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Joined: Wed May 05, 2021 3:20 pm

Consistent USB Symlink for Physical USB Ports Across Hardware Revisions

Fri May 07, 2021 7:59 pm

I (like others) have a desire to assign fixed port names or symlinks to physical USB ports on my pi. This allows us to easily connect support equipment in a repeatable way and be able to make safe assumptions in our system testing. I followed the guidance described in this thread:

The Problem
That solution worked great for my RaspberryPi 3 B. When I used the same UDEV rules on a RaspberryPi 3 B+, I noticed the KERNELS string for the same USB device on the same USB port was different for the different Pi hardware revisions. Here's a side by side comparison of the USB device Kernels string for my Pi 3 B vs B+... (note the reuse of a couple strings for different ports)

Pi 3 B
Top Left USB: "1-1.2"
Bottom Left USB: "1-1.3"
Top Right USB: "1-1.4"
Bottom Right USB: "1-1.5"

Pi 3 B+
Top Left USB: "1-1.1.2"
Bottom Left USB: "1-1-1.3"
Top Right USB: "1-1.3"
Bottom Right USB: "1-1.2"

Because of this difference in KERNELS string in the different hardware, my original udev rules wouldn't work as expected on the Pi 3B+. On the 3B+, if I connected the same USB device to the lower right USB port, it would give me the symlink for top left USB port on the 3B. If I connected the USB device to top left, I would get no symlink at all!

My Solution
I tested a few different Pi 3B+ boards and they all enumerated the same way, so I ended up creating a bash script that checks the hardware revision and determines what to name the resulting symlink depending on the hardware revision and the KERNELS string of the connected device. I then updated my udev rules file to call this script in order to create the symlink properly whenever a ttyUSB device is connected to a specific USB port. Helpful udev rules link: Hardware revisions taken from here:

I attached the udev rules file I'm currently using.
udev_rules.jpg (53 KiB) Viewed 92 times
I attached the bash script that is determining the symlink name depending on hardware revision and KERNELS string.
getUsbComSymlink.jpg (64.09 KiB) Viewed 92 times
My Question
This works consistently now. If I connect the same USB device to the top left USB port on either hardware revision, I get a symlink to that device at /dev/COM1. Bottom left, /dev/COM2. Top Right, /dev/COM3. Bottom Right, /dev/COM4. So now our test can be written to use "/dev/COM1" for serial communication regardless of pi hardware revision. An obvious downside to this, is every time we want this same behavior on different/newer motherboards, we need to update the bash script and potentially the udev if the usb enumeration behaves differently. My solution works, but is there a better way? Is there any documentation around how to predict how USB devices will get connected on the USB bus(es) for the different pi hardware revisions?

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