I have a Radeon HD 6950 connected to two screens on a machine running archlinux.
Recently I changed from the proprietary to the open source driver due to dropped catalyst support. Unfortunately by default the fans are very noisy by default - apparently the power saving mechanisms do not work properly.
low. This seems to work
# cat /sys/kernel/debug/dri/0/radeon_pm_info default engine clock: 800000 kHz current engine clock: 249990 kHz default memory clock: 1250000 kHz current memory clock: 1250000 kHz voltage: 900 mV PCIE lanes: 16
Unfortunately it does only help a little with the fan noise.
I also tried
dynpm which doesn't really seem to have an effect (possibly due to the dual head setup).
Previously, with the catalyst drivers, I had issues with fan noise after suspend, but I was able to use the "ATI Overclocking Utility" to reduce both shader and memory frequency which muted the fans.
Also I have dual boot windows running and there is no issue with fan noise in idle on Windows.
Linux desktux 3.10.7-1-ARCH #1 SMP PREEMPT Thu Aug 15 11:55:34 CEST 2013 x86_64 GNU/Linux
Update: I tried the mainline 3.11 kernel since it is supposed to [improve power management][https://help.ubuntu.com/community/RadeonDriver#Kernel_3.11.x_.28Ubuntu_13.10.2BAC8-Saucy.29_and_Later]. Unfortunately Xorg won't utilize radeon in that case but rather VESA. VESA is actually perfectly fine regarding the fan speed, but does not support dual monitor.