Insert your Ubuntu Desktop 14.04 LTS USB boot stick. Then, boot up Mac Pro without pressing any key. You will see a penguin icon and press it. You will then see a GRUB boot menu, press any arrow key as soon as possible. Move the highlight bar back to "Try Ubuntu without install". Press "e" and insert "
nomodeset radeon.audio=1" between "splash" and "--". After that, press F10 to boot to Ubuntu Live mode.
At the Ubuntu Live mode, click the Install icon to install. When reaching the partition option, select customize. Make sure you do NOT delete the "EFI" partition. You can repartition the existing HFS+ partition or delete it to configure it to your desired Linux partitions, such as / and swap.
When the installation is completed, you will ask to continue testing or restart. Select "continue". Click to the hard drive icon on the left hand side menu bar. Your hard drive (SSD) is mounted. Go to /media/ubuntu/[a serial number]/boot/grub/grub.cfg. Locate the "splash" and insert "
nomodeset radeon.audio=1" and the end of "splash". Do the same thing at /media/ubuntu/[a serial number]/etc/default/grub.
Now, you can reboot your Mac Pro. If everything going fine, you can boot to Ubuntu. After login, you should run "sudo update-grub" to update the GRUB.
At last, do NOT try to install the AMD Radeon proprietary drivers from the Ubuntu repository or AMD official site. It is because you will unable to go to the login screen after installing the proprietary drivers.
That's all! See you.