Open your xorg.conf for editing in any text editor (gedit is used in this example):
gksu gedit /etc/X11/xorg.conf
Now change/add the Driver line in the Device section.
Section "Device" ... #Other directives here Driver "radeon" Option "DRI" "on" #this is the default in recent radeonhd versions Option "AccelMethod" "EXA" #this is the default in recent radeonhd versions EndSection
By default, HDMI audio is disabled. In the Device section of your xorg.conf, you'll need to add the following lines:
Option "Audio" "true" Option "HDMI" "all"
The GPU should automatically drop to low-power mode when the video signal is disabled (i.e. when the monitor turns itself off using DPMS after a specified amount of idle time). You can check/change your DPMS settings on-the-fly with the xset command and you can statically set the appropriate options in xorg.conf. Please read the appropriate man pages (xset and xorg.conf) for more information on that topic. At this time, full dynamic control of clocks and voltages (i.e. ATI PowerPlay) is not implemented in the open-source drivers. However, one can force the GPU to low-power mode at all times by adding the following line in the Device section of xorg.conf:
Option "ForceLowPowerMode" "true"
###—— https://wiki.debian.org/AtiHowTo ——-
Option "AccelDFS" "1"