Dell Touchscreen Display USB Touch Interface not recognized

 I was having some issues setting up a touchscreen kiosk in Ubuntu 24.04 using a Dell 65 4K Interactive Touch Monitor - C6522QT paired with a Dell OptiPlex Micro 7020 installed in the built-in OptiPlex holder tray in the display.

 First I discovered that the instructions have you connect via HDMI 1 and USB 1 ports on the display. Unfortunately, the onboard Intel UHD Graphics 770 won't do 4K over HDMI. The monitor will scale for you but the resolution is lower. The touchscreen input was working in Windows and Linux.

 To get to the full 3840 x 2160 resolution, you need to connect using DisplayPort.

 After switching to DisplayPort and feeling quite chuffed with myself, I had to set the project aside for a few weeks. When I came back to it again, I did some Ubuntu updates, tinkered with trying to get some form of remote desktop working, and then happened to try using the touchscreen again. To my horror, it didn't respond at all. Research led me to learn that the Dell monitor uses an InGlass Display made by a company called FlatFrog that is supported by the hid-multitouch driver that has been in the mainline Linux kernel since version 3.5.

I'll skip the parts where I spend hours messing with my Ubuntu system and pick up where I start to try to enable the hid-multitouch driver and look for the device.

root# modprobe hid-multitouch
root# lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 8087:0033 Intel Corp. AX211 Bluetooth
Bus 001 Device 005: ID 413c:2514 Dell Computer Corp. Dell Universal Receiver
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub

This is when I discover that the device isn't showing at all. I don't know what I'm looking for, but what's there is other stuff. Unplugging everything leaves me with:

root# lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 8087:0033 Intel Corp. AX211 Bluetooth
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub

So I am now wondering why the USB touch device isn't showing up at all.

So I started flailing around some more switching ports on the computer, etc. and eventually wondered if there might be something that I've changed on the monitor. So I then had the brilliant idea to RTFM.

Lo and behold, on page 12 of the Dell C6522QT User's Guide is the magic "Input sources and USB Pairing" section. From this in inferred that because I had switched my display connection from HDMI 1 to DP, I needed to move the USB connection to from USB 1 to USB 3.

Boom:

root# lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 8087:0033 Intel Corp. AX211 Bluetooth
Bus 001 Device 009: ID 0424:2916 Microchip Technology, Inc. (formerly SMSC) USB2916 Smart Hub
Bus 001 Device 010: ID 25b5:00e6 FlatFrog FlatFrog DA-TK65P-20P2WE-M4-00e6
Bus 001 Device 011: ID 0424:284c Microchip Technology, Inc. (formerly SMSC) Hub Feature Controller
Bus 001 Device 012: ID 413c:2514 Dell Computer Corp. Dell Universal Receiver
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 002 Device 003: ID 0424:5916 Microchip Technology, Inc. (formerly SMSC) USB5916 Smart Hub

And the touchscreen started working. Woo!