Debian squeeze gdm / Xserver won’t start without a monitor

The little Atom based computer I was using as a Linux server in the house died.  Well technically, the fan died, which led to the rest of it dying.  It’s a proprietary case and motherboard,  so the fan isn’t something I can just pick up and replace (looks like a laptop fan, squeezed into the case), and I need something working faster than I could repair it.  I had a spare PC upstairs, which isn’t as quiet or as energy efficient as the Atom, but at least it works!  (This was after several days of Raspberry Pi’s failing and trashing SDHC cards, so I was already pretty pissed off with hardware, Linux and the whole building it yourself thing).

Anyway, as it happens, Debian reminded me how exceedingly trivial it is to build a server, and since I had full backups of the Atom PC it didn’t take long to get everything back up and running.  I was also reminded how slow the Atom chips can be, the P4 I’ve replaced it with is a world apart in terms of speed for whatever reason.

The Atom machine was running Ubuntu, but it used to frustrate me when it was quite the same as Debian, and I wanted to go back to a basic Debian build.  Also, since the P4 is more stock than the Atom I don’t need the bleeding edge drivers you get with Ubuntu.  All of this did leave me with one issue though.

When connected to a monitor, the Debian build works fine.  It works if you boot it with a monitor and then remove the monitor as well, but if you boot it without a monitor, it won’t start the X Server.  It tries about a million times [1] and then gives up.

I use Gnome under Debian, and because this machine is sitting physically in my house, I enable the autologon, and remote desktop control, so if I want, I can VNC in from my main machine.  I don’t usually need to do it, and I’m comfortable doing everything I want on that box from the command line, but every now and then it’s nice to use one or two GUI based apps.  Since Gnome supports this out of the box, I don’t feel the need to install VNC and start changing the config – it worked under Ubuntu, and it works under Debian if the monitor is there, I just needed it to work without the monitor.

I did a lot of reading around, there are plenty of suggestions about using VNC instead, some suggestions of modifying xorg.conf with some default display settings, and some other stuff.  I tried setting up an xorg.conf (new versions of X don’t use one by default, so you have to create one), but that didn’t seem to help.  More reading, and more playing around, and then I finally found the exact solution.

You can read the original page here.

Essentially, you need to add an entry to xorg.conf as I had been, but even then, X will probably refuse to start, because it detects modesetting drivers in the kernel and refuses to load the VESA driver.  Here’s the specific section from the error log if you get this,

(II) VESA: driver for VESA chipsets: vesa
(EE) VESA: Kernel modesetting driver in use, refusing to load
(WW) Falling back to old probe method for vesa
(EE) No devices detected.

So, as you can see from the document linked above, you need to disable the modesetting and then X will happily start.  This is the /etc/X11/xorg.conf config file I used (same as the original document),

Section "Device"
Identifier "VNC Device"
Driver "vesa"

Section "Screen"
Identifier "VNC Screen"
Device "VNC Device"
Monitor "VNC Monitor"
SubSection "Display"
Modes "1280x1024"

Section "Monitor"
Identifier "VNC Monitor"
HorizSync 30-70
VertRefresh 50-75

I also used both the i915 modeset change (because I had an i915 config file already), and the Nvidia one, since the machine has an Nvidia card in it.  A quick reboot, gdm and the X Server both started fine.  Very happy!

So the key isn’t just the xorg.conf above, which most people have posted about, you probably have to disable modesetting in the relevant graphics driver as well.

[1] okay, about half a million.

2 thoughts on “Debian squeeze gdm / Xserver won’t start without a monitor

  1. Hey, thanks for the post! i just spent a few hours trying to get debian to run headless, and was about to put the vga > svideo adapter on the back (which i think is kind of lame). But the link you posted to: was just what i was looking for.

    My problem was opposite of yours, I already had the xorg.conf file (saved it from my last install b/c its become a regular thing to do on new installs). But i just couldn’t find out what it wasn’t working. Everything worked with the monitor plugged in, but when it was unplugged only SSH worked and VNC didn’t. To make matters worse, as soon as i plugged the monitor in, the loading mouse icon started to spin and then my background came up. Kind of discouraging that the problem was fixed just by changing the 1 to a 0 in the i915 config file. Thanks again!

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