Hi Robin, On Thu, Mar 25, 2021, at 12:50, Robin Murphy wrote: > On 2021-03-25 07:53, Sven Peter wrote: > > The iommu binding documentation [1] mentions that > > > > The device tree node of the IOMMU device's parent bus must contain a valid > > "dma-ranges" property that describes how the physical address space of the > > IOMMU maps to memory. An empty "dma-ranges" property means that there is a > > 1:1 mapping from IOMMU to memory. > > > > which, if I understand this correctly, means that the 'dma-ranges' for the > > parent bus of the iommu should be empty since the DART hardware can see the > > full physical address space with a 1:1 mapping. > > > > > > The documentation also mentions that > > > > When an "iommus" property is specified in a device tree node, the IOMMU > > will be used for address translation. If a "dma-ranges" property exists > > in the device's parent node it will be ignored. > > > > which means that specifying a 'dma-ranges' in the parent bus of any devices > > that use the iommu will just be ignored. > > I think that's just wrong and wants updating (or at least clarifying). > The high-level view now is that we use "dma-ranges" to describe > limitations imposed by a bridge or interconnect segment, and that can > certainly happen upstream of an IOMMU. As it happens, I've just recently > sent a patch for precisely that case[1]. > > I guess what it might have been trying to say is that "dma-ranges" > *does* become irrelevant in terms of constraining what physical memory > is usable for DMA, but that shouldn't imply that its meaning doesn't > just shift to a different purpose. Should I add a patch to clarify this paragraph for v2 or submit a separate one-off patch? I'm not entirely sure about the process here but I could add a Suggested-by: to the commit if that's fine with you. Best, Sven
