Re: USB serial adapter for serial console

On Fri, Jan 30, 2026 at 12:31:34PM +0000, Stuart Henderson wrote:
> On 2026-01-30, Stuart McKim <stuart@mckims.net> wrote:
> > I have a file server that does not have a serial port, only vga. Since
> > it's normally headless, I'm trying to use a USB serial adapter to
> > 1. login
>
> that should be possible
>
> > 2. display boot messages
>
> that will not be possible with a USB adapter

I was worried that might be the case. Is there a way to get kernel panic
messages via serial in such a case without taking photos of the VGA
console? That's the end goal, as I've been having lockups and until the
last time I didn't have a display connected, but then that time I
goobered it up while trying to take photos.

> > The adapter shows up as:
> > uftdi0 at uhub3 port 1 configuration 1 interface 0 "FTDI USB Serial Converter" rev 2.00/6.00 addr 2
> > ucom0 at uftdi0 portno 1: usb3.0.00001.0
> >
> > I've been able to test that it's functional by connecting to it from my
> > desktop, which has a serial port, using:
> > cu -l /dev/cuaU0
> > And all is well.
> >
> > Starting with goal 1 (login):
> >
> > Trying to add the appropriate entry to /etc/ttys is where I am running
> > into trouble. First I tried:
> > ttyU0 "/usr/libexec/getty std.9600" vt220 on secure
> >
> > When hitting Enter on my desktop end, I get new lines, but no prompt.
> >
> > I changed the entry to:
> > cuaU0 "/usr/libexec/getty std.9600" vt220 on secure
> >
> > Now I get a login prompt, but also receive the following error after
> > logging in:
> > ksh: No controlling tty (open /dev/tty: Device busy)
> > ksh: warning: won't have full job control
> >
> > And it's not really a usable session since I don't have a tty.
> >
> > What am I doing wrong?
>
> It should be ttyU0 not cuaU0 in /etc/ttys. cua is just meant for
> outbound connections, it allows sharing a single device with
> tty for incoming connections - see tty(4) for info.

That was my understanding based on the documentation, but posted what
else I tried in case it was helpful.

> It's possible some of the hardware lines aren't connected up, if so
> and if it's difficult to fix, you could try with 'softcar' and see
> if that gets you a login prompt?

The following modification makes no change in behavior (new lines, but
no login prompt):
ttyU0 "/usr/libexec/getty std.9600" vt220 on secure softcar


--
Stuart A. McKim, P.E.
Idaho Falls, ID