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. 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.
Update: With Kernel 3.15, the radeon.dpm=1 option seems to work regarding the reduction of power consumption. I had to explicitly enable it though even though I thought it was used by default. However, even though the system uses less power than under windows (both idle), the fans are still more noisy. I also measured that the the dual monitor setup uses 50 W more power than a single monitor (idle, similar numbers for Windows/Linux/BIOS).