Setting up USB multiseat with DisplayLink on Linux (GDM up to 2.20)

Four $99 UD-160-A Terminals off a single $299 netbookSoon, we’ll be able to plug inexpensive zero-state USB docks/terminals into new Linux systems, and a new graphical login will pop up in a completely plug and play fashion. Many users can then simultaneously share any single Linux PC. This is great for education, libraries, internet cafes, etc — anywhere where you have clusters of client machines with light 3D/video app demands, and want the simplicity and savings of just one server machine with many terminals connected.

[Update August 8, 2010 – Further work on this has moved to the Google Summer of Code 2010 project for USB Multiseat]

Until this is in the distros, we have to assemble a few pieces ourselves to get everything installed for this scenario. Some major components (GDM/ConsoleKit) are changing the way this kind of thing will be done, so for now the instructions here are designed only for distributions with GDM 2.20 or earlier (Ubuntu < 9.10; Fedora < F12; Debian < 5.0; etc). The instructions have been tested most extensively on Ubuntu 9.04.

gdm –version

will show what version you have on your current system. Any tweaks to make things work with your specific distro are definitely welcome in the comments.

The first step is hardware. You need a USB hub with a DisplayLink USB device and free USB ports for keyboard and mouse. You can get this in one package in the form of devices like the Plugable DC-125, or buy independent USB devices like the UGA-125 combined with any USB hub. USB Audio and other devices can be made to work also, but these instructions just cover basic display, keyboard, and mouse.

With that, you’re ready to start configuring your Linux setup …

1. DisplayLink framebuffer driver

udlfb is in the staging tree of Linux kernels 2.6.32 and later. We’ll install the latest version here. First, make sure you have git installed with “sudo apt-get install git-core”, and create a directory which will host all the source code you download with git (e.g. ~/git/).

sudo apt-get install module-assistant
sudo module-assistant prepare
git clone
cd udlfb
sudo make install
sudo depmod -a

Now, when you plug in a DisplayLink device, you should see a “green screen” as the driver successfully loads and sets the graphics mode to match your monitor.

2. DisplayLink X server

This will get the X server installed, ready for use by later scripts.

sudo apt-get install pkg-config xorg-dev
cd ~git
git clone
cd xf-video-udlfb
sudo make install

We now need to create or modify a few scripts and configuration files. You’ll need to use sudo to edit files in these system directories. You can cut/paste the text below, or download the files with “git clone” and copy each to the right location, and fix up ownership and permissions on the files.

3. udev script

Create a file called /lib/udev/rules.d/50-usbseat.rules owned by user root, with the following contents.

# set all DisplayLink devices to configuration 1
# see for more info
ATTR{idVendor}=="17e9", ATTR{bConfigurationValue}=="2", RUN="/bin/echo 1 > /sys%p/bConfigurationValue"

# aliases for display, kbd, mouse attached to specific hubs

KERNEL=="fb*",SUBSYSTEMS=="usb",PROGRAM="/bin/cat /sys/%p/../../../devnum",SYMLINK+="usbseat/%c/display",RUN+=" %c"
KERNEL=="mouse*", SUBSYSTEMS=="usb", ATTRS{bInterfaceClass}=="03", ATTRS{bInterfaceProtocol}=="02", PROGRAM="/bin/cat /sys/%p/../../../../../devnum",SYMLINK+="usbseat/%c/mouse",RUN+=" %c"
KERNEL=="event*", SUBSYSTEM=="input", ATTRS{bInterfaceClass}=="03", ATTRS{bInterfaceProtocol}=="01",PROGRAM="/bin/cat /sys/%p/../../../../../devnum",SYMLINK+="usbseat/%c/keyboard",RUN+=" %c"
KERNEL=="control*", SUBSYSTEM=="sound", SUBSYSTEMS=="usb", PROGRAM="/bin/cat /sys/%p/../../../../../devnum", SYMLINK+="usbseat/%c/sound"

# Handle when keyboard and mouse are one more hub downstream. Relying on pnp order to have already set up mouse, keyboard on upstream hub if we're daisy-chaining
KERNEL=="event*", SUBSYSTEM=="input", ATTRS{bInterfaceClass}=="03", ATTRS{bInterfaceProtocol}=="01",PROGRAM="/bin/cat /sys/%p/../../../../../../devnum",SYMLINK+="usbseat/%c/keyboard",RUN+=" %c"
KERNEL=="mouse*", SUBSYSTEMS=="usb", ATTRS{bInterfaceClass}=="03", ATTRS{bInterfaceProtocol}=="02", PROGRAM="/bin/cat /sys/%p/../../../../../../devnum",SYMLINK+="usbseat/%c/mouse",RUN+=" %c"

The udev subsystem will run this script automatically each time a USB device is attached.

4. script

Create a file /lib/udev/ owned by user root with the following contents. Make sure to “sudo chmod a+x” to make the file executable.

# takes the "seat number" as parameter $1
# the seat number is the kernel device id of the hub the seat's devices are sitting off of
# called once for every usb device that MIGHT be part of a seat, when they arrive or remove 

if [[ !(-n `/bin/pidof gdm`) ]]; then
    exit 0

seat_running=`/usr/bin/gdmdynamic -l | /bin/sed -n -e "/:$1,/p"`

# $ACTION environment variable is set by udev subsystem
case "$ACTION" in
		if [[ -n "{$seat_running}" ]]; then
			/usr/bin/gdmdynamic -v -d $1
                # A device which might be part of a seat has been added

		# if we already have a running seat for this #, exit
		if [[ -n "${seat_running}" ]]; then
			exit 0

		if [[ -e /dev/usbseat/$1/keyboard && -e /dev/usbseat/$1/mouse && -e /dev/usbseat/$1/display ]]; then

			# We have a newly complete seat. Start it.
			TMPFILE=`/bin/mktemp` || exit 1
			/bin/sed "s/%ID_SEAT%/$1/g" < /lib/udev/usbseat-xf86.conf.sed > $TMPFILE
			/usr/bin/gdmdynamic -v -t 2 -s 1 -a "$1=/usr/X11R6/bin/X -br :$1 -audit 0 -nolisten tcp vt07 -config $TMPFILE"
			/usr/bin/gdmdynamic -v -r $1

exit 0

5. X config file

Create file /lib/udev/usbseat-xf86.conf.sed with contents

Section "ServerFlags"
	Option	"AutoEnableDevices"	"false"
	Option	"AutoAddDevices"	"false"
	Option  "DefaultLayout"		"seat"
	Option	"DontZoom"		"true"
	Option	"DontZap"		"true"
	Option	"AllowMouseOpenFail"	"yes"

Section "Module"
	Load "ddc"

Section "Files"                                                                                                              
    ModulePath      "/usr/lib/xorg/modules"
    ModulePath      "/usr/local/lib/xorg/modules"

Section "Device"
	Identifier "dl"
	driver	   "displaylink"
	Option "fbdev"	"/dev/usbseat/%ID_SEAT%/display"

Section "InputDevice"
	Identifier "keyboard"
	Driver	"evdev"
	Option	"CoreKeyboard"
	Option	"Device"	"/dev/usbseat/%ID_SEAT%/keyboard"
	Option	"XkbModel"	"evdev"
	Option	"XkbLayout"	"us"

Section "InputDevice"
	Identifier "mouse"
	Driver	"mouse"
	Option	"CorePointer"
	Option	"Protocol" "ImPS/2"
	Option	"Device"	"/dev/usbseat/%ID_SEAT%/mouse"
        Option  "Buttons" "5"
	Option	"ZAxisMapping" "4 5"

Section "Monitor"
	Identifier "monitor"

Section "Screen"
	Identifier "screen"
	Device "dl"
	Monitor "monitor"

Section "ServerLayout"
	Identifier "seat"
	Screen	0 "screen" 0 0 
	InputDevice "keyboard" "CoreKeyboard"
	InputDevice "mouse" "CorePointer"

6. fbcon workaround

[update: this step is no longer needed with the latest udlfb from git or in kernels 2.6.37+]

fbcon is a standard Linux kernel module, which aggressively assumes it can open any framebuffer device and take it over for use as a text terminal. Unfortunately, that’s not what we want if we’re going to be using that framebuffer to run its own X server. So we need to add a file /etc/modprobe.d/fbcon.conf to disable fbcon and leave our framebuffers free for other uses.

blacklist font
blacklist tileblit
blacklist bitblit
blacklist fbcon

This file will not take effect until you run

sudo update-initramfs -u

Now, when you reboot and run “lsmod” you should not see fbcon in the loaded modules. And in /sys/class/graphics you should see fb0, instead of fbcon.

7. xrandr workaround

[update: this workaround is not needed when running xf86-video-fbdev or with recent displaylink X servers with this patch]

The DisplayLink X server currently has limited RANDR support, but later versions of GDM assume better. So for the time being, a workaround is required to get GDM applications (including gdmlogin) to display properly within the actual screen area — otherwise they tend to think the screen has a strange rotation, and display themselves completely off it.

Add these lines into /etc/gdm/Init/Default, right after the definition of gdmwhich()

XRANDR=`gdmwhich xrandr`
if [ "x$XRANDR" != "x" ]; then
  $XRANDR -o 0

8. /etc/rc.local script

Add the following lines to your /etc/rc.local script to check for attached usb terminals that were attached (at boot), and udev found them before GDM was running.

for seat in /dev/usbseat/*; do
	set $seat
	/lib/udev/ $4

9. /etc/init.d/gdm patch

In recent versions of X, the system largely assumes that you’ll only run one X server, and it will own all devices. So to support multiseat with multiple X servers easily, we need to have two configurations: normal (using only your primary graphics), and multiseat (where your primary graphics isn’t used). We do this by detecting whether you have a USB terminal attached and configured, and if so coming up with a different gdm configuration.

Add these lines to your /etc/init.d/gdm script, just after the section to “Allow cdd to override the config” (around line 35).

# Allow usbseat to override the config
if [ -f /etc/gdm/gdm-usbseat.conf ]; then
	for usbseat in /dev/usbseat/*; do
		if [ -e "/dev/usbseat/$seatid/keyboard" -a -e "/dev/usbseat/$seatid/mouse" -a -e "/dev/usbseat/$seatid/display" ]; then

10. Create /etc/gdm/gdm-usbseat.conf

This is the alternative gdm.conf that will be used when a USB terminal is present at boot:









Now, when you boot with USB terminal(s) attached, graphical logins will come up on all of those, while your primary display will remain a text console.

usb multiseat with Edubuntu

See my video from Linux Plumbers 2009 for more thoughts and background on USB multiseat.

Unsolved Problems

Beyond reworking for the new GDM/ConsoleKit versions, and getting all this just “built in” to the distributions, there are at least several major problems yet:

input duplication

[Update Feb 19, 2010 – fixed by removing “-sharevts -novtswitch” from the X start line and substituting a specific vt “vt07”. Listing above now has problem resolved]

Input to any of the terminals will be duplicated to your primary console. This has a bunch of very nasty effects, including often duplicating your login to both your session and the console, and possibly causing strange effects like ctrl-c within any terminal will often cause that whole X session to close. This seems to be, which at the bottom reports a workaround involving faketty, which unfortunately didn’t work for me. You can also see a short discussion of this bug at There ought to be a simple solution here — any comments on that welcome.

mouse scroll wheel doesn’t work

[Update Feb 19, 2010 – fixed by changing Protocol from “auto” to “ImPS/2” in the xorg.conf]

This is a problem with some other ways of doing multiseat: Again, probably a small tweak might resolve this.

unplug terminal doesn’t tear down (or reconnect)
Currently, when you physically disconnect a terminal, everything remains running. By default, each user can only be logged in once, which means that you must do gdmdynamic -l to find the zombie instance and sudo gdmdynamic -d $seat to kill it. But the behavior users would really like to see is disconnecting the terminal leaves all apps running, but logging in from that terminal (or any other) reconnects and just transparently starts where the user left off.

Comments on other distros (e.g. yum vs. apt-get), better solutions, and other problems are very much appreciated.

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