IBM Retail Store Solutions dept has certain PS/2 keyboards which extend the
standard PS/2 specification in order to support addition hardware built into the
keyboard (such as a Magnetic Strip Reader, Keylock, Tone generator, extra keys,
extra LEDs, etc).  This addition hardware behaves in a manner that makes it
unusable if driven by a standard PS/2 driver (sadly, due to the fact that its
design is "IP" I can't elaborate on how or why it is incompatible with the
standard PS/2 specification).

In order to use these keyboards, a the standard PS/2 driver needs to behave a
bit differently; thus attached is a modifcation to the PS/2 driver which allows
other drivers to register with the PS/2 driver as 'filters'.  There is a
arbitrary max number of 'filters' set to 1, which is a compile-time define.
The registered drivers are called (in order of registration) for every scancode,
and they may change or consume the scancode (or allow it to pass).  Also the
'filters' are given a function to send an variable-sized buffer to the keyboard
output port; this function is synchronized using a semaphore which also
coordinates with pckbd_leds().

-Dan