The biggest change is better overflow handling, partly ported from the AMD64 
IOMMU  code.  This is important for some 3d drivers who make use of this and can 
map a lot of memory.

First it increases the default memory usage of swiotlb to 64MB.
This was the value used in production systems in 2.4 (e.g. in SLES8) and the default
was far too small.  Note this is only allocated when the swiotlb is needed.

pci_map_single will not panic anymore on an overflow. Instead it returns
a pointer to a scratch area and allows the caller to check using 
dma_mapping_error(). It still panics when the overflow buffer is exceeded.

dma_mapping_error support is implemented.

pci_map_sg will never panic now, but just return 0. Near all callers 
of it seem to have proper error checking for this (IDE will even handle
it without erroring out) 

The handling of passing a NULL device is improved, previously it would 
never map in this case. i386 and other architectures assume NULL means a 4GB
limit instead.   This code does this now too.

I added support for swiotlb=force for easier testing.

Patch supplied by Andi Kleen & Suresh Siddha
Signed-off-by: Tony Luck <tony.luck@intel.com>