How do I access a USB serial port without an entry in /dev?

Question

I have a USB to serial adapter based on the SiLabs CP2101 chipset. I'm trying to use it with Debian 5.0 Lenny (armel architecture).

I seem to be able to get the proper kernel module to recognize the adapter, but it won't create /dev/ttyUSB0.

I ran modprobe usbserial and modprobe cp2101 and then connected the device, resulting in the following output from dmesg:

usb 2-1: new full speed USB device using mv5182_ehci and address 8
usb 2-1: configuration #1 chosen from 1 choice
cp2101 2-1:1.0: cp2101 converter detected
usb 2-1: reset full speed USB device using mv5182_ehci and address 8
usb 2-1: cp2101 converter now attached to ttyUSB0

However, /dev/ttyUSB0 does not exist. There are no new files in /dev.

find / -type f -name ttyUSB0 gives no results.

I did find a bunch of directories named ttyUSB0 that showed up after plugging in the device:

/sys/class/tty/ttyUSB0
/sys/bus/usb-serial/drivers/cp2101/ttyUSB0
/sys/bus/usb-serial/devices/ttyUSB0
/sys/devices/platform/mv5182_ehci.1/usb2/2-1/2-1:1.0/ttyUSB0
 ...

I found a file at /sys/bus/usb-serial/devices/ttyUSB0/ttyUSB0/dev but it does not appear to be a serial port; I can't open it with screen or Python's serial module.

To compare, I tried my Debian 6.0 development machine (which has a newer version of the cp2101 module now called cp210x) and after seeing the "converter now attached to ttyUSB0" line in dmesg, /dev/ttyUSB0 appears.

How do I get the cp2101 module to create /dev/ttyUSB0? If I can't, what file do I open to access the serial port?

Solution

You could try making the device node manually. On my system /dev/ttyUSB0 is major 188 and minor 0. And hopefully, my supposition is correct that it's a type c meaning character device (maybe try u for unbuffered character device if c doesn't work).

mknod /dev/ttyUSB0 c 188 0

If this worked, and the device file was appearing automatically before, something in your udev is messed up. There could also be an issue with USB subsystem settings or drivers.

Comments

[Steven T. Snyder]

This worked. Thanks! I still need to figure out how to get it do this automatically, but at least I can continue with testing.

[Steven T. Snyder]

BTW, this is a fresh Debian 5.0 image on an embedded system. It could just be that the kernel modules (or settings) that came with Debian 5.0 don't work right. Where can I look for settings that might be relevant?