THE SERIAL PORT on my Fedora 15 install, mysteriously refused to be accessed. I discovered that when inserting a USB-to-Serial device, GNU screen would refuse to access the created device /dev/ttyUSB0.

» screen /dev/ttyUSB0 115200
[screen is terminating]

Since I could use screen for serial access as root, and because the newly installed Fedora did have some hiccups in adding my user (the /home/monzool directory already existed from a previous Ubuntu install), I first checked group permissions, but they seemed fine for this situation.

» ll /dev/ttyUSB0
crw-rw----. 1 root dialout 188,  0 Jun  6 08:27 /dev/ttyUSB0
 
» groups
monzool tty wheel uucp dialout tcpdump screen vboxusers

Screen didn’t offer much indication of the problem, but using strace I could see that some of the last things checked for permissions where /var/run/screen. I then removed that directory and recreated the directory setup by starting screen with sudo.

» ll /var/run/screen
drwxrwxr-x. 4 root    root    80 Jun  6 08:27 screen
» rm -rf /var/run/screen
» sudo screen /dev/ttyUSB0
» ll /var/run/screen
drwxrwxr-x. 4 root    screen    80 Jun  6 08:47 screen

This helped nothing! :-(

I then tried minicom, which was more informative about the problem

» minicom
minicom: cannot open /dev/ttyUSB0: Device or resource busy

This would mean that something else had hijacked the port. A quick check confirmed that something called gpsd was using the port.

»  lsof /dev/ttyUSB0
COMMAND PID   USER   FD   TYPE DEVICE SIZE/OFF  NODE NAME
gpsd    883 nobody    8u   CHR  188,0      0t0 11408 /dev/ttyUSB0
»  ps ax | grep gpsd
  883 ?        S<s    0:00 gpsd -n -F /var/run/gpsd.sock

Now gpsd is for handling GPS devices, but it made no sense to trigger this daemon for a simple USB-to-Serial adapter.

Knowing what was causing the hazzle, I found this bug rapport https://bugzilla.redhat.com/show_bug.cgi?id=663124. In it, it is proposed to set USBAUTO=no in /etc/sysconfig/gpsd.

» echo "USBAUTO=no" >> /etc/sysconfig/gpsd

And sure enough, this fixed the problem. The USB-to-Serial adapter could now be accessed by any serial terminal.