Kernel configuration
====================
Add the following two lines to your kernel configuration file:

  device puc
  device uart

Also be sure to remove or comment out the "device sio" line in your
kernel configuration, otherwise sio(4) will get used!


/boot/loader.conf configuration
===============================
Add the following lines to /boot/loader.conf, which will enable uart(4)
for two serial ports (COM1 and COM2), and disable COM3 and COM4:

  # For use with uart(4)
  hint.uart.0.at="isa"
  hint.uart.0.port="0x3F8"
  hint.uart.0.flags="0x10"
  hint.uart.0.irq="4"
  hint.uart.1.at="isa"
  hint.uart.1.port="0x2F8"
  hint.uart.1.irq="3"
  hint.uart.2.disabled="1"
  hint.uart.3.disabled="1"

NOTE #1:
I'm not even sure these hints are needed.  I would need to look at the
actual driver to determine that.  It may be that some features are
determined based on /boot.config or other loader.conf settings (such
as set console="comconsole", etc.).

NOTE #2:
Some people have recommended you add these to /boot/device.hints, but I
recommend otherwise.  You can add hints to loader.conf which is a
"local" configuration file; device.hints(5) is supposed to be updated by
the FreeBSD driver development folks as needed, and in my opinion should
only be updated via mergemaster(8).


/etc/ttys configuration
=======================
The uart(4) driver uses a different naming convention for the serial
ports (both callin and callout).  With sio(4), the callin ports are
ttydX while with uart(4) they are ttyuX.

So, add some entries to /etc/ttys:

  # Serial terminals (for uart(4))
  ttyu0   "/usr/libexec/getty std.9600"   dialup  off secure
  ttyu1   "/usr/libexec/getty std.9600"   dialup  off secure
  ttyu2   "/usr/libexec/getty std.9600"   dialup  off secure
  ttyu3   "/usr/libexec/getty std.9600"   dialup  off secure

Make appropriate changes the above as you see fit (such as enabling
ttyu0, setting a different port speed, etc.)


Rebuild the kernel
==================
Now rebuild your kernel and reboot.

  cd /usr/src
  make buildkernel
  make installkernel
  reboot


That's it!
==========
You should see a line similar to the following in your dmesg on bootup
(usually right after the keyboard/system console section):

  uart0: <16550 or compatible> at port 0x3f8-0x3ff irq 4 flags 0x10 on isa0


Caveats/Issues
==============
One issue I've encountered with uart(4) is characters on the serial port
being either lost, or entire lines/paragraphs duplicated, even when
hardware RTS/CTS flow control is used.  I've also seen this happen
with the sio(4) driver, but not as often.

Example (induced by hitting <Enter> a few times):

  login:
  FreeBSD/i386 (medusa.parodius.com) (ttyu0)

  login:
  login:
  )
  (medusa.parodius.com) (ttyu0)

  login:

  login:
  (medusa.parodius.com) (ttyu0)

  login:
  FreeBSD/i386 (medusa.parodius.com) (ttyu0)

This only happens at the console login prompt (e.g. getty + login), and
does not happen once the user has logged in and spawned a shell.  Thus,
stty(1) must be doing something to the tty that makes it behave
properly.  Making a gettytab entry which included the "hw" gettytab(5)
entry did not fix the problem, nor did using XON/XOFF flow control.

I reported this issue to Marcus Moolenaar, who had the following to say:

>  Oddly enough, this is normal behaviour. What happens is that login(1)
>  terminates when you hit enter. getty(8) re-initializes the line and
>  starts up a login(1) again. This causes the TTY buffers to prune and
>  data to be lost. This all happens in layers above uart(4). It's not
>  (necessarily) an indication of a flow-control problem.