X99 WS-E won't boot up when > 2 GTX1080 and CSM

ggsato wrote: Hi, I have a box with 4 GTX 1080s on X99 WS-E. I noticed, if CSM is enabled(default) and more than 2 GTX 1080s are installed, it won't boot up with error codes, 99(PCIe error) or 62(chipset initialisation error).  When any 2 1080s are installed on PCIe7 and PCIe3, the box works fine. But whenever 3 1080s are installed on PCIe7, PCIe3, and PCIe5, it fails to boot. I confirmed this by installing 1080s one by one, and swapping in case of errors. It proved no 1080 has any problem, but this is simply related to the number of graphics cards and if CSM is enabled or not. I did some research, but didn't find why this could happen. In my environment, I need to set CSM disabled, so this problem is not an issue. But since I took a couple of days to sort this out, just want to know why. Does any one have an idea? Thanks

I'm not clear about how you sorted out the problem.

I think it is, if you set CSM to Disabled, then the PC can POST fine with all four of the 1080s in the board, is that correct?

Did you install your OS (Windows?) with CSM set to Disabled? I assume you have a HEDT processor with 40 PCIe 3.0 lanes?

CSM, the Compatibility Support Module causes the UEFI firmware our boards now have to work in emulated legacy BIOS mode. That happens with CSM enabled. Legacy BIOS works in 16 bit mode and is able to only use 1MB of the PC's memory, among other limitations. This is done to insure compatibility with older hardware. Older video cards for example cannot work the UEFI Graphics Output Protocol, which will cause a black screen if CSM is disabled.

When CSM is disabled, the UEFI firmware operates normally and can use much more of the PC's memory. I assume that with CSM enabled, there was not enough memory to initialize all four of the video cards during POST. We see that happen with bit coin mining PCs, the user adds another video card, say from three to four, and the PC won't POST or the video card is not recognized.

I know that is a simple, general answer, but I doubt any more detail would be useful, and I can't be 100% certain what actually happened.