API change for bus_dma

[Andrew Gallatin]

Scott Long writes:
 > 
 > I'm not familiar with Solaris DDI.  bus_dmamem_alloc() is guaranteed to
 > give you contiguous memory that doesn't require bouncing (or ENOMEM if
 > that's not possible).  I can't imagine what DDI_DMA_STREAMING is.

Most sparc's have 2 different sorts of DMA modes.  One is cache
coherent (aka DDI_DMA_CONSISTENT) -- this is what we all know and love
from PC, alphas, macs, etc.

The other mode (DDI_DMA_STREAMING) allows non cache coherent DMA.
This requires you to call ddi_dma_sync() between your last touch of
the data and you starting a DMA read from a device.  And vice-versa
for a DMA write.

The reason people use DDI_DMA_STREAMING is because coherent DMA
bandwith tends to be abysmal on most sparcs.   Using DDI_DMA_STREAMING
upgrades the bandwith from abysmal to just bad.  Here are some
examples: 

	 For u80, UltraSPARC II, using chip "Psycho",
	     98 MBytes/s consistent vs. 150 MBytes/s streaming.
	 For sunfire, UltraSPARC III, using chip "Schizo",
	     70 MBytes/s consistent vs. 173 MBytes/s streaming.

(compare to 450MB/sec for most intel 64-bit/66MHz PCI slots)..

Drew