Jeff Garzik wrote:

That's the main limitation on request size right now...  libata limits 
S/G table entries to 128[1], so a perfectly aligned, fully merged 

    ...

[1] because even though the block layer properly splits on segment 
boundaries, pci_map_sg() may violate those boundaries (James B and 
others are working on fixing this).  So...  for right now the driver 
must check the s/g entry boundaries after DMA mapping, and split them 
(again) if necessary.  IDE does this in ide_build_dmatable().