it once again (e.g. after the new child has been registered)
+ * to recover from the race condition. This method is executed for all
+ * kinds of suspend transitions and is followed by one of the suspend
+ * callbacks: @suspend(), @freeze(), or @poweroff().
+ * The PM core executes @prepare() for all devices before starting to
+ * execute suspend callbacks for any of them, so drivers may assume all of
+ * the other devices to be present and functional while @prepare() is being
+ * executed. In particular, it is safe to make GFP_KERNEL memory
+ * allocations from within @prepare(). However, drivers may NOT assume
+ * anything about the availability of the user space at that time and it
+ * is not correct to request firmware from within @prepare() (it's too
+ * late to do that). [To work around this limitation, drivers may
+ * register suspend and hibernation notifiers that are executed before the
+ * freezing of tasks.]
+ *
+ * @complete: Undo the changes made by @prepare(). This method is executed for
+ * all kinds of resume transitions, following one of the resume callbacks:
+ * @resume(), @thaw(), @restore(). Also called if the state transition
+ * fails before the driver's suspend callback (@suspend(), @freeze(),
+ * @poweroff()) can be executed (e.g. if the suspend callback fails for one
+ * of the other devices that the PM core has unsuccessfully attempted to
+ * suspend earlier).
+ * The PM core executes @complete() after it has executed the appropriate
+ * resume callback for all devices.
+ *
+ * @suspend: Executed before putting the system into a sleep state in which the
+ * contents of main memory are preserved. Quiesce the device, put it into
+ * a low power state appropriate for the upcoming system state (such as
+ * PCI_D3hot), and enable wakeup events as appropriate.
+ *
+ * @resume: Executed after waking the system up from a sleep state in which the
+ * contents of main memory were preserved. Put the device into the
+ * appropriate state, according to the information saved in memory by the
+ * preceding @suspend(). The driver starts working again, responding to
+ * hardware events and software requests. The hardware may have gone
+ * through a power-off reset, or it may have maintained state from the
+ * previous suspend() which the driver may rely on while resuming. On most
+ * platforms, there are no restrictions on availability of resources like
+ * clocks during @resume().
+ *
+ * @freeze: Hibernation-specific, executed before creating a hibernation image.
+ * Quiesce operations so that a consistent image can be created, but do NOT
+ * otherwise put the device into a low power device state and do NOT emit
+ * system wakeup events. Save in main memory the device settings to be
+ * used by @restore() during the subsequent resume from hibernation or by
+ * the subsequent @thaw(), if the creation of the image or the restoration
+ * of main memory contents from it fails.
+ *
+ * @thaw: Hibernation-specific, executed after creating a hibernation image OR
+ * if the creation of the image fails. Also executed after a failing
+ * attempt to restore the contents of main memory from such an image.
+ * Undo the changes made by the preceding @freeze(), so the device can be
+ * operated in the same way as immediately before the call to @freeze().
+ *
+ * @poweroff: Hibernation-specific, executed after saving a hibernation image.
+ * Quiesce the device, put it into a low power state appropriate for the
+ * upcoming system state (such as PCI_D3hot), and enable wakeup events as
+ * appropriate.
+ *
+ * @restore: Hibernation-specific, executed after restoring the contents of main
+ * memory from a hibernation image. Driver starts working again,
+ * responding to hardware events and software requests. Drivers may NOT
+ * make ANY assumptions about the hardware state right prior to @restore().
+ * On most platforms, there are no restrictions on availability of
+ * resources like clocks during @restore().
+ *
+ * All of the above callbacks, except for @complete(), return error codes.
+ * However, the error codes returned by the resume operations, @resume(),
+ * @thaw(), and @restore(), do not cause the PM core to abort the resume
+ * transition during which they are returned. The error codes returned in
+ * that cases are only printed by the PM core to the system logs for debugging
+ * purposes. Still, it is recommended that drivers only return error codes
+ * from their resume methods in case of an unrecoverable failure (i.e. when the
+ * device being handled refuses to resume and becomes unusable) to allow us to
+ * modify the PM core in the future, so that it can avoid attempting to handle
+ * devices that failed to resume and their children.
+ *
+ * It is allowed to unregister devices while the above callbacks are being
+ * executed. However, it is not allowed to unregister a device from within any
+ * of its own callbacks.