API change for bus_dma

[Andrew Gallatin]

Scott Long writes:
 > 
 > The approach taken with busdma is that you don't assume coherency.  The

Unfortunately, in our application we must assume coherency in some
situations.  We have kernel memory mmap'ed into user space for
zero-copy io of small messages.  Doing a system call to force the dma
sync would add unacceptable latency. (we're talking sub 10us latencies
here, without syscalls). 

 > idea is to call bus_dmamap_sync() with the appropriate opcode to signal
 > pre|post read|write, and have that take care of the platform-specific
 > magic.  On i386 when bouncing does not occur, these are NOOP, otherwise
 > the actual bouncing bcopy() takes place.  On sparc64 it looks like it
 > does the appropriate IOMMU and memory barrier magic.

Sure, but we're a 64-bit card and never bounce.  If we've bounced, we
might as well take the ball and go home, so to speak ;)

Anyway, this has saved me a lot of time.  Its now apparent that
there's no point in our using busdma, since the main gain would have
been to enable us to run on sparc64.  Directly using physical
addresses works great everywhere else..

Drew