diff --git a/include/linux/pm.h b/include/linux/pm.h
index 096fb6f754cf..6b27e07aef19 100644
--- a/include/linux/pm.h
+++ b/include/linux/pm.h
@@ -142,29 +142,61 @@ typedef struct pm_message {
 } pm_message_t;
 
 /*
- * There are 4 important states driver can be in:
- * ON     -- driver is working
- * FREEZE -- stop operations and apply whatever policy is applicable to a
- *           suspended driver of that class, freeze queues for block like IDE
- *           does, drop packets for ethernet, etc... stop DMA engine too etc...
- *           so a consistent image can be saved; but do not power any hardware
- *           down.
- * SUSPEND - like FREEZE, but hardware is doing as much powersaving as
- *           possible. Roughly pci D3.
+ * 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).
  *
- * Unfortunately, current drivers only recognize numeric values 0 (ON) and 3
- * (SUSPEND).  We'll need to fix the drivers. So yes, putting 3 to all different
- * defines is intentional, and will go away as soon as drivers are fixed.  Also
- * note that typedef is neccessary, we'll probably want to switch to
- *   typedef struct pm_message_t { int event; int flags; } pm_message_t
- * or something similar soon.
+ * One transition is triggered by resume(), after a suspend() call; the
+ * message is implicit:
+ *
+ * ON		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 will rely on while
+ * 		resuming.  On most platforms, there are no restrictions on
+ * 		availability of resources like clocks during resume().
+ *
+ * Other transitions are triggered by messages sent using suspend().  All
+ * these transitions quiesce the driver, so that I/O queues are inactive.
+ * That commonly entails turning off IRQs and DMA; there may be rules
+ * about how to quiesce that are specific to the bus or the device's type.
+ * (For example, network drivers mark the link state.)  Other details may
+ * differ according to the message:
+ *
+ * SUSPEND	Quiesce, enter a low power device state appropriate for
+ * 		the upcoming system state (such as PCI_D3hot), and enable
+ * 		wakeup events as appropriate.
+ *
+ * FREEZE	Quiesce operations so that a consistent image can be saved;
+ * 		but do NOT otherwise enter a low power device state, and do
+ * 		NOT emit system wakeup events.
+ *
+ * PRETHAW	Quiesce as if for FREEZE; additionally, prepare for restoring
+ * 		the system from a snapshot taken after an earlier FREEZE.
+ * 		Some drivers will need to reset their hardware state instead
+ * 		of preserving it, to ensure that it's never mistaken for the
+ * 		state which that earlier snapshot had set up.
+ *
+ * A minimally power-aware driver treats all messages as SUSPEND, fully
+ * reinitializes its device during resume() -- whether or not it was reset
+ * during the suspend/resume cycle -- and can't issue wakeup events.
+ *
+ * More power-aware drivers may also use low power states at runtime as
+ * well as during system sleep states like PM_SUSPEND_STANDBY.  They may
+ * be able to use wakeup events to exit from runtime low-power states,
+ * or from system low-power states such as standby or suspend-to-RAM.
  */
 
 #define PM_EVENT_ON 0
 #define PM_EVENT_FREEZE 1
 #define PM_EVENT_SUSPEND 2
+#define PM_EVENT_PRETHAW 3
 
 #define PMSG_FREEZE	((struct pm_message){ .event = PM_EVENT_FREEZE, })
+#define PMSG_PRETHAW	((struct pm_message){ .event = PM_EVENT_PRETHAW, })
 #define PMSG_SUSPEND	((struct pm_message){ .event = PM_EVENT_SUSPEND, })
 #define PMSG_ON		((struct pm_message){ .event = PM_EVENT_ON, })