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r/linuxhardware•
TomorrowPlusX

Dell XPS13 9300 display-port over USBC/Thunderbolt question

Support

I have a 4k monitor, and a mid-range thunderbolt/USBC power delivery hub with display port output. My work-issued macbook pro can use the hub for power delivery, and drive the 4k monitor at 60hz over the hub's display port, which is awesome - one plug does it all.

However, my personal XPS 13 (running ubuntu 20.04) can only drive that monitor at 30hz when using the hub's display port. Fortunately, I can plug a separate usbc/thunderbolt-to-displayport cable into the XPS13's other usbc/thunderbolt plug and drive the monitor at 60hz.

This tells me a couple things. First, the hub clearly can drive the monitor at 60hz because it works with the macbook pro. Secondly, the XPS13 can drive the monitor at 60hz with a dedicated usbc/thunderbolt-to-displayport cable. But the XPS13 can't do it via the hub, whereas the mac can.

I will confess - I don't know where the issue is likely to be. It could be the XPS13 has a lower bandwidth thunderbolt port than the mac and can't multiplex or whatever with the hub? It could be the mac is doing some kind of on-the-fly video compression to stream at 60 hz.

Any ideas?

(I also recognize that this is a first-world problem, apologies)

Followup: This is the hub: https://www.amazon.com/gp/product/B07X1J7R34/

Resolution: My hub's USBC, not thunderbolt! It's actually working as expected on the Dell, it's macOS which is doing something funky.

Posted May 11, 2020, 8:28 PM UTC · edited

86
blackghost58•

I happened to come across this the other day as I have a 5K monitor and nothing to power it (it was free), and realised I didn't understand it all enough.

https://www.bigmessowires.com/2019/05/19/explaining-4k-60hz-video-through-usb-c-hub/

4
hiboux918•

You mention that, "... mid-range thunderbolt/USBC power delivery hub with display port output.".

Can you provide the brand and specific model number of the device you are using?

2
TomorrowPlusXOP•

Of course, I should have in the writeup: https://www.amazon.com/gp/product/B07X1J7R34/

It's a non-name chinese hub, but like I mentioned, works great under macOS.

2
hiboux918•

The specifications as listed by the seller are not bad, but they do have some ambiguity.

That said, based on the information provided the adapter you link to appears to have an internal DisplayPort MST hub within the unit which complicates determining its specific capabilities (and effects its ability to function under macOS, since macOS does not support DisplayPort MST)

Speaking in general, in my experience a USB-C device that provides not only DisplayPort Alternate Mode video output but also data transfer (in the form of USB ports, Ethernet, memory card slots) is limited to a maximum of 4K resolution at a 30Hz refresh rate when connected to a host system that supports the DisplayPort 1.2 standard via USB-C (direct 'video only' connections tend to support 4K@60Hz).

This is because of USB-C device bandwidth restrictions that limit the amount of video information that can be sent in conjunction with data transfers (even if data transfers are not happening, the bandwidth for them is automatically allocated).

To put that another way, I would have expected you to get 4K@30Hz via the device's DisplayPort video output when connected to your Dell system.

I would not have expected you to get 4K@60Hz via the device's DisplayPort video output when connected to your Apple Mac system.

That you have found otherwise is unexpected (in my opinion at least :-) and I am afraid I do not have an explanation to offer.

** I know both of your system's have Thunderbolt 3 support, however the adapter you are using is a USB-C device. Hence, it is bound by the capabilities of the USB-C standard, and cannot take advantage of either host systems Thunderbolt 3 capability. **

3
TomorrowPlusXOP•

Thanks for the explanation! This clarifies a lot - It certainly seems like if I want power and 4k@60hz over a single port I need a true Thunderbolt3 hub, and at $250+, I may wait a bit before diving in.

1
hereforthepix•

(I must own 6-7 "no-name Chinese USB-C hubs", but that Tiergrade is the best. Highly recommended.)

2