Perhaps the Linux vendors were supplied with only the Aspeed and Polaris chips to work with to get 64 bit DMA. Since (it looks like) the WX 5100 also works, and has the Polaris, this may be the case.

I don't know where the software falls down. Could be in the Linux kernel, Xorg, or even in the early stages of boot. I'll just bet somebody is working on it.