Several functions in the serial drivers can be called from bottom
half or interrupt context. They must use the GFP_ATOMIC flag for
calls to kmalloc() and usb_submit_urb().
  
Functions which must use GFP_ATOMIC:
1. All *_callback() functions.
2. Any code which is inside a spinlock.
3. write(), throttle(), unthrottle(), which may be called by
   the line discipline in bottom half context.
  
Functions which can use GFP_KERNEL:
1. open(), close(), startup(), shutdown(), set_termios().