DisplayLink

DisplayLink devices on Linux still only have experimental support. While some people have had success in using them, it is generally not an easy process and not guaranteed to work. The steps on this page describe the generally most successful methods of using external monitors with DisplayLink.

Installation

Note: Modern USB 3.0 DisplayLink adapters are not supported as of October 2014.[1] Older DisplayLink chips (USB 2.0) are supported.

Install the xf86-video-fbdev package, which provides framebuffer video for X.org.

Hardware-level support is provided by the kernel module udlfb, which should be loadable by default in Arch.

udl

The kernel DRM driver for DisplayLink is udl, a rewrite of the original udlfb driver. It allows configuring DisplayLink monitors using Xrandr.

First, the setup and installation:

  • Blacklist the old kernel module, udlfb, which may attempt to load itself first.

After that, run:

# xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x49 cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 2 outputs: 8 associated providers: 0 name:Intel
Provider 1: id: 0x13c cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 0 name:modesetting

In the above output, we can see that provider 0 is the system's regular graphics provider (Intel), and provider 1 (modesetting) is the DisplayLink provider. To use the DisplayLink device, connect provider 1 to provider 0:

# xrandr --setprovideroutputsource 1 0

and xrandr will add a DVI output you can use as normal with xrandr. This is still experimental but supports hotplugging and when works, it's by far the simplest setup. If it works then everything below is unnecessary.

Configuration

These instructions assume that you already have an up and running X server and are simply adding a monitor to your existing setup.

Load the framebuffer device

Before your system will recognize your DisplayLink device, the udl kernel module must be loaded. To do this, run

# modprobe udl

If your DisplayLink device is connected, it should show some visual indication of this. Although a green screen is the standard indicator of this, other variations have been spotted and are perfectly normal. Most importantly, the output of dmesg should show something like the following, indicating a new DisplayLink device was found:

usb 2-1.1: new high-speed USB device number 7 using ehci-pci
usb 2-1.1: New USB device found, idVendor=17e9, idProduct=03e0
usb 2-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 2-1.1: Product: Lenovo LT1421 wide
usb 2-1.1: Manufacturer: DisplayLink
usb 2-1.1: SerialNumber: 6V9BBRM1
[drm] vendor descriptor length:17 data:17 5f 01 00 15 05 00 01 03 00 04
udl 2-1.1:1.0: fb1: udldrmfb frame buffer device
[drm] Initialized udl 0.0.1 20120220 on minor 1

Furthermore, /dev should contain a new fb device, likely /dev/fb1 if you already had a framebuffer for your primary display.

To automatically load udl at boot, create the file udl.conf in /etc/modules-load.d/ with the following contents:

/etc/modules-load.d/udl.conf
udl

For more information on loading kernel modules, see Kernel modules#Loading.

Configuring X Server

There are three popular ways people use DisplayLink devices with X on desktop Linux computers:

  • With xrandr and udl (recommended)
  • Xinerama with a single X server
  • Two separate X servers, linked together in some way

While Xinerama is probably the more desirable setup, it is also less likely to work. Both methods are described below.

xrandr

The udl kernel driver is the only one that works wiith xrandr. Once the driver is loaded, the DisplayLink monitor is listed as an output provider:

$ xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x43 cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 2 outputs: 2 associated providers: 1 name:Intel
Provider 1: id: 0xcb cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 1 name:modesetting

In the above example, provider 1 is the DisplayLink device, and provider 0 is the default display. Running xrandr --current gives a list of available screens:

$ xrandr --current
Screen 0: minimum 320 x 200, current 1600 x 900, maximum 8192 x 8192
LVDS1 connected 1600x900+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   1600x900       60.0*+   40.0  
   1440x900       59.9  
   1360x768       59.8     60.0  
   1152x864       60.0  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 disconnected (normal left inverted right x axis y axis)
DVI-1-0 connected (normal left inverted right x axis y axis)
   1366x768       60.0 +
   1368x768_59.90   59.9  
  1368x768_59.90 (0xd0)   85.7MHz
        h: width  1368 start 1440 end 1584 total 1800 skew    0 clock   47.6KHz
        v: height  768 start  769 end  772 total  795           clock   59.9Hz

If the above does not list the DisplayLink screen, then you will need to offload DisplayLink to the main GPU:

xrandr --setprovideroutputsource 1 0

Once the screen is available, refer to Xrandr for info on setting it up. For automating the configuration process, see displaylink.sh.

Xinerama setup

You must update or create an xorg.conf with a properly configured ServerLayout to use a DisplayLink monitor, as Xorg will prefer internal monitors by default. The DisplayLink device is normally only usable if it is set as screen0 and the internal display as screen1.

Add this to the bottom of your xorg.conf:

/etc/X11/xorg.conf
 ################ DisplayLink ###################
 Section "Device"
        Identifier      "DisplayLinkDevice"
        Driver          "fbdev" 
        BusID           "USB"               # needed to use multiple DisplayLink devices 
        Option          "fbdev" "/dev/fb0"  # change to whatever device you want to use
 #      Option          "rotate" "CCW"      # uncomment for rotation
 EndSection
 
 Section "Monitor"
        Identifier      "DisplayLinkMonitor"
 EndSection
 
 Section "Screen"
        Identifier      "DisplayLinkScreen"
        Device          "DisplayLinkDevice"
        Monitor         "DisplayLinkMonitor"
        DefaultDepth    16
 EndSection

Then edit your server layout to look something like this

Screen		0	"DisplayLinkScreen"
Screen		1	"Internal" RightOf "DisplayLinkScreen"
Option		"Xinerama" "on"

Change Internal to your main display, then restart X.

Dual X setup

Refer to the steps covered in the Gentoo Wiki for details on this setup.

Troubleshooting

Screen redraw is broken

If you are using udl as your kernel driver and the monitor appears to work, but is only updating where you move the mouse or when windows change in certain places, then you probably have the wrong modeline for your screen. Getting a proper modeline for your screen with a command like

gtf 1366 768 59.9

where 1366 and 768 are the horizontal and vertical resolutions for your monitor, and 59.9 is the refresh rate from its specs. To use this, create a new mode with xrandr like follows:

xrandr --newmode "1368x768_59.90"  85.72  1368 1440 1584 1800  768 769 772 795  -HSync +Vsync

and add it to Xrandr:

xrandr --addmode DVI-0 1368x768_59.90

Then tell the monitor to use that mode for the DisplayLink monitor, and this should fix the redraw issues. Check the Xrandr page for information on using a different mode.

X crashes or keeps blank

If X crashes, or nothing is shown at load, try to start X only using the external display by updating xorg.conf as follows:

 Screen		0	"DisplayLinkScreen"
 #Screen		1	"Internal" RightOf "DisplayLinkScreen"
 #Option		"Xinerama" "on"

There are reported instances in which Xinerama requires all its Screen color depths to be the same. In both Screen sections, try changing the DefaultDepth to 16.

Section "Screen"
    ...
    DefaultDepth    16
    ...
EndSection

Note: With fbdev this is not true anymore, because fbdev provides virtual 24 bit support. Everything may be used with DefaultDepth 24. However, USB 2.0 has less bandwidth than a standard monitor, so a performance update may be seen from setting it to 16 bit mode.

Cannot start in framebuffer mode. Please specify busIDs for all framebuffer devices

With two monitors configured in Xinerama mode, /var/log/Xorg.0.log will sometimes display the following error:

Fatal server error:
Cannot run in framebuffer mode. Please specify busIDs for all framebuffer devices

As indicated by this Gentoo Forums thread, KMS might be incompatible with fbdev. They also suggested the possible fix of running startx -- -retro which will allow X to run even when, to X itself, it doesn't appear to be working.

Note: A bug report has been filed for this issue.

DisplayLink refresh rate is extremely slow with gnome 3

If once you set up your DisplayLink your entire desktop becomes slow, and maximizing a window on the displaylink screen reduces the lag, try adding the screen manually via xrandr:

xrandr -q
xrandr --output [screen_here] --right-of [other_screen] --auto

See Also