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.