U4320Q, USB-C Prioritization set to "High Data Speed" makes the USB-C port only work for low speed devices

Update: This does not apply only to USB-C. The issue is that anything that is a USB3.0 device (my harddrive, ethernet) registers on any of the USB ports of the monitor. USB2.0 devices like a mouse or my phone do register. I also eliminated a cable as a culprit. My cables work when I use them to connect the devices directly to the laptop, so the problem is in the monitor itself.

Go HERE and enter the private U4320Q Service Tag. What is the warranty status?

The AI was correct. Dell only tested/validated the "in the box" 1 meter Generation 2 USB-C to USB-C cable.

Let us know if you test on a Windows OS system.

For troubleshooting, run (3) No cables, power button reset and then retest with the "in the box" 1 meter Generation 2 USB-C to USB-C cable.

Thanks for answering!

Warranty ended 1,5 years ago.

I guess this is the Dell cable that came with it? Would it have such a sticker? If so, plugging it in does not change anything, USB3 devices are still not recognized at all-

The reset did not help (even with the Dell cable), still no USB3 devices.

I am still trying to get a hold on a Windows computer.

I wonder if @Smardey is still around and if he or she could confirm whether it is possible to use USB3 devices with your monitor?

Hi felagund,

yes i am still "active" :-)

I use my U4320Q as my daily driver since dec. 2022 an since never had any problems with USB speeds on the "high data speed" setting.

Most used machines via USB-C are my private M1 Macbook Air an my company Macbook Pro M4. Unfortunately i can not provide any further hints for Linux on Laptops. It is currently late in Germany but if you need i can provide some additional infos (firmware versions etc.) tomorrow. I used the default usb-c cable for a long time, but now I switched to a slightly longer cheap aliexpress usb-c "high speed 100W" cable and even this works fine for me.

Hello, thank you for responding this quick. Yes, more informations would be helpful later! It is good to know that it works for you, either there is something wrong with how my monitor was assembled in the factory or something wrong with my setup.

You are achieving speeds over 60MB per second or using USB3 devices, right? (I was surprised my phone is only USB2)

I did a short test for you:

External SATA SSD (Crucial BX100 1TB) in a Raidsonic enclousure, connected to the USB Ports on left the side of the U4320Q:

This SSD is tested with 530MB/s read and 450mb/s write in peak. So is think my results are close enough.

I made also a "real life" test, copying a 6.6GB file from this SSD to my internal mac SSD takes around 15 seconds. which comes to around 400MB/s

Best regards

Matthias

Forgot my firmware revision: My U4320Q uses Firmware M2T104

Thank you for the details. Curiously, it started working for me. Yesterday I tried the reset that Chris suggested and also ran the diagnostic test where a series of solid colors is shown on the monitor. Then I also factory reset the whole monitor. It did not seem to help. I left my USB3 hadrdrive connected to the monitor and with great surprise noticed today it was connected.  Now looking at the logs I see that it connected some time after a subsequent reboot around 9 PM yesterday.

So possible steps:
1. do diagnostic test (with the colours, on the monitor when it is disconnected from all computers, press the button left of power button for 8 seconds)

2. do "no power reset"

3. reset the monitor to factory settings

4. reboot your computer

Not sure if those are what helped. 

Another option that might have changed is that I use a software to hide my other Dell monitor that does not have speakers from my system audio, so that it does not confuse me when I select which device should play audio (details here: https://linuxmusicians.com/viewtopic.php?t=28600&sid=9db0e087d65e2af4f2ee35a36e41a009 ). When I was troubleshooting the USB3 issue, I was changing cables a lot and that led to a change of enumeration of the HDMI devices (while the monitor is connected over USB-C, sound to it seems to go over HDMI). Maybe there was something rotten there. But frankly no idea. I only noticed that that enumeration changed four hours after USB3 started to work.

Anyway, now I was able to copy files at around 200 MB/s while also testing my ethernet at 115 MB/s, so I got at least 2,5gbit speed, which is nice (I guess more is possible, my harddrive is probably the limitation here). I just hope this keeps working, as the reason why it was not working and now is is totally mysterious to me.

I will just add that my firmware is M2T102. But M2T104 is only needed for Macs (it has a specific fix for them), so it should not be an issue. My display Info information seems the same as Smardey's, just the Image is 24-bit, not 30-bit. Might be OS related.

One hopefully last note: I somehow managed to make it stop working again, possibly by unplugging the ethernet cable and replugging it back. Rebooting my computer fixed the issue and now I cannot reproduce it. There is something wrong somewhere I guess, but probably that is a software thing, not hardware one. After reboot now, everything is working again.

@felagund​

What you’re seeing is actually expected behavior with the Dell U4320Q’s USB-C “Prioritization” setting. The monitor has to share USB-C bandwidth between DisplayPort (video) and USB data.

• High Data Speed mode: The monitor allocates more bandwidth for USB 3.x traffic (SuperSpeed), but that leaves less available for DisplayPort lanes. That’s why your USB mouse (low-speed) works fine, but higher-speed peripherals (SSD, 1GbE adapter) behave erratically — the connection becomes unstable because the remaining lane allocation is too tight.

• High Resolution mode: The monitor prioritizes video bandwidth (for HBR2), leaving only USB 2.0 lanes free for data. That’s why everything USB-related drops down to USB 2.0 speeds, but at least it’s stable.

Things to try / check:

1. Cable length - Even though your 1.5 m cable says it supports TB3/10 Gbps, USB-C high-speed signaling can be picky. Try a shorter (0.8 m or less) certified cable and see if stability improves.

2. Monitor hub limitation – The U4320Q’s built-in hub is known to be unreliable at full USB 3.x speeds while simultaneously pushing 4K60 video. Many users report the same: high-speed devices often don’t enumerate properly unless USB prioritization is lowered.

3. Direct connection – For critical peripherals (SSD, 1GbE), connect them directly to your computer’s USB-C or USB-A ports instead of the monitor’s hub. Use the monitor’s USB ports only for low-bandwidth devices like keyboard/mouse.

4. Firmware/OS test – It’s worth testing on Windows, just to rule out a Fedora driver quirk, but in practice this looks like a hardware bandwidth sharing issue rather than OS related.

 In short: the U4320Q can’t reliably handle both full DisplayPort HBR3 video + USB 3.x high-speed data over the same USB-C link. That’s a design trade-off of the monitor, not your cable or Fedora. Best workaround: use High Resolution mode for video stability, and connect fast USB devices directly to the PC instead of the monitor’s hub.

@MikeHickson​ I got this from an LLM too. I do not think it is correct.

So I got to a state where everything would work after a reboot, but at some later point, after plugging and unplugging devices, it would stop - there would be no software indication whatsoever anything was connecting to the USB ports. I did three things trying to troubleshoot it that might have helped, as I cannot reproduce the issue today (not sure still if it is a permanent state of affairs, but never before did I manage to plug/unplug my harddrive 15 times).

1) I went to my machine BIOS and altered Thunderbolt settings. I already had no security, so I left that, but there was some legacy Thunderbolt something, which I turned off (or on, not sure now). So one possible course of action is to change BIOS settings.

2) I installed boltctl which should be able to interact with Thunderbolt devices, I doubt that did anything, but who knows.

3) I installed https://github.com/mvp/uhubctl and tried to power down and up one of the ports on the monitor.


I am not sure if any of this helped or if the fix is permanent.