[a system with a printer hooked up on the serial port]
> > hangs on boot trying to set ttyflags.
> 
> Where did it hang before? I thought it always hung at boot?

I'm not sure of the relevancy of this here, but I've been seeing boot
"hang" when setting tty flags, too.  I don't know why the kernel does
this, but I know why it appears to hang.  When setting tty flags, the
serial driver somehow asserts the DTR line, and it tells my modem to
spit out whatever in its output buffer to the computer (usually "NO
CAREER").  And the serial driver echoes back the characters which in
turn are echoed right back at the serial port by the modem.  This
dribble of characters between the serial port and the modem eats up
the CPU time and makes boot look like it hangs.  Power cycling the
modem stops the dribble and let the kernel proceed the bootstrap.
I've been seeing this behavior in kernels made of the source of at
least the last couple of months up to 2nd of this month.

Could it be the cause of the "boot hang" problem with printers?

ken