USB Legacy support
  ~~~~~~~~~~~~~~~~~~
  
  Vojtech Pavlik <vojtech@suse.cz>, January 2004
  
  
  Also known as "USB Keyboard" or "USB Mouse support" in the BIOS Setup is a
  feature that allows one to use the USB mouse and keyboard as if they were
  their classic PS/2 counterparts.  This means one can use an USB keyboard to
  type in LILO for example.
  
  It has several drawbacks, though:
  
  1) On some machines, the emulated PS/2 mouse takes over even when no USB
     mouse is present and a real PS/2 mouse is present.  In that case the extra
     features (wheel, extra buttons, touchpad mode) of the real PS/2 mouse may
     not be available.
  
  2) If CONFIG_HIGHMEM64G is enabled, the PS/2 mouse emulation can cause
     system crashes, because the SMM BIOS is not expecting to be in PAE mode.
     The Intel E7505 is a typical machine where this happens.
  
  3) If AMD64 64-bit mode is enabled, again system crashes often happen,
     because the SMM BIOS isn't expecting the CPU to be in 64-bit mode.  The
     BIOS manufacturers only test with Windows, and Windows doesn't do 64-bit
     yet.
  
  Solutions:
  
  Problem 1) can be solved by loading the USB drivers prior to loading the
  PS/2 mouse driver. Since the PS/2 mouse driver is in 2.6 compiled into
  the kernel unconditionally, this means the USB drivers need to be
  compiled-in, too.
  
  Problem 2) can currently only be solved by either disabling HIGHMEM64G
  in the kernel config or USB Legacy support in the BIOS. A BIOS update
  could help, but so far no such update exists.
  
  Problem 3) is usually fixed by a BIOS update. Check the board
  manufacturers web site. If an update is not available, disable USB
  Legacy support in the BIOS. If this alone doesn't help, try also adding
  idle=poll on the kernel command line. The BIOS may be entering the SMM
  on the HLT instruction as well.