For a little more info you can head over there.
You just need to install some AMDGPU drivers
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Written by: Nick CongletonRequired Packages
There are still some packages that need to be installed and some things that need to be done to get the drivers working. The first thing you need to do is enable thecontrib and non-free repositories in Debian. Yes, the drivers are open source, but they do require firmware blobs to function properly.
In your
/etc/apt/sources.list add contrib non-free after main on every line. When that's done, close your text editor and run: # apt update && apt upgradeYou should be using
apt instead of apt-get by now. apt is the newer and better command. In creating this guide, kernel 4.9 was being kept back by apt-get but installed perfectly fine with apt. Linux Firmware
Alright, so now you're ready to install the single package that makes this possible. Because Debian doesn't ship firmware blobs by default, the required firmware is available but not installed. Install it with the command below.# apt install firmware-linuxThat's really it. Now, don't restart just yet. There is another important part.
LLVM
LLVM is required to build Mesa, and it is used by some of the graphical packages. You may be able to get by without it, but it's a good idea to make sure that your system has the right version installed, just in case.In order for Mesa to be built with AMDGPU support, LLVM 3.9 or greater is needed. At the time of this article, 3.9 is still the latest version, but 4.0 is available as a release client.
Meanwhile, Stretch is still shipping 3.7 and 3.8 as well as 3.9. Install 3.9 or 4.X now to make sure that the older versions are not pulled in.
# apt install llvm-3.9 clang-3.9
Restart and Test
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After this procedure, my 2nd display worked.
