Why can't I load the radeon kernel module after X server and Unity desktop environment have started?

[Teo En Ming]

Hi,

For purposes of Xen VGA passthrough, I have blacklisted the radeon 
kernel module in /etc/modprobe.d/blacklist.conf. I have also uninstalled 
the lightdm display manager.

# blacklist radeon kernel module
blacklist radeon
# blacklist snd_hda_intel kernel module
blacklist snd_hda_intel

When I am not doing VGA passthrough, I would like to load the radeon 
kernel module so that I can play MPEG-2 and MPEG-4 videos smoothly. But 
I can't load the radeon kernel module after X server and Unity desktop 
environment have started. The screen will turn blank.

Now I can only load the radeon kernel module in the following sequence.

sudo modprobe radeon
sudo apt-get install lightdm
sudo service lightdm start

Do you know why I can't load the radeon kernel module after X server and 
Unity desktop environment have started?

Thank you.

-- 
Yours sincerely,

Teo En Ming
Singapore