SLI/Crossfire vs. GX2/X2

[parityboy]

I apologise if this has been covered elsewhere, but I am extremely confused as to how ATI's X2 and nVidia's GX2 series relate to Crossfire and SLI respectively - with regard to drivers that is.

I understand that to get two discrete cards to work together, you need SLI/Crossfire support in the driver - after all, the cards are completely separate, with separate bus IDs, and have to be managed in order to get them to work together.

However, for single-card, dual GPU setups (the GX2s and X2s of this world), why do you need special support in the driver? Should this not be handled in hardware, and be completely transparent to the driver, in the same way that an SATA hard disk can be "hidden" behind a Firewire bridge chip, and only expose the firewire personality to the host computer?

With a X2/GX2 are two bus IDs exposed onto the PCIe bus, or just one? Am I just confused and talking rubbish?

[bridgman]

Nope, it's a good question. An X2 card appears to the driver as two different GPUs, just like if you had two separate cards, complete with different bus IDs.

There are benefits to an X2 card, however :

(a) you don't have to install and connect the two cards,

(b) you don't need a special motherboard and chipset with dual PCIE slots,

(c) you can have more/wider/faster connectivity between the GPUs than is possible with two discrete cards.

[parityboy]

Nope, it's a good question. An X2 card appears to the driver as two different GPUs, just like if you had two separate cards, complete with different bus IDs.

There are benefits to an X2 card, however :

(a) you don't have to install and connect the two cards,

(b) you don't need a special motherboard and chipset with dual PCIE slots,

(c) you can have more/wider/faster connectivity between the GPUs than is possible with two discrete cards.

So what about the nVidia GX2-type cards? Do they appear as discrete GPUs also?

[bridgman]

I haven't played with them myself, but my understanding is that they also appear as two GPUs to the driver.

http://forums.nvidia.com/index.php?act=Print&client=html&f=68&t=74768