USB: documentation update for the pre_reset method

This patch (as1459) updates the documentation for the pre_reset method
in struct usb_driver.  When a driver is notified of an impending
reset, it must cancel all outstanding I/O and not start any new I/O
until it has been notified that the reset is complete.

As far as I know, most existing drivers that implement pre_reset do
this now.  The major exceptions appear to be the SpeedTouch and
CDC-WDM drivers.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/usb/callbacks.txt

@@ -95,9 +95,11 @@ pre_reset
 
 int (*pre_reset)(struct usb_interface *intf);
 
-Another driver or user space is triggering a reset on the device which
-contains the interface passed as an argument. Cease IO and save any
-device state you need to restore.
+A driver or user space is triggering a reset on the device which
+contains the interface passed as an argument. Cease IO, wait for all
+outstanding URBs to complete, and save any device state you need to
+restore.  No more URBs may be submitted until the post_reset method
+is called.
 
 If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
 are in atomic context.

include/linux/usb.h

@@ -806,8 +806,10 @@ struct usbdrv_wrap {
  * @resume: Called when the device is being resumed by the system.
  * @reset_resume: Called when the suspended device has been reset instead
  *	of being resumed.
- * @pre_reset: Called by usb_reset_device() when the device
- *	is about to be reset.
+ * @pre_reset: Called by usb_reset_device() when the device is about to be
+ *	reset.  This routine must not return until the driver has no active
+ *	URBs for the device, and no more URBs may be submitted until the
+ *	post_reset method is called.
  * @post_reset: Called by usb_reset_device() after the device
  *	has been reset
  * @id_table: USB drivers use ID table to support hotplugging.