Description of problem:

William Reich noticed the following inconsistency between the DMA-API.txt file
and the DMA-mapping.txt file.  The DMA-mapping.txt file was backported into RHEL4.


Version-Release number of selected component (if applicable): 77.EL

Additional info:

pci_alloc_consistent() calls dma_alloc_coherent()

dma_alloc_coherent() executes the following code:

     memory = dma_alloc_pages(dev, gfp, get_order(size));
                .
                .
                .
                int high, mmu;
                bus = virt_to_bus(memory);
                high = (bus + size) >= dma_mask; <<< by bumping the dma_mask,
                                                     you are simply increasing
                                                     this value.  Therefore,
                                                     high = 0 (always).
                mmu = high;
                if (force_iommu && !(gfp & GFP_DMA)) 
                        mmu = 1;
                if (no_iommu || dma_mask < 0xffffffffUL) { 
                        if (high) {
                                if (!(gfp & GFP_DMA)) { 
                                        gfp |= GFP_DMA; 
                                        free_pages((unsigned long)memory,
                                                   get_order(size));
                                        goto again;
                                }
                                goto free;
                        }
                        mmu = 0; 
                }       
                memset(memory, 0, size); 
                if (!mmu) { 
                        *dma_handle = virt_to_bus(memory);
                        return memory;  <<< we always return here because mmu=0
                }


In the case that that high evaluates to 1, we end up at another method of
getting a dma area,

        *dma_handle = dma_map_area(dev, bus, size, PCI_DMA_BIDIRECTIONAL, 0);

The call to dma_map_area *does not guarantee* any alignment.  It simply queries
the iommu for an empty area and returns pointers to the area (virtual and a dma
handle).