I don't know anything about the i915 warning, I use a 6600 myself and didn't notice anything like it, but I had the same experience you did: Live CD was running smoothly, but the installed system was very slow. Using top
I could see xorg (the graphical server) using 100% of the CPU's power, a sign it wasn't using proper hardware acceleration..
Ubuntu 16.04 comes with the 4.4 kernel, which doesn't come with support for the amdgpu drivers the RX series cards use.
You basically have three options:
Install Ubuntu 16.10, which comes with the 4.7 kernel which should
support the Rx 480 properly (although I had troubles making HDMI
audio work on 16.10 even with the proprietary drivers).
Update just your kernel. I haven't tried this one so I won't cover it here.
Install the proprietary AMDGPU-PRO drivers. This is probably the easiest one. There's an installation guide on the official site for the drivers (at http://support.amd.com/en-us/kb-articles/Pages/AMDGPU-PRO-Install.aspx) but I'll be giving a slightly more detailed explanation here.
Installing the AMDGPU-PRO Drivers
First, open a terminal. You can use Ctrl+Alt+T. If you think your UI is too slow to do this procedure, you can drop to text-only mode with Ctrl+Alt+F1 instead and login with your username/password (if you want to go back to graphical mode, use Ctrl+Alt+F7).
First, lets update our packages. Run sudo apt-get update
and input your password if asked to download an up-to-date package list, and after that's finished run sudo apt-get upgrade
to download and install the most recent version of all your packages. Confirm any prompt given.
Now we need to download the AMDGPU-PRO drivers. These can be found at http://support.amd.com/en-us/kb-articles/Pages/AMDGPU-PRO-Driver-for-Linux-Release-Notes.aspx (unfortunately I wasn't able to download them through command line, so you'll have to either use a browser via the graphical interface or download through another computer/OS and transfer the file).
I'll assume you downloaded the 16.60 version and it's now on your Downloads folder. Now we need to extract it. Go into your downloads folder with cd ~/Downloads
. Then type tar xf amdgpu-pro-16.60-379184.tar.xz
(hint: you can type just tar xf amd
and use the Tab key to auto-complete the name). Now we go into the extracted directory with cd amdgpu-pro-16.60-379184
(hint: you can use Tab to auto-complete again).
Now we run chmod u+x ./amdgpu-pro-install
to mark the installer as executable and finally we run it with ./amdgpu-pro-install
. After that's done we need to reboot our system to reload the graphical interface. In the command line this can be done with Ctrl+Alt+Del
.
This should Ubuntu run with proper hardware acceleration, getting rid of the terrible choppiness you were experiencing.