I find this situation about unlockable and not unlockable cards to be very interesting.
I think the main unlock indicator is PCI ID change (from 67B1 to 67B0). It may be a driver who reads this ID and further locks out some of cores.
Why only Powercolor and some XFX are unlockable so far is very strange.
To make things a little more understandable, I'd like to ask to add some more information about the cards you try to unlock:
1. Its VERY interesting to see a result of flashing the real 290X card with stock 290 BIOS. All 290X cards should lock to 67B1 ID. It would be very strange if 290X will not change its ID. Since the cards are dual BIOS and of the same reference design, I consider its a safe experiment, but I don't have any of 290 or 290X in hands now. If would be wonderful if someone with 290X will try that.
2. Please post a details of your stock BIOS 290 (non-X) together with unlock results - is it the same as listed on techpowerup?
3. Post some lot numbers, production date or serial numbers from XFX cards, both unlockable and non-unlockable. Its possible what only a specific batch of 290 chips is unlockable and card's manufacturing date is the key. The photo of chip's face would be the best, so if you're going to watercooling please make chips photos. Chip marking and date could be meaningful.
There are some theories possible why some cards are unlocked but others don't:
- The worst: some batches of 290 cards may be populated with 290X chips due to shortage of real 290 chips. Doubtful, but still possible.
- A little bit better: some (the very first?) batch of 290 chips was HW protected somehow from ID change, but later chips were not protected to simplify production process.
- The best: there are some difference in boards (like tiny bootstrap jumpers or even resistors like used by NVIDIA). 290 boards must have somewhat different set of bootstraps, but some manufacturers may omit the changes to simplify production. If so, it would be possible to find a difference between 290 and 290X boards and fix it to unlock these not unlockable cards.
If the most of chips have HW locked ID, the lock should be added at the very last stage of manufacturing. Of course, if they indeed test & select 290 / 290X chips basing on chip quality. It shouldn't be possible to run real tests on uncut die or bare crystals because of enormous power dissipation. Chips should all be ready to use and mounted on their BGA substrates to be heatsinked for testing. That leaves only surface methods of HW locks like laser cutting or jumper strapping. So far I saw nothing like that on chip photos.
Will the unlocked cores indeed do their work is another question. Being known bad or untested, extra cores could drive the chip to instability or provide erroneous computational results. For games that's not so bad, so I'f you're not going to use unlocked card for GPGPU it should be safe. The other possible side effect is increased power consumption, but power controls should easily counter it.