Today I discovered AMD no longer supports Radeon HD 5000 & 6000 Series cards [0] What surprises me that I've built catalyst 15.12 with Radeon 6850 on Debian 8.2 without problems. (maybe because 15.12 still supports it?? [1])
╭─taco@ sourcream ~
╰─$ inxi -G
Graphics: Card: Advanced Micro Devices [AMD/ATI] Barts PRO [Radeon HD 6850]
Display Server: X.Org 1.16.4 driver: fglrx Resolution: 1920x1080@ 60.00hz
GLX Renderer: AMD Radeon HD 6800 Series GLX Version: 4.5.13397 - CPC 15.20.1046
This leads me to the premise that anything above 15.12 will be worse the open source driver [2]. I've prepared a configuration to optimize the open source driver, feel free to pitch in your own configs.
# /etc/X11/xorg.conf.d/20-radeon.conf
Section "Device"
Identifier "ATI Radeon HD6850"
Driver "radeon"
Option "DRI3" "on"
Option "SWcursor" "off"
Option "TearFree" "on"
Option "AccelMethod" "Glamor" # [3]
Option "RenderAccel" "on"
Option "ColorTiling" "on"
EndSection
[0]
http://www.anandtech.com/show/9815/amd-moves-pre-gcn-gpus-to-legacy
[1]
http://support.amd.com/en-us/kb-articles/Pages/AMDRadeonSoftwareCrimsonEdition15-12LINReleaseNotes.aspx
[2]
https://www.phoronix.com/scan.php?page=article&item=amd_hd6000_open
[3]
https://www.phoronix.com/scan.php?page=news_item&px=MTYzMDM