Ytdyklly

I built a new computer to dual boot Windows and Ubuntu and am using the AMD R9 380 Graphics card. Windows installation went fine. After installing Ubuntu 14.04 I found that I can't change my screen resolution. I tried installing the proprietary drivers from the AMD website and then I couldn't boot the OS. I reinstalled Ubuntu and stayed with the open source drivers but the problem remains.

The solutions I found online used xrandr to set resolutions but when I run it I always get "Failed to get size of gamma for default output". When I try to change the system display settings I can only use one resolution (1024x768) and only uses built-in display. Clicking on detect displays doesn't do anything.

Does anyone know a resolution?

I just completed a build of new desktop using the Radeon R9 380 graphics card and got the exact same problem.
I fixed this by going into:

System Settings .. Software & Updates .. Additional Drivers

I chose the AMD graphics accelerators from fglrx updates

I hope that this works for you as well

After doing the above update to my machine the display looked fine but movies would not play very well. So after numerous re-installs of Ubuntu 15.04 I followed the install instructions on the amd site. Note this is for my AMD radeon R9 380 gpu. Please scan through the install instructions that you might find on the site below. It should be as I have it here.
http://support.amd.com/en-us/download

Note that there isn't just 1 file to download. You need to download 4 files, in my case yesterday they're as follows:

fglrx-core_15.201-0ubuntu1_amd64_UB_14.01.deb,

fglrx_15.201-0ubuntu1_amd64_UB_14.01.deb,

fglrx-dev_15.201-0ubuntu1_amd64_UB_14.01.deb,

fglrx-amdcccle_15.201-0ubuntu1_amd64_UB_14.01.deb.

Then check that there are no legacy drivers on your system with the follwing cmd

sudo dpkg -l fglrx fglrx-core fglrx-amdcccle fglrx-dev

You can use the following to remove them. Note after the below cmd might not be a good time to reboot your machine ;)
sudo dpkg -r fglrx fglrx-core fglrx-amdcccle fglrx-dev

My terminal session gave an error message that fglrx-dev was unknown to my system, so I removed that from the above command

I placed all the files into a single folder, in terminal I navigated to that folder and once in that folder I ran the following

sudo dpkg -i fglrx-core_15.201-0ubuntu1_amd64_UB_14.01.deb fglrx_15.201-0ubuntu1_amd64_UB_14.01.deb fglrx-dev_15.201-0ubuntu1_amd64_UB_14.01.deb fglrx-amdcccle_15.201-0ubuntu1_amd64_UB_14.01.deb

There were no errors reported in ubuntu 15.04, which was a newly installed build.

I can now watch movies in VLC, which for some reason now plays better than xbmc and kodi. Strange as my old underspecced dual core machine was the other way around. After installing steam we played 2 games, not very gpu intensive, i.e. Dota and TerraTech. I can see that Ubuntu is not getting the most from the gpu. Will still be trying out other games, but good luck