Mini DP port not working in XPS 15 l521x

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http://askubuntu.com – I've installed Ubuntu 12.10 in my XPS 15 l521x. I had some issues (i.e. HidPoint is quite unstable for my Logitech wireless mouse/keyboard and works pretty randomly), but now the big one that is stopping me is that I'm unable to make my Mini Display-Port working. I have a Mini DP to VGA (branded Apple) connected to an external monitor. The monitor is working and recognized in Windows 8, but there's no way I can make it work through Ubuntu (it doesn't even detect the display). $ xrandr -q Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192 LVDS1 connected 1920x1080+0+0 (norma (HowTos)