On 11/13/2013 07:38 AM, Will Deacon wrote: > On Tue, Nov 12, 2013 at 11:34:20PM +0000, Stephen Warren wrote: >> On 11/11/2013 01:31 AM, Hiroshi Doyu wrote: >>> An "IOMMU device" on the bus is poplulated first, "IOMMU'able devices" >>> are done later. >>> >>> With CONFIG_OF_IOMMU, "#stream-id-cells" DT binding would be used to >>> identify whether a device is IOMMU'able or not. If a device is >>> IOMMU'able, we'll defer to populate that device till an iommu device >>> is populated. Once an iommu device is populated, "dev->bus->iommu_ops" >>> is set in the bus. Then, those defered IOMMU'able devices are >>> populated and configured as IOMMU'abled with help of the already >>> populated iommu device via iommu_ops->add_device(). >> >> This looks fairly neat and clean. >> >> I'm still worried about using #stream-id-cells in DT nodes though. While >> I do understand that the *Linux* device model currently only allows each >> struct device to be affected by a single IOMMU, I worry that encoding >> that same restriction into DT is a mistake. I'd far rather see a >> property like: >> >> SMMU: >> smmu: smmu@xxxxxx { >> #smmu-cells = <1>; >> } >> >> Affected device: >> smmus = <&smmu 1>; >> (perhaps with smmu-names too) >> >> That would allow the DT to represent basically arbitrary HW configurations. >> >> The implementation of this patch would then be almost as trivial; you'd >> just need to walk the smmus property to find each phandle in turn, just >> like any other phandle+specifier property, and validate that the SMMU >> driver was already probe()d for each. > > There are a few problems with that: > > 1.) It assumes all devices sharing an SMMU have the same number of > "smmu cells" The SMMU HW defines how many cells are required to represent the client stream IDs. So, this isn't a problem. Are you thinking of cases where an SMMU client issues transactions with multiple different stream IDs? That is supported by having multiple entries in the smmus property. (Assuming that #smmu-cells in the SMMU is <1>) HW that issues with 1 stream ID: smmus = <&smmu 123>; HW that issues with 2 stream IDs: smmus = <&smmu 123 &smmu 345>; > 2.) It moves SMMU-specific data out to the device, which makes it Yes, but I think it's really SMMU-client-specific data not SMMU-specific. The SMMU doesn't dictate the stream IDs that "clients" include with their transactions; the "client" HW dictates that. > impossible to describe more complicated topologies where IDs can be > remapped/remastered, potentially by multiple SMMUs and/or bus bridges. I don't think so. The SMMU "clients" can indicate what stream IDs they use to issue transactions. The SMMU DT node or driver itself can still include a table that describes how it re-writes those stream IDs when forwarding transactions to the next step. I don't believe there's any requirement that the SMMU list all its clients and this mapping table in the same property. So, the SMMU could easily have: (entries are this SMMU's #stream-id-cells, assumed to be 1 here, then the next SMMU's #stream-id-cells, assumed to be 2 here) smmu-stream-id-translations = <123 1 987>, <345 0 765>; > When writing the binding for the ARM SMMU driver, I originally started with > something similar to what you're suggesting, but was later forced down a > different route when I realised what sort of systems were being built. > > We will have similar problems for PCIe RIDs and GIC DIDs (I spoke about this > at the ARM mini-summit). They are not fixed across the system: they > originate from a device, but can change as they traverse the system > topology. -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html