Getty on USB serial port

["William D. Jones" <thor0505%comcast.net@localhost>]

Hello all,

I'm trying to add a secondary serial port to my Raspberry Pi 1 Model A for 
debugging reasons (I want to repurpose the main UART for `pppd` as an 
experiment). Right now, I'm stuck on trying to spawn a getty on the USB 
serial port.

According to dmseg, my USB-to-serial device is a Prolific device:

```
ucom0 at uplcom0
uplcom0 at uhub0 port 1
uplcom0: Prolific Technology Inc. (0x67b) USB 2.0 To COM Device (0x2303), 
rev 1.10/3.00, addr 2
```

According to `man uplcom` and `man ucom`, `ucom` devices are treated as 
`ttys` and are available under `/dev/ttyU?`, where I presume `?` matches the 
number assigned to `ucom?`. So, using the above `dmesg` output, I add  an 
entry to `/etc/ttys`, expecting it to be picked up on reboot:
```
ttyU0   "/usr/libexec/getty default"    vt100   on secure
```

However, this change had no effect and I didn't see any output on the system 
attached at the other end of the USB serial port upon rebooting. At this 
point I figured it's possible that the baud rate is incorrect, but I figured 
I should debug from a root shell first.

However, no matter which arguments I invoke the getty program_, getty 
_insists_ on using the main console, to dump input and output. This leads to 
fun things that I had no idea were possible like, for example (when invoking 
using `/usr/libexec/getty default ttyU0`):

* Getting a login prompt printed over the UART associated with `console` 
that says "(`ttyU0`)", when `ttyU0` should be associated with the USB serial 
port.
* Logging into said login prompt, invoking `tty` command, and the output 
saying my current tty is a psuedoterminal: `/dev/pts/0`

At this point I'm stumped. What would cause me to have the above conditions? 
And what is the proper command I should use from a root shell (after logging 
in over `console` UART) to spawn a login prompt over the USB serial port?

As always, thanks all for any help.

Sincerely,

--
William D. Jones
thor0505%comcast.net@localhost