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So far I've been using Windows 10 Lenovo T590 with Dell USB-C docking station supporting 2 screens each of 2K resolution (2560x1440) without any issue at all.

After I decided to migrate to Ubuntu Linux, Dell USB-C docking station appeared to not supporting any of these screens on Linux. Then I handled to get different docking station of the same brand that laptop is and now ONLY ONE screen could be set to it's native resolution. Second one is detected but is not able to generate any output unless, I set both monitors on lower resolution (1920x1080) then both of them are working but as you can guess, it's not the best way, seeing not sharp picture.

I am wondering, assuming that Windows 10 was able to handle both of 2K screens, where is the bottleneck in my setup? The docking station, or GPU driver, or something else?

Is there any way to reduce Hz frequency to get less usage of bandwidth (using xrandr tool for example) , but having these 2 screen on their native resolution?

lspci reports my card as:

00:00.0 Host bridge: Intel Corporation Coffee Lake HOST and DRAM Controller (rev 0c)
00:02.0 VGA compatible controller: Intel Corporation WhiskeyLake-U GT2 [UHD Graphics 620] (rev 02)
Robert Siemer
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Marek
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1 Answers1

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I’ve written a couple of answers for multi-monitor xrandr setups. Let me recap:

  • Intel UHD 620 GT2 supports DisplayPort 1.2 → total bandwidth is 17.28 Gbit/s
  • USB-C Alternate Mode for DisplayPort does either
    • use 2 lanes for DP (8.64 Gbit/s) and leave 2 lanes for USB 3
    • use 4 lanes for DP (17.28 Gbit/s) and have no USB 3 (USB 2 is always available)
  • 2x1080p requires only 6.4 Gbit/s
  • 2x1440p requires more than 11 Gbit/s

Can you dual boot into Windows or similar? Can you confirm that Windows could not drive USB 3 speeds over the dock while running 2 monitors at 1440p?

I do not know if and how the operating system decides over which mode to use (2 vs. 4 lanes). On some Acer(?) laptops a BIOS setting (or “firmware setup utility”) decided over which mode is used. The option was hidden behind some confusing name like “High-Res USB-C mode” or similar.

Some information is missing in your report. First, you should always be able to switch one monitor off and the other into maximum resolution, like this:

xrandr --output <SECOND_MON> --off --output <FIRST_MONITOR> --auto

Which error do you get from xrandr and which from the kernel (new last lines from dmesg) if you switch the second monitor on to full?

xrandr --output <SECOND_MON> --auto

Now get a list of supported modes via xrandr with no arguments. Set the second monitor to a super low resolution and ramp up one common resolution to the next. Start with

xrandr --output <SECOND_MON> --mode 640x480

...then 800x600, 1024x768 etc. Which is the last one working? Which does not work any more?

With this data we can make a very educated guess what bandwidth seems to be the limit here. If we get stuck at 8.64 Gbit/s then it is pretty clear that the connection to the dock uses only 2 lanes for DisplayPort on Linux, while it used all 4 with Windows.

And: hooking up one monitor via another port (not over the same USB-C connection) should also allow both monitors a full resolution.

Can you confirm that?

Robert Siemer
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