Documentation update for the driver model core

This patch (as1509) documents two important points regarding the use
of device structures in the driver model:

	Structures must be initialized to all 0's before they are
	passed to device_initialize().

	Structures must not be passed to device_add() or
	device_register() more than once.

Although these restrictions have applied ever since the driver model
was first created, they have not been mentioned anywhere.
Signed-off-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/base/core.c

@@ -632,6 +632,11 @@ static void klist_children_put(struct klist_node *n)
  * may be used for reference counting of @dev after calling this
  * function.
  *
+ * All fields in @dev must be initialized by the caller to 0, except
+ * for those explicitly set to some other value.  The simplest
+ * approach is to use kzalloc() to allocate the structure containing
+ * @dev.
+ *
  * NOTE: Use put_device() to give up your reference instead of freeing
  * @dev directly once you have called this function.
  */
@@ -930,6 +935,13 @@ int device_private_init(struct device *dev)
  * to the global and sibling lists for the device, then
  * adds it to the other relevant subsystems of the driver model.
  *
+ * Do not call this routine or device_register() more than once for
+ * any device structure.  The driver model core is not designed to work
+ * with devices that get unregistered and then spring back to life.
+ * (Among other things, it's very hard to guarantee that all references
+ * to the previous incarnation of @dev have been dropped.)  Allocate
+ * and register a fresh new struct device instead.
+ *
  * NOTE: _Never_ directly free @dev after calling this function, even
  * if it returned an error! Always use put_device() to give up your
  * reference instead.
@@ -1090,6 +1102,9 @@ int device_add(struct device *dev)
  * have a clearly defined need to use and refcount the device
  * before it is added to the hierarchy.
  *
+ * For more information, see the kerneldoc for device_initialize()
+ * and device_add().
+ *
  * NOTE: _Never_ directly free @dev after calling this function, even
  * if it returned an error! Always use put_device() to give up the
  * reference initialized in this function instead.