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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.

I set /sys/class/drm/card0/device/power_method to profile and /sys/class/drm/card0/device/power_profile to 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.

Kernel Version: 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.

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dynpm works with kernels >= 3.11 and it does not work with dual displays. –  Tim Sep 10 '13 at 18:11
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