pci_set_consistent_dma_mask explanation

Could somebody explain the difference between 
pci_set_consistent_dma_mask and pci_set_dma_mask?

So far what I gather is that pci_set_dma_mask tells the PCI layer that
some hardware can only use DMA on addresses denoted by the mask that
is passed in. As for what the "consistent" means, I haven't been able
to find out.

thanks.