Multiseat (IV): evdev and Xephyr

(...comes from step III)




What is evdev?

evdev is the generic input event device driver. It passes the events generated in the kernel (since version 2.6.8) straight to the program, with timestamps.

The evdev driver is able to separate and distinguish events coming from different keyboards and mouses plugged into the computer. The xorg supports evdev input driver module since version 6.9/7.0 . This way, it can get separated input events coming from every seat (keyboard and mouse pair) on the system.


This is a great step compared to previous multiseat solutions, since it makes possible to have multiterminal systems without having to patch neither the kernel nor the X server.

What is Xephyr?

Xephyr is a kdrive based X Server which targets a window on a host X Server as its framebuffer. In other words, it is a piece of software which creates a X server on top of another X server. That will allow us to run as many Xephyr instances as seats we have in our system. Every Xephyr instance will cover the area shown in a given display, and finally, evdev will allow us to separate the different input devices.

The good news

Xephyr is included in the universe repository from Ubuntu.

The bad news

The Xephyr version included in the Ubuntu repository, does not support evdev. I hope in a near future Xephyr comes with evdev upport.

Right now, you will have either to patch the Xephyr sources and compile it, or download an already patched solution. Download links for both solutions are available here.

# wget http://www.c3sl.ufpr.br/multiterminal/howtos/xephyr-precompiled.tar.bz2
# tar -xjf xephyr-precompiled.tar.bz2 --directory /usr/local/

Now it is time to plug your second set of keyboard and mouse. The evdev driver will access the input coming from every device by means of a given event in the form of /dev/input/eventNN. You must obtain the event id associated to every input device.

A new problem arises, since the event number assigned can change (in fact, it changes) between different boots. To solve this problem we well use a wrapper script. The script is the same as proposed here, but modified in order to obtain the event number given the physical connection of the device. We are assuming that the devices will be always plugged in the same position (you can label the back connectors to remember the correct position). Since the physical device addresses do not change between boots, the script can obtain the corresponding event id and match it properly with one physical keyboard or mouse.

Input devices and events, once plugged the second keyboard and mouse:

# cat /proc/bus/input/devices
I: Bus=0011 Vendor=0001 Product=0001 Version=ab41
N: Name="AT Translated Set 2 keyboard"
P: Phys=isa0060/serio0/input0
S: Sysfs=/class/input/input0
H: Handlers=kbd event0
B: EV=120013
B: KEY=4 2000000 3802078 f840d001 f2ffffdf ffefffff ffffffff fffffffe
B: MSC=10
B: LED=7

I: Bus=0011 Vendor=0002 Product=0005 Version=0000
N: Name="ImPS/2 Generic Wheel Mouse"
P: Phys=isa0060/serio1/input0
S: Sysfs=/class/input/input1
H: Handlers=mouse0 event1 ts0
B: EV=7
B: KEY=70000 0 0 0 0 0 0 0 0
B: REL=103

I: Bus=0010 Vendor=001f Product=0001 Version=0100
N: Name="PC Speaker"
P: Phys=isa0061/input0
S: Sysfs=/class/input/input2
H: Handlers=kbd event2
B: EV=40001
B: SND=6

I: Bus=0003 Vendor=0d62 Product=001c Version=0202
N: Name="Darfon USB Combo Keyboard"
P: Phys=usb-0000:00:1d.1-1/input0
S: Sysfs=/class/input/input5
H: Handlers=kbd event4
B: EV=120003
B: KEY=10000 7 ff87207a c14057ff febeffdf ffefffff ffffffff fffffffe
B: LED=7

I: Bus=0003 Vendor=0d62 Product=001c Version=0202
N: Name="Darfon USB Combo Keyboard"
P: Phys=usb-0000:00:1d.1-1/input1
S: Sysfs=/class/input/input6
H: Handlers=kbd event5
B: EV=3
B: KEY=3078 d801d101 1e0000 0 0 0

I: Bus=0003 Vendor=046d Product=c03f Version=2000
N: Name="Logitech USB-PS/2 Optical Mouse"
P: Phys=usb-0000:00:1d.1-2/input0
S: Sysfs=/class/input/input8
H: Handlers=mouse1 event3 ts1
B: EV=7
B: KEY=f0000 0 0 0 0 0 0 0 0
B: REL=103

The Name section will help us to identify the device. The Phys section is the parameter that we will send to the wrapper script in order to start every Xephyr instance.

The Xephyr wrapper

Now, it is time to create the wrapper script. The original one is modified to obtain the event numbers from the physical addresses

# vi /usr/sbin/Xephyr.sh

This is the final wrapper script:

#!/bin/bash

# 20060905 - josean - added get_event() function to obtain eventNN from a physical address
# Original version:
# http://en.wikibooks.org/wiki/Multiterminal_with_Xephyr
# http://www.c3sl.ufpr.br/multiterminal/howtos/Xephyr.sh

trap "" usr1

XEPHYR=/usr/local/bin/Xephyr

get_event()
{
  evento=`grep -A2 $1 /proc/bus/input/devices | grep 'H: Handlers=' | grep --only-matching -e 'event[0-9]*'`
}

args=()

while [ ! -z "$1" ]; do
 if [[ "$1" == "-xauthority" ]]; then
   shift
   if [ ! -z "$1" ]; then
     export XAUTHORITY="$1"
   fi
 elif [[ "$1" == "-display" ]]; then
   shift
   if [ ! -z "$1" ]; then
     export DISPLAY="$1"
   fi
 elif [[ "$1" == "-kbdphys" ]]; then
   shift
   if [ ! -z "$1" ]; then
     get_event $1
     args=("${args[@]}" "-keyboard")
     args=("${args[@]}" "/dev/input/$evento")
   fi
 elif [[ "$1" == "-mousephys" ]]; then
   shift
   if [ ! -z "$1" ]; then
     get_event $1
     args=("${args[@]}" "-mouse")
     args=("${args[@]}" "/dev/input/$evento,5")
   fi
 else
   if ! expr match $1 'vt[0-9][0-9]*' >/dev/null; then
     args=("${args[@]}" "$1")
   fi
 fi

 shift
done

# echo $XEPHYR "${args[@]}" >> /tmp/logXephyr

exec $XEPHYR "${args[@]}"

Finally, be sure to allow the script to be executed.

# chmod +x /usr/sbin/Xephyr.sh

(continues on step V...)