For optimus and powerxpress setups where we can explicitly power off the GPU I'd like to do this under control of the driver. Now that I've added X server support for secondary GPUs, it makes sense to start powering them down more often if we can.
I've tested this code on two laptops, one intel/nouveau one intel/radeon It works, powertop says I use 5-6W less power.
Caveats: There is a race at X server start with platform probing code, if the secondary device is off, we the wrong PCI info for it, I've got a patch that works around this I'll send to the xorg-devel.
Audio seems to get screwed at least on one machine, we power up and the alsa callbacks into snd_hda_intel get called but it can't find the hw properly, need to investigate a bit further.
Dave.