The SGI IOC4 PCI device is a bit of a strange beast, so some notes on
  it are in order.
  
  First, even though the IOC4 performs multiple functions, such as an
  IDE controller, a serial controller, a PS/2 keyboard/mouse controller,
  and an external interrupt mechanism, it's not implemented as a
  multifunction device.  The consequence of this from a software
  standpoint is that all these functions share a single IRQ, and
  they can't all register to own the same PCI device ID.  To make
  matters a bit worse, some of the register blocks (and even registers
  themselves) present in IOC4 are mixed-purpose between these several
  functions, meaning that there's no clear "owning" device driver.
  
  The solution is to organize the IOC4 driver into several independent
  drivers, "ioc4", "sgiioc4", and "ioc4_serial".  Note that there is no
  PS/2 controller driver as this functionality has never been wired up
  on a shipping IO card.
  
  ioc4
  ====
  This is the core (or shim) driver for IOC4.  It is responsible for
  initializing the basic functionality of the chip, and allocating
  the PCI resources that are shared between the IOC4 functions.
  
  This driver also provides registration functions that the other
  IOC4 drivers can call to make their presence known.  Each driver
  needs to provide a probe and remove function, which are invoked
  by the core driver at appropriate times.  The interface of these
  IOC4 function probe and remove operations isn't precisely the same
  as PCI device probe and remove operations, but is logically the
  same operation.
  
  sgiioc4
  =======
  This is the IDE driver for IOC4.  Its name isn't very descriptive
  simply for historical reasons (it used to be the only IOC4 driver
  component).  There's not much to say about it other than it hooks
  up to the ioc4 driver via the appropriate registration, probe, and
  remove functions.
  
  ioc4_serial
  ===========
  This is the serial driver for IOC4.  There's not much to say about it
  other than it hooks up to the ioc4 driver via the appropriate registration,
  probe, and remove functions.