GPUDirect is very specific it require implementation on the PCIe switch (some modification to IOMMU), UEFI/BIOS and card, as well as the OS Kernel and the display drivers.
Because it requires both very specific software and hardware configurations only some features work on some systems.
Also calling this DMA is kind of misleading it does require the CUDA software layer to work, and for the most part it's a lot of hacks coupled together to form a feature set.
So I would still currently stand by what I said that there isn't a standard and generic way for GPU's to have DMA access.
The GPU does have DMA to RAM
References:
https://stackoverflow.com/questions/19242711/how-can-i-use-g...
http://docs.nvidia.com/cuda/gpudirect-rdma/
https://developer.nvidia.com/gpudirect