This patch supports bidirectional I/O (read and write) for printers that
can transmit status information back to the host over the parallel port.

This includes HP printers released since 1992 (e.g. Laserjet 4 and up, 
Deskjet 1200 and up).

It also includes a streamlined interrupt-driven write.  It uses MUCH less
CPU time on printers with fast interfaces.  I would be very interested
to hear whether it works on old and cranky printers.


===== Installation ====================

   $ cd /usr/src
   $ patch -p < lp-bidir.patch


===== Using bidirectional I/O ====================

It's pretty simple: 

    $ cat /dev/lp1
    @PJL INFO STATUS
    CODE=10001
    DISPLAY="00 READY"
    ONLINE=TRUE
    

At the system call level, open("/dev/lp1", O_RDONLY) succeeds and then
read() returns what the printer has to say.  It never returns EOF, just
waits until the printer sends something and returns that.  O_NONBLOCK
can be used to check whether the printer has anything to say.


===== Using interrupt-driven write ====================

No change, still

    # tunelp /dev/lp1 -i 7

or whatever.  

As a benchmark, I tried sending a continuous stream of NULs (ascii 0)
to a Laserjet 5.  This is about as close as I can get to testing the
raw interface speed.

The standard lp.c driver sends about 115k/sec using 100% of the CPU.

The new driver sends about 97 k/sec using 2% of the CPU, but that's
very misleading since it ignores time spent in servicing interrupts.
I haven't been able to measure it satisfactorily, but I estimate it's
using about 50-75% of the CPU at that speed.

In actually printing ghostscript output (raster graphics), the printer
bursts data at about 50k/sec for a few seconds, then the interface is
idle for 20-30 seconds.

At any rate, there is a noticeable effect on response time with the
old driver, and none with the new driver.  Printing isn't any faster.
With a non-laser printer there probably will be little difference in
CPU use or response time, since they consume data more steadily
instead of in huge gulps.