[SOLVED] Have multiple USB<->RS2332 devices. Which /dev/ttyUSBx goes to each device?

intestinal fortitude · 08-12-2022, 08:50 AM

I have multiple USB<->serial devices. They use the FTDI chip. As each USB device is plugged in, they are correctly detected by the kernel and given a serial port, /dev/ttyUSB0, /dev/ttyUSB1, etc.

Devices can be plugged in any order, they can be unplugged, and plugged in again, etc. How do I map which serial port is which USB device? i.e. I want to ask the kernel which /dev/ttyUSBx device it assigned to a specific USB<->serial device.

This question has come up a few times on stackexchange. Using shell scripting, it can be done by following directory links in /sys/bus/usb/devices, looking for a line in uevent files, etc. I want to do this from C or C++, and was wondering if there's system calls I can use to query the kernel/driver, instead of searching directories for files, parsing links and open/reading/parsing text files.

I am familiar with libusb, so I already know how to get the list of USB devices and find my devices: bus #, device #, ID, manufacturer string, serial number string, etc.

Thanks!

dugan · 08-12-2022, 04:12 PM

/sys is the interface.

Remember, everything is a file, yadda yadda.

michaelk · 08-12-2022, 05:10 PM

You should be able to write a udev rule based on the serial number to assign a particular device ID to each adapter.

Code:

SUBSYSTEM=="tty", SUBSYSTEMS=="usb", ATTRS{idVendor}=="xxxx", ATTRS{idProduct}=="xxxx", ATTRS{serial}=="xxxx", SYMLINK+="ttyUSBX"

Using udevadm you can see all the attributes for each particular device.

udevadm info --name=/dev/ttyUSB0 --attribute-walk

intestinal fortitude · 08-16-2022, 10:59 AM

Thanks for the help!

IMO, having a separate /dev/ttyUSBx for each device is the cleaner solution. I am not allowed to make any changes to system configuration files (for reasons of work policy).

I will parse the files in /sys.

GREYES71 · 11-01-2022, 05:23 PM

Quote:

Originally Posted by intestinal fortitude

Thanks for the help!

IMO, having a separate /dev/ttyUSBx for each device is the cleaner solution. I am not allowed to make any changes to system configuration files (for reasons of work policy).

I will parse the files in /sys.

Hi

I have a question.
What application do you use to communicate the host with the device?
Some derivative of SynCE? What version of Linux do you use? Device?

Thanks in advance

intestinal fortitude · 11-02-2022, 06:45 AM

I am writing the application that communicates with the devices. The application is written in C++.

One of the devices is the Thorlabs FW102C filter wheel. When plugged into the USB port, its USB chip is detected by the kernel as a USB<->RS232 bridge, and the FTDI device-driver creates a device-file named /dev/ttyUSBx (where 'x' is the next available number). The device-file can be opened and used to communicate with the device as if it was a classic serial port.

The system is Redhat Enterprise Linux, but the application I'm developing is distribution-agnostic.

sundialsvcs · 11-02-2022, 08:45 AM

Remember that "directories" such as /sys, /dev, /proc, are actually kernel interfaces. They do not correspond to any physical resource although they appear to be "directories" containing "files." It's all done by the kernel. Some of the files can be written to, usually only by root, and when this happen it causes changes to occur. I have never before seen anything as simply elegant as this.

intestinal fortitude · 08-27-2024, 08:30 PM

Shortly after originally posting, the project got shelved for a while. I've recently been able to continue work on it. This is some pseudo-code on how I resolved the problem:

Code:

Use libusb to enumerate all of the attached USB devices
Loop through device list:
    Search the list of attached devices for one with matching Vendor ID and Hardware ID
    Read the serial number.  Move to next device if it doesn't match.

If there is a matching entry in the USB device list:

Call libusb_get_bus_number() to get the device's bus number
Call libusb_get_port_number() to get the device's port number
Call opendir("/sys/bus/usb-serial/devices")
Each entry in the directory is a symbolic link into the corresponding USB device's hierarchy
Loop through the directory contents with readdir():
    Use readlink() on each entry to get its target
    Look for an entry that is a symbolic link to the USB device with matching bus & port numbers
    If there's a matching entry, then the name of the file in /sys/bus/usb-serial/devices is the name of the USB-tty device.

Example: After plugging in the device and searching the list of devices, the program determines it is USB Bus 1, Port 8. Directory listing of /sys/bus/usb-serial/devices

Code:

localhost$ ls -l /sys/bus/usb-serial/devices
total 0
lrwxrwxrwx 1 root root 0 Aug 27 19:59 ttyUSB0 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8:1.0/ttyUSB0

As you can see, ttyUSB0 points to USB device Bus 1, Port 8. So I know that /dev/ttyUSB0 is the serial port that corresponds to my USB device. An alternative directory that I could have used is /dev/serial/by-path:

Code:

% ls -l /dev/serial/by-path
total 0
lrwxrwxrwx 1 root root 13 Aug 27 19:59 pci-0000:00:14.0-usb-0:8:1.0-port0 -> ../../ttyUSB0

The directory /dev/serial/by-path has the opposite mapping as /sys/bus/usb-serial/devices (USB-to-tty vs. tty-to-USB). I decided to stick with /sys/bus/usb-serial/devices because the fact it is in /sys makes me feel a bit more warm and fuzzy.

sundialsvcs · 08-28-2024, 11:51 AM

I think that your solution is certainly the most "robust." And, easiest to manage. (Because, the entire resolution is handled "within your code," versus dependency upon some external configuration which "your code" could not check.)

michaelk · 08-28-2024, 06:10 PM

If the same cable/device was always plugged into the same port I believe ../usb-serial/devices would work but I assumed by your first post by any order that was not the case.