[power] Update documentation.

Documentation/power/devices.txt

@@ -70,3 +70,74 @@ System devices will only be suspended with interrupts disabled, and
 after all other devices have been suspended. On resume, they will be
 resumed before any other devices, and also with interrupts disabled.
 
+
+Runtime Power Management
+
+Many devices are able to dynamically power down while the system is
+still running. This feature is useful for devices that are not being
+used, and can offer significant power savings on a running system. 
+
+In each device's directory, there is a 'power' directory, which
+contains at least a 'state' file. Reading from this file displays what
+power state the device is currently in. Writing to this file initiates
+a transition to the specified power state, which must be a decimal in
+the range 1-3, inclusive; or 0 for 'On'.
+
+The PM core will call the ->suspend() method in the bus_type object
+that the device belongs to if the specified state is not 0, or
+->resume() if it is. 
+
+Nothing will happen if the specified state is the same state the
+device is currently in. 
+
+If the device is already in a low-power state, and the specified state
+is another, but different, low-power state, the ->resume() method will
+first be called to power the device back on, then ->suspend() will be
+called again with the new state. 
+
+The driver is responsible for saving the working state of the device
+and putting it into the low-power state specified. If this was
+successful, it returns 0, and the device's power_state field is
+updated. 
+
+The driver must take care to know whether or not it is able to
+properly resume the device, including all step of reinitialization
+necessary. (This is the hardest part, and the one most protected by
+NDA'd documents). 
+
+The driver must also take care not to suspend a device that is
+currently in use. It is their responsibility to provide their own
+exclusion mechanisms.
+
+The runtime power transition happens with interrupts enabled. If a
+device cannot support being powered down with interrupts, it may
+return -EAGAIN (as it would during a system power management
+transition),  but it will _not_ be called again, and the transaction
+will fail.
+
+There is currently no way to know what states a device or driver
+supports a priori. This will change in the future. 
+
+
+Driver Detach Power Management
+
+The kernel now supports the ability to place a device in a low-power
+state when it is detached from its driver, which happens when its
+module is removed. 
+
+Each device contains a 'detach_state' file in its sysfs directory
+which can be used to control this state. Reading from this file
+displays what the current detach state is set to. This is 0 (On) by
+default. A user may write a positive integer value to this file in the
+range of 1-4 inclusive. 
+
+A value of 1-3 will indicate the device should be placed in that
+low-power state, which will cause ->suspend() to be called for that
+device. A value of 4 indicates that the device should be shutdown, so
+->shutdown() will be called for that device. 
+
+The driver is responsible for reinitializing the device when the
+module is re-inserted during it's ->probe() (or equivalent) method. 
+The driver core will not call any extra functions when binding the
+device to the driver. 
+