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Update

since the latest release of displaylink drivers this issue was solved - it was a driver issue

I'm connecting a Lenovo Thinkvision Displaylink USB external monitor to a Lenovo Thinkpad T450s.

It works fine when I connect the screen to an AC powered USB hub connected, but when I connect the screen directly to the laptop it doesn't draw enough energy and the screen won't work well (flickers, doesn't refresh properly, doesn't paint windows etc.). It works perfectly when connected to the hub attached to the laptop.

The screen came with a usb "y" cable (you know the one that is splitted in two plugs on one end). I think it's precisely meant to draw more power when not used on AC powered hubs. However, I do think that Ubuntu is not sending any power there on the 2nd port and when connected on the laptop it only draws power from a single port, which is insufficient. Tried with two different cables, to no avail. Tried also to make sure that the laptop will charge usb devices (always-on usb 3.0 feature, the screen is usb 2.0 anyway).

I use Ubuntu 15.10.

How would you address this?

Updates

Looks like the monitor can work with just 1 usb from the laptop, but I've found more weirdness going on:

  1. if I start ubuntu, then connect the display with either 1 or 2 usb, it will work (i can see the refresh rate is better with 2)

  2. if I disconnect the display, and reconnect it, it won't work. It may turn on and all, but it will refresh very slowly, to the point of being unusable.

  3. if anything related to screen displays refreshes/restarts, such as the displaylink service or for example if I update my software for display calibration (DisplayCal), the screen will flash and will enter in this state where it doesn't paint properly

  4. if I start my laptop with my display connected already (either to 1 or 2 usb) it won't work, it will only function in this bad state - sometimes it will even keep the same appearance as I left it since last reboot (same partially painted areas)

  5. disconnecting and reconnecting again after the screen has entered in this state, no matter what, it will always bring it back in the same unusable mode. I have to restart ubuntu with the screen disconnected, then connect the screen and hope it won't hang

  6. very interestingly, whenever the screen is in this failed state, it will show 2 extra displays in my "Screen" settings - one correctly labeled Lenovo Thinkvision 14" and the other labeled as FNM 0"

this will only show up when display doesn't work correctly

all this leads me to believe is something that could be fixed within ubuntu

unfulvio
  • 1,655

2 Answers2

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You might try the tlp package for Thinkpad power management, it provides some USB options:

http://linrunner.de/en/tlp/docs/tlp-configuration.html#usb

Read the install instructions carefully, it is incompatible with laptop-mode-tools (uninstall first).

# tlp stat| grep -i USB

shows the info about USB device manamement then.

MoreIT
  • 111
1

As has been said earlier, software can't control what power is sent out to what USB ports, however depending on the spec, to which the port was build, will depend on what the electroncis will kick out of it. It sounds like you need to get back to basics to work out what power you have/need.

I've had similar problems with USB and Raspberry pi. The standard USB (2.0) spec says a USB port should kick out 0.6A (600mA), but the max a device can draw is 500mA. USB 3.0 increased that, so some devices (and hence some power supplies) can kick out 2A (2000mA), but devices should draw no more than 900mA.

To illustrate my problem, my Rpi, requires (according to spec) 700mA. If I connect it to a nokia phone charger, it works fine. But when I start using the network, it crashes. The reason, I have a 600mA supply and without networking, that's fine.

I have a 2A supply, and can in theory run both my LCD display and my Rpi from it. However if I connect the Display to the Rpi, and power the Rpi with the supply, it won't work, because the total power draw, would excede the limit on the Rpi. I need a usb hub, that can take the 2A supply and split the power to both devices to get it to work.

So it could be simply that you can't do it, with your laptop. I suggest you check the small print. If the Laptop is USB2, there is a good chance it won't supply more than 1A over 2 devices, that may not be enough for your display.

sibaz
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