Underclock GPU Memory below "Recommended" Limits on Linux

Question

I'd like to try underclocking my GPU Memory. Based on what I've read, this is frequently done to reduce heat without sacrificing hashrate, and can sometimes even boost the hashrate if done carefully.

I seems that those running on Windows have a tool called "Afterburner" that allows them to over/underclock beyond the manufacturer's recommended limits, without flashing their BIOS.

However, under Linux, aticonfig --odsc=XXX,YYY won't allow setting the memory clock any lower than the range displayed by aticonfig --odgc. Other tools I've investigated, such as AMDOverdriveCtrl, appear to just be front-ends for aticonfig, and therefore limited to what it can do.

I've seen some mentions that flashing on a new third party BIOS rectifies this, but that makes me more than a little nervous, so I would prefer to avoid that.

Would someone please explain, either...

A) How to underclock the memory on ATI GPUs (6850 and 6950, specifically) on Linux beyond the manufacturer's recommended limits, without flashing the BIOS.

... or ...

B) How it's possible for tools running on Windows to have more control over the hardware than those running on linux. (This feels like a slap in the face to me.)

Answer 1

aticonfig respects the settings set in your video card's firmware by your card's manufacturer (frequently, although incorrectly, called a BIOS). However, AMDOverdriveCtrl can clock memory down much farther, depending on the card.

Radeon 5xxx, 67xx, and 68xx series cards should be clocked down to 1/3rd of the core speed (ie, 750mhz should have a memory speed of 250), 69xx cannot clock down far enough to be worth it, and GCN (79xx, 78xx, 77xx) seem to have a limit of 150 mhz less than core (however, although it is not worth underclocking on 69xx, it is worth underclocking on GCN due to automatic memory voltage control done by the hardware/driver; on my 7950 @ 1125mhz, going from 1375 down to 975 dropped my GPU temps 3-4 degrees celsius).

5xxx and 6xxx series cards also have a problem where they will instantly lock up if memory speed is set between around 400mhz to around 600mhz (depending on core speed), so your mileage may vary as always.