Hi. On Fri, Jul 29, 2011 at 11:24 PM, Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> wrote: > Hello, > > On Friday, July 29, 2011 12:54 PM Joerg Roedel wrote: > >> On Fri, Jul 29, 2011 at 12:14:25PM +0200, Marek Szyprowski wrote: >> > > This sounds rather hacky. How about partitioning the address space for >> > > the device and give the dma-api only a part of it. The other parts can >> > > be directly mapped using the iommu-api then. >> > >> > Well, I'm not convinced that iommu-api should be used by the device drivers >> > directly. If possible we should rather extend dma-mapping than use such > hacks. >> >> Building this into dma-api would turn it into an iommu-api. The line >> between the apis are clear. The iommu-api provides direct mapping >> of bus-addresses to system-addresses while the dma-api puts a memory >> manager on-top which deals with bus-address allocation itself. >> So if you want to map bus-addresses directly the iommu-api is the way to >> go. This is in no way a hack. > > The problem starts when you want to use the same driver on two different > systems: > one with iommu and one without. Our driver depends only on dma-mapping and the > fact > that the first allocation starts from the right address. On systems without > iommu, > board code calls bootmem_reserve() and dma_declare_coherent() for the required > memory range. Systems with IOMMU just sets up device io address space to start > at the specified address. This works fine, because in our system each device has > its own, private iommu controller and private address space. > > Right now I have no idea how to handle this better. Perhaps with should be > possible > to specify somehow the target dma_address when doing memory allocation, but I'm > not > really convinced yet if this is really required. > What about using 'dma_handle' argument of alloc_coherent callback of dma_map_ops? Although it is an output argument, I think we can convey a hint or start address to map to the IO memory manager that resides behind dma API. Of course, it is unable to map a specific physical address with the dma address with the idea. I think the problem can be solved for some application with overriding alloc_coherent callback in the machine initialization code. Still the above idea cannot answer when a physical address is needed to be mapped to a specific dma address with 'dma_map_*()'. DMA API is so abstract that it cannot cover all requirements by various device drivers;; Regards, Cho KyongHo. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>