On Mon, May 5, 2014 at 8:42 PM, James Bottomley <jbottomley@xxxxxxxxxxxxx> wrote: > On Mon, 2014-05-05 at 17:01 -0600, Bjorn Helgaas wrote: >> On Fri, May 02, 2014 at 10:42:18AM +0200, Arnd Bergmann wrote: > >> > I don't know about NCR_Q720, but all others are only used on machines >> > where physical addresses and bus addresses are in the same space. >> >> In general, the driver doesn't know whether physical and bus addresses >> are in the same space. At least, I *hope* it doesn't have to know, >> because it can't be very generic if it does. > > The API was designed for the case where the memory resides on a PCI > device (the Q720 case), the card config gives us a bus address, but if > the system has an IOMMU, we'd have to do a dma_map of the entire region > to set up the IOMMU before we can touch it. The address it gets back > from the dma_map (the dma_addr_t handle for the IOMMU mapping) is what > we pass into dma_declare_coherent_memory(). The IOMMU (if any) is only involved for DMA to system memory, and there is no system memory in this picture. The device does DMA to its own memory; no dma_map is required for this. We use dma_declare_coherent_memory() to set things up so the CPU can also do programmed I/O to the memory. > The reason it does an > ioremap is because this IOMMU mapped address is now physical to the CPU > and we want to make the region available to virtual space. Essentially > the memory the allocator hands out behaves as proper virtual memory but > it's backed by physical memory on the card behind the PCI bridge. Yep, the programmed I/O depends on the ioremap(). But I don't think it depends on any IOMMU mapping. > I'm still not that fussed about the difference between phys_addr_t and > dma_addr_t, but if the cookie returned from a dma_map is a dma_addr_t > then that's what dma_declare_coherent_memory() should use. If it's a > phys_addr_t, then likewise. > > James > -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
