On Monday 05 May 2014 14:45:21 Bjorn Helgaas wrote: > [+cc Ben, Chris] > > On Fri, May 2, 2014 at 12:59 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Friday 02 May 2014 10:54:59 Bjorn Helgaas wrote: > >> > +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). Ah, so the only question is whether to store it in dev_archdata or in device. I think the argument for using struct device directly was that it lets us access this field from architecture independent code. It's not important to the overall design though: we could easily put it in dev_archdata and call an architecture specific function to set it, if there are good reasons for keeping it out of the generic device structure. Arnd -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html