Allocate DMA buffers in first 4GB
I tried to boot DragonFlyBSD on my AMD notebook, unfortunately it doesn't work. The problem is that on modern AMD systems, if you don't properly support AMD IOMMU, you have to allocate DMA buffers in the first 4 gigabytes of memory. That's how OpenBSD (and probably others) is doing this.
I've attached patch that fixes it in busdma_machdep.c. With this patch applied, DragonFlyBSD boots fine on my machine (and probably other modern AMD computers).
As a bonus, it simplifies code a bit, but I'm not really sure if it's a good thing. I just don't quite understand why this code wasn't using contigmalloc() all along, which seems to be more fitting for this kind of stuff.
Looks like previous code used kmalloc() when needed memory block was small enough, and then prayed for proper alignment. It retried if alignment wouldn't turn out to be OK. If requested memory block was larger, it would've used contigmalloc(). It just doesn't feel right.
I'm no expert on this stuff, so any comments are welcome!