DisplayLink and x2x brings back Zaphod mode

Ever since work issued me a Lenovo T61 and I installed Fedora on it, I have lamented the loss of something that X afficionados referred to as “Zaphod mode”.  By gluing together a few different software and hardware components I managed to get close to the old Zaphod mode days — but first some background…

Usually when you set up a multi-monitor installation you get a single desktop that spans all the screens.  This is great when you have a single desktop, but on Linux multiple desktops are the norm.  When I started using multiple screens in Linux, I loved the extra screen real estate but the fact that switching virtual desktops caused *all* the windows on all the screens to switch really bugged me.  I wanted the ability to have something — like an email program, or a web browser — to stay on one screen while I switched between desktop views on the other screen.  Or better still, the ability for both screens to have virtual desktops that were independent of each other.

Enter “Zaphod mode”, named for Zaphod Beeblebrox from the Hitchhikers Guide to the Galaxy by Douglas Adams.  Beeblebrox, who was President of the Galaxy before he stole the Starship Heart Of Gold, had two heads that were independent of each other.  In X server terms, multiple display devices are often referred to as “heads”.  So you can probably deduce that “Zaphod mode” refers to an operating mode of the X server where the multiple “heads” or display devices function as different displays.

Go back far enough and you get to a point where that was the standard mode of operation of X.  The X extension “Xinerama” was developed to provide the merging of different X displays into a single screen.  NVidia also had a hardware/firmware based equivalent called TwinView, where multiple heads on an NVidia card (and even sometimes heads on different cards) could be joined.  These extensions came not without their problems however: it was common for windows and dialog boxes to get confused about what display to appear on.  You would almost always see dialog boxes that are meant to display in the middle of the screen being split across the two physical displays.  Also, there was the multiple desktop “inconvenience” of not being able to switch the desktops independently.

Zaphod mode fixed these problems.  Because the screens were separate, windows and dialog boxes always appeared in the centre of the physical screen.  You could leave a web browser on one screen while you switched between an e-mail client, an IRC client, and an SSH session in the other.  It wasn’t all beer-and-skittles though, since in Zaphod mode it was not possible to move an application from one screen to the other.  Plus, some applications like Firefox could not have windows running on both screens (the second one to start could not access the user profile).

Zaphod mode largely “went away” during the transition from XFree to Xorg.  The servers dropped support for multiple separate displays in the one server, and only gradually added it back in (with the Intel driver being one of the last to do so, and probably still has not).  Since laptops were the only place I still used multiple screens, and the laptops I used all had Intel integrated graphics, I had to do without Zaphod mode.

Today, I hardly use dual monitors at all.  I used to have a desktop system with a 21″ CRT flanked by 17″ LCDs on either side, but that all got replaced by a single 24″ LCD.  At work we don’t have assigned desks, so setting up a screen to plug the laptop into isn’t going to happen.  I guess I learned to live without Zaphod mode by just going back to a single screen.  I still remember my Zaphod-powered dual-screen days fondly though, and with almost every update to Xorg I would scan the feature list looking for something like “Support for configuration of multiple independent displays (Zaphod mode)”.

A while back I bought a DisplayLink USB to DVI adapter.  I didn’t really know what to do with it at the time, but recently I dug it out and tried setting it up.  Googling for “DisplayLink Fedora” sent me to a couple of very helpful pages and it didn’t take long to get the “green screen of life” that indicates that the DisplayLink driver was active.  It was when I was looking at how to make it work as an actual desktop — part of the process involves setting up a real xorg.conf (that’s right, something about the DisplayLink X server means it can’t be configured by the Xorg auto configuration magic) — that I realised I could do something wonderful.  Instead of making a config file that contained both my standard display and the DisplayLink device (and probably cause havoc for the 90% of times I boot without an additional screen) I would create a config file with *just* the DisplayLink device and start it as a second server.  Run a different window manager in there, and I would have two independent desktops — Zaphod mode!

I did a couple of little experiments just starting an xterm in the second X, and it worked fine (the more alert of you will realise that I’m taking a bit of artistic license with the word “fine” here, and know that three little letters in the title of this post are a clue to what wasn’t yet working…) with the desktop and the xterm appearing in the second monitor.  I installed XFCE, and configured it to start as the window manager of the second X server, which also worked well.

Something was missing though: there was no mouse input to the second screen.  In Zaphod mode, even though the two screens were separate X displays they were managed by the same server.  This meant that the input devices were shared between the two displays.  In this configuration, I was careful to exclude any mouse and keyboard devices from my second display config to avoid any conflicts.  So how was I to get input device data into the second server?  A second display is not much good if you can’t click and type on the applications that run on it…

I remembered about an old program called x2x that could transfer the mouse and keyboard events to a different X server when you moved the mouse to the edge of your display (and, inexplicably, I forgot all about a much younger program called Synergy that can do the same thing).  Since x2x isn’t built for Fedora I found the source and built it and started it up…  and it worked first time!  When I moved the mouse to the edge of the screen, it appeared on the other screen!  I could start apps and type into them exactly as I wanted.

It wasn’t perfect, however.  I found that when I returned the mouse to the primary screen, the second screen was still getting keyboard events.  I figured this would be particularly inconvenient when, for example, I was entering user and password details into an app on the primary screen while an editor or terminal program had focus on the second screen…  I checked the Xorg.1.log file, and found that even though I had not specified a “keyboard” input device Xorg was automatically defining one for me.  I turned off the udev options, but it still happened.  My initial enthusiasm was starting to fade.

What fixed it was to manually define a “dummy” keyboard device.  There must be some logic in Xorg that it refuses to allow a configuration with no configured keyboard (which makes sense), so in this rather unusual case where I don’t want a keyboard I have to define one but give it a dummy device definition.  Defining the dummy keyboard stopped Xorg from defining its automatic one, and everything worked as expected!  Even screensavers work more-or-less as designed (although I haven’t actually spent much time in front of the setup yet so haven’t had to unlock the screen that often).

I’m away from the computer in question right now, otherwise I would post configs and command lines (and even a pic of the end result).  I’ll update this post with the details — leave a comment if you think I need to hurry up!  🙂