Port Shifts Solution with udev

Using udev to Create Persistent Device Paths

1. Problem Statement: USB Port Shifting

In Linux, kernel-level device enumeration (e.g., for USB-to-Serial devices) is non-deterministic. A device like a Pixhawk may be assigned /dev/ttyACM0 on one boot and /dev/ttyACM1 on another, especially when other serial devices (like a RealSense camera) are present. This breaks static configurations in applications like ROS.

2. Solution: Persistent udev Symlinks

The udev subsystem manages the /dev directory. We can create a custom rule to identify a device by its unique, permanent hardware attributes (like idVendor and idProduct) and create a stable, persistent symbolic link (symlink) to it (e.g., /dev/pixhawk).

3. Prerequisites

• Root/sudo access on an Ubuntu system.
• The physical device (e.g., Pixhawk).
• A terminal.

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Step-by-Step Guide

Step 1: Identify Device Hardware Attributes

1. Connect the device to your computer.
2. Identify its current kernel-assigned path.

   ls /dev/ttyACM*
   

   (This might also be ttyUSB* for other devices. Note the path, e.g., /dev/ttyACM0).

3. Use udevadm to query the device’s attributes. Replace /dev/ttyACM0 with your device’s path.

   udevadm info -a -n /dev/ttyACM0 | grep -E "idVendor|idProduct|serial"
   

4. Analyze the output. Find the attribute block for the main TTY device. Record the ATTRS{idVendor} and ATTRS{idProduct} values.

   # Example Output
   ATTRS{idVendor}=="26ac"
   ATTRS{idProduct}=="0032"
   ATTRS{serial}=="00000000001A"
   

   You must use the values from your command output.

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Step 2: Create udev Rule File

1. Create a new rules file in /etc/udev/rules.d/. The 99- prefix ensures it runs after system defaults.

   sudo nano /etc/udev/rules.d/99-pixhawk.rules
   

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Step 3: Define the Rule

1. Paste the following line into the file, replacing <VENDOR_ID> and <PRODUCT_ID> with the values you found in Step 1.

   KERNEL=="ttyACM*", ATTRS{idVendor}=="<VENDOR_ID>", ATTRS{idProduct}=="<PRODUCT_ID>", MODE:="0666", GROUP:="dialout", SYMLINK+="pixhawk"
   

   • Example Rule:

       KERNEL=="ttyACM*", ATTRS{idVendor}=="26ac", ATTRS{idProduct}=="0032", MODE:="0666", GROUP:="dialout", SYMLINK+="pixhawk"
     

2. Rule Breakdown:
   • KERNEL=="ttyACM*": Match any device the kernel names ttyACM....
   • ATTRS{...}: Filter to match only the device with this specific Vendor and Product ID.
   • MODE:="0666": Set file permissions to read/write for all users.
   • GROUP:="dialout": Assign group ownership to dialout. ROS/Mavros users are typically in this group for serial port access.
   • SYMLINK+="pixhawk": Create the persistent symlink at /dev/pixhawk.
3. Save and exit the editor (in nano: Ctrl+O, Enter, Ctrl+X).

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Step 4: Apply and Verify the New Rule

1. Reload the udev rules:

   sudo udevadm control --reload-rules
   

2. Trigger the new rules to apply them to currently connected devices:

   sudo udevadm trigger
   

3. Test by hotplugging: Unplug your device, wait a few seconds, and plug it back in. This is the most reliable way to verify the rule.
4. Check if the symlink was created:

   ls -l /dev/pixhawk
   

   • Success: lrwxrwxrwx 1 root root ... /dev/pixhawk -> ttyACM0 (or -> ttyACM1)
   • Failure: ls: cannot access '/dev/pixhawk': No such file or directory (See Troubleshooting).

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Step 5: (Optional) Configure User Permissions

If your user (running ROS) does not have permission, ensure it is in the dialout group.

1. Add your user to the group:

   sudo usermod -a -G dialout $USER
   

2. IMPORTANT: Group changes only apply after you log out and log back in or reboot.

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Step 6: Update Application Configuration (ROS Example)

Modify your ROS launch file (e.g., mavros/px4.launch) to use the persistent path.

• Before:

    <param name="fcu_url" type="string" value="serial:///dev/ttyACM0:57600" />
  

• After:

    <param name="fcu_url" type="string" value="serial:///dev/pixhawk:57600" />
  

  This ensures your application reliably connects to the Pixhawk, regardless of its kernel-assigned ttyACMx path.

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Troubleshooting & Advanced Cases

• Rule Not Working:
  1. Double-check idVendor/idProduct for typos in your .rules file.
  2. Ensure you reloaded rules (udevadm control --reload-rules) and re-plugged the device.
  3. Check permissions. Ensure your user is in the dialout group (and you’ve logged out/in).

• Multiple Identical Devices: If you have two identical Pixhawks, idVendor and idProduct are not unique. In this case, use the ATTRS{serial} value (from Step 1) to differentiate them. ``` # Rule for device A KERNEL==”ttyACM*”, ATTRS{idVendor}==”26ac”, ATTRS{idProduct}==”0032”, ATTRS{serial}==”00000000001A”, MODE:=”0666”, GROUP:=”dialout”, SYMLINK+=”pixhawk_A”

  # Rule for device B KERNEL==”ttyACM*”, ATTRS{idVendor}==”26ac”, ATTRS{idProduct}==”0032”, ATTRS{serial}==”00000000001B”, MODE:=”0666”, GROUP:=”dialout”, SY