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Old 08-12-2022, 08:50 AM   #1
intestinal fortitude
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Have multiple USB<->RS2332 devices. Which /dev/ttyUSBx goes to each device?


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!
 
Old 08-12-2022, 04:12 PM   #2
dugan
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/sys is the interface.

Remember, everything is a file, yadda yadda.
 
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Old 08-12-2022, 05:10 PM   #3
michaelk
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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
 
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Old 08-16-2022, 10:59 AM   #4
intestinal fortitude
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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.
 
  


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