bug#80660: Can't set baud rate to 600

The "genspeed" script looks for all the common baud speeds in
/usr/include/$(uname -m)-linux-gnu/bits/termios.h; if B600 were missing
from there, you'd see a different error message, along the lines “stty:
invalid argument ‘600’”.

The error “stty: /dev/ttyS0: unable to perform all requested operations”
suggests that the kernel is rejecting the tcsetattr call; have you by any
chance upgraded the kernel recently?

-Martin