Cannot get monitor above 4k@30 or 1440p@60

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1

Hello Fedora Forums!

Bought a used laptop a few weeks ago together with an USB-C dock and a used 4K monitor.
I’ve been using Arch for about 8 years, but this laptop is for my wife so wanted something more reliable so installed Fedora Silverblue 42 on it.
The HDMI output is stuck at 4k@30, most likely due to not having HDMI 2.0, so have been trying to get DP over USB-C working at 4k@60 with the dock.
The display settings in Gnome maxes out at 1440p@60 for my monitor when using DP over the USB-C dock. 4K was not an option at all.
Assumed that this was due some bad EDID in the monitor, so investigated how to override that and found this very nice thread on the fedora forums: Cannot choose 4k60 option for external display

So I put some different EDID files under /usr/lib/firmware/edid by using rpm-ostree usroverlay and then applying them by doing e.g. echo 'edid/DELL P2715Q_1095185228.edid' > /sys/module/drm/parameters/edid_firmware as root. Been using edid-decode to print the settings of each edid file (though, I don’t really understand what most fields mean…). When changing the edid firmware and connected my monitor, I’ve been looking at dmesg to check for errors and it looks fine (except for when I e.g. passed the wrong path to the firmware a few times).

When doing this, some EDID files allowed me to at least 4k@30 on the monitor over DP, but I have yet been unsuccessful at getting 4k@60. Without the EDID firmware override, I’ve only been able to get 4k@30 using HDMI so I was onto something at least.

I’ve been testing a few different EDID files downloaded from the website edid.tv, I searched on that website first for the same model number of my monitor but with an 4k@60 “maximum resolution” such as this edid entry. Then I tried some other newer 27" 4K dell monitor EDID such as this one. These custom EDIDs just allowed me to get 4k@30 under gnome-settings.
I also tried some crazy custom 4k@60 EDID from some random github repo, but when applying that Gnome settings only allowed me to use some strange 4:3 ~1000x1000 resolution…

At this point I have concluded that the EDID on my monitor is probably wrong, but even with that in mind there seems to be something else limiting me to get 60hz considering how many different EDID files I’ve been testing… Any idea on what more to investigate?

Specs of computer:

Lenovo ThinkPad X1 Carbon 7th
Intel® Core™ i5-8265U
Fedora Linux 42.20250516.0 (Silverblue)

Specs of monitor:

Dell P2715Q
Maximum resolution over DP: 3840x2160@60
Maximum resolution over HDMI: 3840x2160@30

Specs of USB-C dock

Lenovo ThinkPad USB-C Dock Gen2
DP 1.4
HDMI 2.0
Citation from Lenovos website: “Dock has the ability to support DisplayPort™ 1.4 and HDMI™ 2.0 for max 2 x 4K @ 60 Hz displays, but the real video output would depend on the system’s output.”
Tested both original Lenovo USB-C cable with the dock, as well as another third party USB-C cable that is supposed to support “4K” and “20Gbps”

I also blamed my DP cable for not being able to do 4k@60, so I bought a brand new one with DP2.0, which I in the end realized wasn’t necessary, the old cable was fine…

Some other nice links I’ve been looking at

2

the system is limited to DP1.2!
Such a system cannot support 4K@60Hz via a USB-C dock.
Either get a thunderbolt3 dock or connect the monitor directly to the system with a USB-C to DP cable.
Connecting the monitor to the system with a USB-C cable can expose the same limitations (4K@30Hz) unless you disable USB3 in monitor’s config.

see also Display and Video Output Configurations - Docking Stations

3

Ugh, so it’s that simple?

I assumed it would work since DP1.2 has support for 4k@60, but according to that link it seems like you’re right that the dock only supports 4k@30 or 1440p@60 when the laptop has DP1.2…

How come you mean that its possible to have 4k@60 if I disable USB3 and have a straight USB-C → DP cable? And what do you mean “disabling USB in monitors config”? The monitor itself doesn’t have much configuration? I don’t really care about USB3, just need a keyboard and mouse at the desk.

4

some systems like HP had an option to prioritize high resolutions over USB transfer speed. This would disable USB3 on the USB-C port and use USB2 for system ↔ dock.
Some OEM docks do have a switch to block USB3 as well.
Many usb-c displays do also have an option to disable USB-3

Lenovo didn’t care.

Look at a USB-C pin layout. USB3 use the same pins as DP.
In a dock configuration 2 lanes are configured for DP and 2 for USB3 data. DP1.2 system need all 4 lanes configured for DP to support 4k@60Hz.

a USB-C to DP cable can transfer only DP. So you don’t need to care about USB3.
This is only a concern with USB-C monitors and docks.

In TB mode, system configures all 4 lanes for TB and transfers USB + DP over TB to the TB dock.

5

That fully explains it, thanks.

In that case I’ll probably buy an USB-C → DP cable and a cheap USB 2.0 hub for connecting the mouse and keyboard instead and completely skip the dock.

Thanks a lot for the help, I’ve been experimenting with this for hours so really appreciate it :slight_smile:

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I thought the monitor has a built-in usb3 hub?
You’ll probably need only a usb3 USB-A to USB-B cable.

7

There is a USB hub in the monitor yes. Looked at that earlier and buying a USB-A to USB-B cable, but for some reason they are hard to come by locally nowadays for a decent price (~€20). An USB-C to USB-B was just €10 though, but only have one USB-C port with data. Found a used Thunderbolt 3 hub online for cheap (€35) including an extra charger so bought that instead actually.

8

Hm, got my “Lenovo Thinkpad Thunderbolt 3 Dock (40AC)” now, has the exact same issue that it does not allow me to go above 4k@30hz.

Also brought home my work laptop “HP EliteBook 830 G6” running Ubuntu 24.04, has the same issue… Apparenly has the exact same processor as well lol.

dmesg doesn’t seem to give me any hints, it seems happy. Have tested two different USB-C cables as well without any luck.

Under the “Thunderbolt” tab in gnome-settings it is listed as a “thunderbolt device” and marked as “Connected and authorized” so it does seem to connect via thunderbolt.

In general though the TB3 dock seems a bit less reliable, takes much longer time for it to detect my mouse and display when plugging it in and sometimes I need to reboot to make it get detected at all. Will probably just revert back to the USB-C dock until I find some better solution.

9

Is the monitor connected to the HDMI port? I think the HDMI port supports 4k@30Hz only :slight_smile:

Can you see the link speed between host and dock? I think it should be at least 20GBit/s, and up to 40GBit/s.
Otherwise make sure you use a TB3 compatible USB-C cable. Just because it has a USB-C plug does not mean that the cable meets the requirements.

10

No, I’ve stopped trying with HDMI as I also saw that it was limited at 4k@30.

I tried two cables:

  1. The original cable for the Lenovo ThinkPad USB-C Dock Gen 2
  2. An USB-C cable from Amazon supposed to support 20Gbit/s and 4K@60

I’m having a hard time finding the exact “Gbit/s” value of the thunderbolt connection, but this is what “lspci -vv” gives me when the Thunderbolt 3 dock is connected.

It was too long to be in a reply on this forum, so had to upload to pastebin.

11

The original cable for the Lenovo ThinkPad USB-C Dock Gen 2

that’s probably not a TB3 cable! The part number differs from the tb3 cable included with the 40AC dock.

A TB3 certified cable has a thunderbolt symbol and sometimes a 3 , 4 or 5 for TB3/4/5 printed on the plugs

03X7134%20Cable_20191219204147878
03X7134%20Cable_20191219204147878430×266 19.9 KB

I think boltctl should be able to report the link speed to a device.

at least you get some TB3 link. IIRC the old 40AC dock doesn’t have a USB-C failback mode.

what you could try is to connect the monitor with a USB-C to DP cable to the TB port at the front of the dock. Also disable 10bit color support. I think 4k@60Hz is only supported with 8bit color depth.

12

boltctl tells me 20Gb/s, so the cable seems to be alright.

 ● Lenovo ThinkPad Thunderbolt 3 Dock
   ├─ type:          peripheral
   ├─ name:          ThinkPad Thunderbolt 3 Dock
   ├─ vendor:        Lenovo
   ├─ uuid:          00a62908-3505-0801-ffff-ffffffffffff
   ├─ generation:    Thunderbolt 3
   ├─ status:        authorized
   │  ├─ domain:     c4010000-00a0-9d18-2390-752372e08824
   │  ├─ rx speed:   20 Gb/s = 2 lanes * 10 Gb/s
   │  ├─ tx speed:   20 Gb/s = 2 lanes * 10 Gb/s
   │  └─ authflags:  none
   ├─ authorized:    ons 21 maj 2025 13:56:22
   ├─ connected:     ons 21 maj 2025 13:56:22
   └─ stored:        tis 20 maj 2025 15:00:10
      ├─ policy:     auto
      └─ key:        yes

Not sure how to disable 10bit color, but my 4K monitor is quite old so I highly doubt that it supports that.

13

I mis-remembered. 4k@60Hz with 10bit color depth is also supported with DP1.2.

I would not completely rule out the cable. Passive TB cables do have a defined length.
Try to get a proper TB4 cable.

According to this document you need at least 25Gbit/s to enable one 4K display + usb3 + ethernet.

image
image960×450 34.8 KB

you should expect 40Gbit/s in each direction.

 ├─ rx speed:   40 Gb/s = 2 lanes * 20 Gb/s
 ├─ tx speed:   40 Gb/s = 2 lanes * 20 Gb/s

also important:

image
image661×208 12.7 KB

–
USB-C: Just cos it fits don’t mean it works. :slight_smile:

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