serial port O_SYNC functionality in 2.4.5

[James R Bruce <bruce+@andrew.cmu.edu>]

As far as I can tell from observed operation and from perusing the
code, O_SYNC doesn't seem to be supported by the serial driver in
2.4.5.  Writes are forced as far as the serial.c's circular queue, but
O_SYNC seems to be ignored from there on, so any application trying to
do small synchronous writes to the serial port will end up backing it
up PAGE_SIZE bytes rather than getting synchronous operation (which is
incidentally what I was trying to do when I ran into this :).

I'm unfamiliar with the serial driver so I'm not sure the right way to
fix it is, but perhaps using a smaller value for WAKEUP_CHARS from
drivers/char/serial.c when the port is opened with O_SYNC
functionality might do the trick (i.e. 0 rather than 256).

Hopefully someone familiar with that part of the code please can tell
me what should be done... or if a patch or configuration option can
already get that functionality in another way.

Thanks,
  Jim Bruce