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I use an external monitor with my laptop. The monitor has HDMI and USB-C ports. I haven't been able to get display-via-USB-C to work reliably with this monitor (on any computer, Ubuntu or Windows), so I use HDMI for display. However, it is convenient to use the USB-C connection to the monitor for power and to access the monitor's built-in USB hub. The problem is that then (sometimes) the laptop thinks there are two external monitors, and this wreaks havoc with my display setup.

So, how can I block Ubuntu 20.04 from recognizing the USB-C monitor--without interfering with its use of USB-C for power, USB-C for USB, and HDMI for display?

The laptop is a Lenovo X390, and the display is a Samsung S24H850QFN (2560x1440).

Thanks.

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  • Look at the logs! sudo journalctl -b 0 and see how the system recognizes the USB-C display. Then you'll know which process/daemon/whatever you need to meddle with.
    – waltinator
    Nov 26, 2020 at 0:53
  • have you considered different hubs, it's hit and miss.. also xrandr ... see xrandr --output eDP1 --off from askubuntu.com/questions/961441/… .. so do you have different eDP1 etc for 2 monitors?
    – pierrely
    Nov 27, 2020 at 5:31
  • Thanks, both. Much appreciated. The monitor started working again, so I'll need to wait till the next time it breaks to follow up.
    – Bolio
    Nov 27, 2020 at 17:21

1 Answer 1

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You can try a different cable.

You likely already know this but I'll review for the rest of the class, the USB-C port contains these 3 independent parts, USB 2.0, power, and the 4 "super speed" lanes. To stop the DP video you can connect the monitor to the computer with a cable that lacks the high bandwidth data lanes required for video. If you have a USB-C USB 2.0 cable lying about then try that to see if it resolves the video problem but keeps the USB ports working. Obviously the lack of "super speed" means a lack of USB 3.x, but if all you are concerned about is having the hub for mouse and keyboard then the loss of USB 3.x will not matter. Some monitors use all 4 of those lanes for video anyway so you lose nothing but frustration.

Where can you find such a cable for this experiment? Do you have a USB-C cell phone and charger? Is your laptop an Apple? Use the USB-C charge cord from either to connect the display. If that doesn't work then do you have a USB-C to USB-A (male) cable and a USB-C to USB-A (female) adapter? Plug the two together and see how the USB functions work. This setup will preserve USB 3.x data but limit USB power to 5 volts and something like 1.5, 2.4, or maybe 3.0 amps.

I don't know if there is an easy solution that preserves USB 3.x and power while filtering out DisplayPort. Some kind of hub or dock might do that for you. Maybe that falls under "easy" but not "cheap".

Oh, I think I might have to make a note about power. These USB-C "charge only" cables, which tend to actually be USB 2.0 cables, can be rated for 3 amps or 5 amps. The cables Apple includes with their laptops and sell on their website are 5 amp cables. Most cell phones will likely be 3 amp cables. Depending how much power your monitor takes 3 amps may be enough. If you are using an Apple laptop or some other laptop that charges from USB-C then it can likely operate just fine on 3 amps from the charger. For some laptops and chargers they can't source or sink more than 3 amps. A simple fix may be just swapping the cables around. Take the "full featured" (I think that's the technical name) USB-C cable from the monitor and plug that into your charger, then take the "charge only" cable (again, likely wired for USB 2.0 in actuality) that you just unplugged from the charger and plug that into the monitor. If you and your laptop are happy with 3 amps from the charger then the problem is solved.

Well, solved unless there is an issue of cable length. USB 2.0 cables can be 3 meters long while USB 3.x cables can be 2 meters long. A Thunderbolt cable can be used to fill in for the USB 3.x cable in this experiment but those cost more and are shorter yet, under 1 meter.

Once you prove this works then you may want to consider this a permanent solution or get your phone charger cable back by buying a new one. A quality store brand 1 meter cable is probably about $8. Or keep the short phone charger cable on the monitor and treat yourself to a name brand 2 meter cable for the phone at about $20.

One last thing, if you have an actual "charge only" USB-C cable then you will lose USB 2.0 to your monitor when using it to connect the monitor to the computer. I have such a cable, and depending on how they are built they can be very nice. They tend to cost more than a USB-C USB 2.0 cable because of whatever extra feature they offer necessitating the absence of USB 2.0. If all you care about is power and not seeing the OS wig out over some odd DisplayPort thing, and you don't need it for anything else, then problem solved for no cost.

Testing this theory should likely cost nothing, fixing it permanently should not cost more than $20.

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