[Xpert]Dual-head Gnome / GDM non-Xinerama working with Matrox G450 on RedHat7.3

[Dietmar May]

I searched for information about running multi-head Gnome GDM (or XDM) sessions with XFree, but couldn't really find anything to indicate that it was possible. As a matter of fact, some of the gnome mailing lists basically said, Wait for gnome 2.2.

However, I did manage to get it working. There may be a cleaner way, but here's what I did. I hope this can help someone else get it going too. (Who knows, that someone may be me in 4 months after I've forgotten exactly what changes to make :-)).

These changes apply to RedHat 7.3 with Gnome (and no KDE) installed. From what I've read dual-head with separate window manager sessions should work almost out of the box using KDE (unconfirmed). Unfortunately, all of RedHat's tools are based on Gnome, RedHat doesn't update their KDE RPMs, and KDE doesn't support stable RedHat releases. So, I'm stuck using Gnome, not having the time or inclination to hack around trying to get a known good KDE environment running on RH 7.3. But all that has no bearing on the technical aspects; it's just why someone might bother to try.

Your mileage may vary.

1. Edit XF86Config-4. Add a second Screen section, and a second Display section. Both should be a duplicate of the existing sections.

Next, change the Identifier for the second Display and Screen sections to be unique.

Use scanpci to obtain the BusID for the display adapter. If you have two PCI adapters or a separate PCI and AGP adapter, the BusID for the two Display sections will be different. If you have a single dual-head G450, it should be identical. In this latter case, add a Screen line to both Display sections - the first being Screen 0 and the second being Screen 1.

Make any other obviously necessary changes (like display resolutions, color depths, monitor parameters, etc).

Here are the sections from my XF86Config-4 file:

-------

Section "Device"
Identifier "Matrox G450 Head1"
Driver "mga"
VendorName "Matrox"
BoardName "Millennium G450 AGP"
BusID "PCI:1:0:0"
Screen 0
EndSection

Section "Device"
Identifier "Matrox G450 Head2"
Driver "mga"
VendorName "Matrox"
BoardName "Millennium G450 AGP"
BusID "PCI:1:0:0"
Screen 1
EndSection

Section "Screen"
Identifier "Screen0"
Device "Matrox G450 Head1"
Monitor "Dell P991"
DefaultDepth 16
Subsection "Display"
Depth 16
# demonstrates different display resolutions on one screen
# e.g. for a 15" LCD and a Multi-scan monitor
Modes "1024x768" "800x600" "640x480"
# Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubsection
EndSection

Section "Screen"
Identifier "Screen1"
Device "Matrox G450 Head2"
Monitor "Dell P991"
DefaultDepth 16
Subsection "Display"
Depth 16
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubsection
EndSection

-------

Finally, add a second Screen line to the ServerLayout, referencing the newly added Screen section.

-------

Section "ServerLayout"
Identifier "Anaconda Configured"
Screen 0 "Screen0" 0 0
Screen 1 "Screen1" LeftOf "Screen0"
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
EndSection

-------

2. At this point, you should be able to bring up X (startx), and see the gdm or xdm login screen on Screen0, and a gray screen with the X cursor on Screen1. After logging in, Gnome should start on Screen0. Screen1 should still be gray.

Now edit the file /etc/X11/xinit/Xclients. The key here is to invoke gnome-session twice, passing --display=:0.1 to one of the sessions. There are cleaner ways to do this, but this has minimal impact on the system configuration (in other words, if you don't have a second screen configured, it should work like the stock install). Note that the script greps for LeftOf or RightOf in the config file -- you could add Above, Below, and Relative as well if needed.

-------

#!/bin/bash
# Copyright 1999 - 2001 Red Hat, Inc.
# License: GNU General Public License v2

# check to see if the user has a preferred desktop
PREFERRED=
if [ -f /etc/sysconfig/desktop ]; then
if [ -n "`grep -i GNOME /etc/sysconfig/desktop`" ]; then
PREFERRED=gnome-session
elif [ -n "`grep -i KDE /etc/sysconfig/desktop`" ]; then
PREFERRED=startkde
fi
fi

if [ -n "$PREFERRED" ] && which $PREFERRED >/dev/null 2>&1; then
PREFERRED=`which $PREFERRED`
if "$PREFERRED" = "gnome-session" ; then
if [ -n "`grep "LeftOf\|RightOf" /etc/X11/XF86Config-4`" ]; then
$PREFERRED --display=:0.1 &
exec $PREFERRED --display=:0.0
else
exec $PREFERRED
fi
fi
fi

# now if we can reach here, either no desktop file was present,
# or the desktop requested is not installed.

if [ -z "$PREFERRED" ]; then

GSESSION=gnome-session
STARTKDE=startkde

# by default, we run GNOME.
if which $GSESSION >/dev/null 2>&1; then
if [ -n "`grep "LeftOf\|RightOf" /etc/X11/XF86Config-4`" ]; then
`which $GSESSION` --display=:0.1 &
exec `which $GSESSION` --display=:0.0
else
exec `which $GSESSION`
fi
fi

# if GNOME isn't installed, try KDE.
if which $STARTKDE >/dev/null 2>&1; then
exec `which $STARTKDE`
fi
fi

# Failsafe.
# Nothing of interest after this...

-------

A simpler way to accomplish all of this is to simply create a .xclients file in your home directory.

-------

#!/bin/bash
gnome-session --display=:0.1 &
exec gnome-session --display=:0.0

-------

3. Now, after restarting X, gnome should come up in both windows. A couple of strange behaviors occur: if logging in as root, two warning message boxes pop up, both in the primary display. Also, the splash screen is displayed in the second screen as expected; however, it displays itself perpetually until you click on it with the mouse. Wait a few seconds (OK, maybe longer if you don't have a really fast machine) and eventually the gnome desktop and launcher bar appear. I've seen the launcher bar occasionally hang for some seconds after it is initially displayed. When logging out, one of the two will simply clear the screen, the other will close out both sessions (it seems somewhat random, like it could be based on a race condition, depending on which initialized first). Other than these few oddities, everything seems to work fine for me. Hopefully, it will for you too.

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