Serial control protocol character driver implementation

[Super user]

I'm in need of a good method/model under which to implement
a fully reentrant character driver of constantly variable
"synchronousness".  The device is a smart "realtime serial control
protocol" interface running at roughly 30KB (typical mapping: CTRL,
STAT, DATA), and as such, many data expected by this interface
(xx_write()) simply cannot be anticipated prior to knowing what it
wants next (xx_read()).  However, certain sequences of TYPES of data
can be anticipated, allowing for the driver be asynchronous under many
states, even if it means multiple-queueing likely candidate data streams
(or templates) for immediate access by xx_intr().  The target system is
SCO Xenix/Unix 386 2.3.2 and higher.  While the device DOES generate
interrupts at all logical points, the driver MUST be able to guarantee an
acceptable response to any and all data requests at full bandwidth.
Buffering on the device itself is limited to several data, and the
serial control protocol offers absolutely NO type of flow control or
error detection/correction.

So.    Under the premise that the driver MUST meet its obligations:
  
  1) Are there similar current driver models available for study?
  2) Is it wise to include a substantial amount of logic in xx_intr()?
  3) Is it possible to realistically assume that a driver could be
     designed to allow more than one user full access to this
     interface?
  4) Any good one-liners to fend off the WP and Lotus-jockey-type
     people if I have to go fully synchronous?

--Dsc--