I am the happy owner of a new netbook with an AMD Fusion E-450 APU, which includes a Radeon graphics card. I am using the open-source driver on it, that is a 3.2-rc7 kernel for KMS, and xserver-xorg-video-radeon package from sid. I have to say I am not really happy about the performances.
No I don't speak about the graphical performances that are pretty good (especially compared to my Intel Atom N450 based previous netbook) but about the power consumption. With this setup and with the original battery I get 2h30 of autonomy. Switching to UMS and adding some power management options in xorg.conf improves it to 2h40, but breaks suspend to ram/disk (a pity for a netbook) and switch between VT. I then tried the non-free fglrx driver, it also suffers from the suspend to ram/disk issue, in addition to crashing xorg when playing videos... On the other hand I get an impressive 3h30 of autonomy, and additionally a silent netbook (contrary to the open-source driver, the fan doesn't spin at idle).
I have tried plenty of options, ranging from adding some power
management options to xorg.conf, to passing dynclks=1 to the radeon
module, including setting
dynpm. Right now I have worked around the issue by buying a bigger
battery which brings me 5h30 of autonomy, but I would really appreciate
any software way to improve it with the open-source driver.