I recently upgraded my system to Ubuntu 14.10.
Now when testing HDMI, I hit some bug.

My system is an optimus laptop.
The HDMI is connected to a nvidia card.

Now the HDMI has signal, and while logging in (so when Xorg is starting) I can shortly move my cursor over the external display. But when login is complete, the external display is 1 artifact, and doesn't show any updates anymore.

So I started bisecting the version from 14.10 to 14.04 (where it was working).

And I found out the following commit caused the issue:
commit 975b9798be77b30cbed485583d0ccb48318708f7
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed May 21 08:58:24 2014 +0100

    sna: Add support for Present
    
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>


Now I recompiled the intel driver with --disabled-dri3, and that fixes the issue also.

My hardware
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)
01:00.0 VGA compatible controller: NVIDIA Corporation GF108M [GeForce GT 540M] (rev a1)

The driver I use for the nvidia card is nouveau.