On Friday 02 May 2014 05:58 AM, Arnd Bergmann wrote: > On Thursday 01 May 2014 14:12:10 Grant Likely wrote: >>>> I've got two concerns here. of_dma_get_range() retrieves only the first >>>> tuple from the dma-ranges property, but it is perfectly valid for >>>> dma-ranges to contain multiple tuples. How should we handle it if a >>>> device has multiple ranges it can DMA from? >>>> >>> >>> We've not found any cases in current Linux where more than one dma-ranges >>> would be used. Moreover, The MM (definitely for ARM) isn't supported such >>> cases at all (if i understand everything right). >>> - there are only one arm_dma_pfn_limit >>> - there is only one MM zone is used for ARM >>> - some arches like x86,mips can support 2 zones (per arch - not per device or bus) >>> DMA & DMA32, but they configured once and forever per arch. >> >> Okay. If anyone ever does implement multiple ranges then this code will >> need to be revisited. > > I wonder if it's needed for platforms implementing the standard "ARM memory map" [1]. > The document only talks about addresses as seen from the CPU, and I can see > two logical interpretations how the RAM is supposed to be visible from a device: > either all RAM would be visible contiguously at DMA address zero, or everything > would be visible at the same physical address as the CPU sees it. > > If anyone picks the first interpretation, we will have to implement that > in Linux. We can of course hope that all hardware designs follow the second > interpretation, which would be more convenient for us here. > not sure if I got your point correctly but DMA address 0 isn't used as DRAM start in any of the ARM SOC today, mainly because of the boot architecture where address 0 is typically used by ROM code. RAM start will be at some offset always and hence I believe ARM SOCs will follow second interpretation. This was one of the main reason we ended up fixing the max*pfn stuff. 26ba47b {ARM: 7805/1: mm: change max*pfn to include the physical offset of memory} > > [1] http://infocenter.arm.com/help/topic/com.arm.doc.den0001c/DEN0001C_principles_of_arm_memory_maps.pdf > Regards, Santosh -- 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