[+cc Ben, Chris]

On Fri, May 2, 2014 at 12:59 PM, Arnd Bergmann [off-list ref] wrote:

On Friday 02 May 2014 10:54:59 Bjorn Helgaas wrote:

quoted

quoted

+static void of_dma_configure(struct device *dev)
+{
+     u64 dma_addr, paddr, size;
+     int ret;
+
+     dev->coherent_dma_mask = DMA_BIT_MASK(32);
+     if (!dev->dma_mask)
+             dev->dma_mask = &dev->coherent_dma_mask;
+
+     /*
+      * if dma-coherent property exist, call arch hook to setup
+      * dma coherent operations.
+      */
+     if (of_dma_is_coherent(dev->of_node)) {
+             set_arch_dma_coherent_ops(dev);
+             dev_dbg(dev, "device is dma coherent\n");
+     }
+
+     /*
+      * if dma-ranges property doesn't exist - just return else
+      * setup the dma offset
+      */
+     ret = of_dma_get_range(dev->of_node, &dma_addr, &paddr, &size);
+     if ((ret == -ENODEV) || (ret < 0)) {
+             dev_dbg(dev, "no dma range information to setup\n");
+             return;
+     }
+
+     /* DMA ranges found. Calculate and set dma_pfn_offset */
+     dev->dma_pfn_offset = PFN_DOWN(paddr - dma_addr);
+     dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);

Is this effectively the same as an IOMMU that applies a constant offset
to the bus address?  Could or should this be done by adding a simple IOMMU
driver instead of adding dma_pfn_offset to struct device?

We currently have two dma_map_ops variants on ARM (plus another set for
coherent/noncoherent differences, but we can ignore that for the sake
of this discussion): one that handles linear mappings and one that
handles IOMMUs by calling into the linux/iommu.h APIs.

I guess what you mean by 'a simple IOMMU driver' would be another
dma_map_ops implementation that is separate from real IOMMUs, right?

I suppose so; it seems like the offset could be managed inside
arm_dma_ops.  My idea of an IOMMU is something that maps bus addresses
to physical memory addresses.  That's what we're doing here; it's just
that the mapping function is very simple.  So why add something new in
struct device for it?

I think powerpc and tile do something similar in dma_direct_map_page()
and tile_pci_dma_map_page() (they store the offset in struct
dev_archdata).

Bjorn