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I have more than 2 serial devices enumerated by FTDI driver as /dev/ttyUSB0, /dev/ttyUSB1, /dev/ttyUSB2 etc. On reboot, these may get jumbled up in any other order. Also, I may replace/swap these devices physically among these or with some other similar device.

Now, I want a persistent enumeration for these. I want the device names to be enumerated according to the physical USB port (I may connect to the USB ports on the motherboard of the PC directly or use a USB hub) that the device is connected to - say if the devices are connected to the USB hub, port 1 should be reserved to named as ttyUSB0, port 2 as ttyUSB1 and so on.

After some basic reading, I figured (as mentioned here) that /dev/serial/by-path/ lists the devices as sort of a symlink. So, I created a file /etc/udev/rules.d/101-usb-serial.rules with the following:

KERNEL=="ttyUSB[0-9]*", SUBSYSTEM=="tty", DRIVERS=="ftdi_sio", PATH=="pci-0000:00:14.0-usb-0:10.1:1.0", SYMLINK+="ttyUSB000"
KERNEL=="ttyUSB[0-9]*", SUBSYSTEM=="tty", DRIVERS=="ftdi_sio", PATH=="pci-0000:00:14.0-usb-0:10.2:1.0", SYMLINK+="ttyUSB001"
KERNEL=="ttyUSB[0-9]*", SUBSYSTEM=="tty", DRIVERS=="ftdi_sio", PATH=="pci-0000:00:14.0-usb-0:10.3:1.0", SYMLINK+="ttyUSB002"

But this doesn't work. On doing ls /dev/ttyUSB* I'm unable to see the new symlinks I have created. What could possibly be going wrong?

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  • 1
    There is no PATH key. Look at the output for udevadm info /dev/ttyUSB0 while it is plugged in. You can then probably use the ID_PATH value. Since this is prefixed by E: you need to look for it with ENV{...}.
    – meuh
    Commented Sep 11, 2019 at 19:09
  • I tried replacing PATH with ENV{ID_PATH}. It didn't work as well.
    – skrowten_hermit
    Commented Sep 12, 2019 at 4:59

1 Answer 1

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Taking cues from the comment above I managed to find a solution to the above.

udevadm info /dev/ttyUSB0 | grep "ID_PATH="

The above lists the the sysfs path for the port that /dev/ttyUSB0 is connected to. Use this value to create rules for as many devices (ttyUSB1, ttyUSB2....) as you want in the rules file, say /etc/udev/rules.d/101-usb-serial.rules as follows:

SUBSYSTEM=="tty",ENV{ID_PATH}=="pci-0000:00:14.0-usb-0:10.1:1.0",SYMLINK+="ttyUSB001"
SUBSYSTEM=="tty",ENV{ID_PATH}=="pci-0000:00:14.0-usb-0:10.2:1.0",SYMLINK+="ttyUSB002"
SUBSYSTEM=="tty",ENV{ID_PATH}=="pci-0000:00:14.0-usb-0:10.3:1.0",SYMLINK+="ttyUSB003"

Once done making changes or creating the file, run the following:

sudo udevadm control --reload-rules
sudo /etc/init.d/udev restart

P.S. : The above example scenario (the one that I am using) is using a 4-port Belkin USB hub. Device 1 is connected to the port 1 of the hub Device 2 is connected to the port 2 and so on.

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  • The answer from @skrowten_hermit is correct, with one caveat: Using filename 101-... forces the rule to be fired in the beginning. This will not work, as ID_PATH is not available yet. Please ensure you add this rule using filename 99-usb-serial.rules. The ID_PATH environment variable is set by /usr/lib/udev/rules.d/60-serial.rules IMPORT{builtin}="path_id", so the custom rule must be fired after that one.
    – Ruben Faelens
    Commented Jul 12, 2023 at 9:52

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