Hi Robin,
On Wed, Mar 24, 2021, at 16:29, Robin Murphy wrote: > On 2021-03-20 15:19, Sven Peter wrote: > > > > I have just noticed today though that at least the USB DWC3 controller in > > host > > mode uses *two* darts at the same time. I'm not sure yet which parts seem to > > require which DART instance. > > > > This means that we might need to support devices attached to two iommus > > simultaneously and just create the same iova mappings. Currently this only > > seems to be required for USB according to Apple's Device Tree. > > > > I see two options for this and would like to get feedback before > > I implement either one: > > > > 1) Change #iommu-cells = <1>; to #iommu-cells = <2>; and use the first > > cell > > to identify the DART and the second one to identify the master. > > The DART DT node would then also take two register ranges that would > > correspond to the two DARTs. Both instances use the same IRQ and the > > same clocks according to Apple's device tree and my experiments. > > This would keep a single device node and the DART driver would then > > simply map iovas in both DARTs if required. > > This is broadly similar to the approach used by rockchip-iommu and the > special arm-smmu-nvidia implementation, where there are multiple > instances which require programming identically, that are abstracted > behind a single "device". Your case is a little different since you're > not programming both *entirely* identically, although maybe that's a > possibility if each respective ID isn't used by anything else on the > "other" DART? That would be possible. The only difference is that I need to program ID 0 of the first DART and ID 1 of the second one. Both of these IDs are only connected to the same USB controller. > > Overall I tend to view this approach as a bit of a hack because it's not > really describing the hardware truthfully - just because two distinct > functional blocks have their IRQ lines wired together doesn't suddenly > make them a single monolithic block with multiple interfaces - and tends > to be done for the sake of making the driver implementation easier in > terms of the Linux IOMMU API (which, note, hasn't evolved all that far > from its PCI-centric origins and isn't exactly great for arbitrary SoC > topologies). Yes, the easier driver implementation was my reason to favour this option. > > > 2) Keep #iommu-cells as-is but support > > iommus = <&usb_dart1a 1>, <&usb_dart1b 0>; > > instead. > > This would then require two devices nodes for the two DART > > instances and > > some housekeeping in the DART driver to support mapping iovas in > > both > > DARTs. > > I believe omap-iommu.c supports this setup but I will have to read > > more code to understand the details there and figure out how to > > implement > > this in a sane way. > > This approach is arguably the most honest, and more robust in terms of > making fewer assumptions, and is used by at least exynos-iommu and > omap-iommu. In Linux it currently takes a little bit more housekeeping > to keep track of linked instances within the driver since the IOMMU API > holds the notion that any given client device is associated with "an > IOMMU", but that's always free to change at any time, unlike the design > of a DT binding. Sounds good. I'll read those drivers and give it a try for v2. Thanks, Sven
