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    SubjectRe: [PATCH 0/3] Apple M1 DART IOMMU driver
    > Date: Tue, 23 Mar 2021 14:53:46 -0600
    > From: Rob Herring <robh@kernel.org>
    >
    > On Sun, Mar 21, 2021 at 05:00:50PM +0100, Mark Kettenis wrote:
    > > > Date: Sat, 20 Mar 2021 15:19:33 +0000
    > > > From: Sven Peter <sven@svenpeter.dev>
    > > >
    > > > Hi,
    > > >
    > > > After Hector's initial work [1] to bring up Linux on Apple's M1 it's time to
    > > > bring up more devices. Most peripherals connected to the SoC are behind a iommu
    > > > which Apple calls "Device Address Resolution Table", or DART for short [2].
    > > > Unfortunately, it only shares the name with PowerPC's DART.
    > > > Configuring this iommu is mandatory if these peripherals require DMA access.
    > > >
    > > > This patchset implements initial support for this iommu. The hardware itself
    > > > uses a pagetable format that's very similar to the one already implement in
    > > > io-pgtable.c. There are some minor modifications, namely some details of the
    > > > PTE format and that there are always three pagetable levels, which I've
    > > > implement as a new format variant.
    > > >
    > > > I have mainly tested this with the USB controller in device mode which is
    > > > compatible with Linux's dwc3 driver. Some custom PHY initialization (which is
    > > > not yet ready or fully understood) is required though to bring up the ports,
    > > > see e.g. my patches to our m1n1 bootloader [3,4]. If you want to test the same
    > > > setup you will probably need that branch for now and add the nodes from
    > > > the DT binding specification example to your device tree.
    > > >
    > > > Even though each DART instances could support up to 16 devices usually only
    > > > a single device is actually connected. Different devices generally just use
    > > > an entirely separate DART instance with a seperate MMIO range, IRQ, etc.
    > > >
    > > > 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.
    > > >
    > > > 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.
    > > >
    > > > I currently prefer the first option but I don't understand enough details of
    > > > the iommu system to actually make an informed decision.
    >
    > Please don't mix what does the h/w look like and what's easy to
    > implement in Linux's IOMMU subsytem. It's pretty clear (at least
    > from the description here) that option 2 reflects the h/w.

    Apple does represent these as a single node in their device tree. The
    two instances share an interrupt and share power/clock gating. So
    they seem to be part of a single hardware block.

    > > > I'm obviously also open to more options :-)
    > >
    > > Hi Sven,
    > >
    > > I don't think the first option is going to work for PCIe. PCIe
    > > devices will have to use "iommu-map" properties to map PCI devices to
    > > the right iommu, and the currently implementation seems to assume that
    > > #iommu-cells = <1>. The devictree binding[1] doesn't explicitly state
    > > that it relies on #iommu-cells = <1>, but it isn't clear how the
    > > rid-base to iommu-base mapping mechanism would work when that isn't
    > > the case.
    > >
    > > Now the PCIe DARTs are simpler and seem to have only one "instance"
    > > per DART. So if we keep #iommu-cells = <1> for those, you'd still be
    > > fine using the first approach.
    > >
    > > As I mentioned before, not all DARTs support the full 32-bit aperture.
    > > In particular the PCIe DARTs support a smaller address-space. It is
    > > not clear whether this is a restriction of the PCIe host controller or
    > > the DART, but the Apple Device Tree has "vm-base" and "vm-size"
    > > properties that encode the base address and size of the aperture.
    > > These single-cell properties which is probably why for the USB DARTs
    > > only "vm-base" is given; since "vm-base" is 0, a 32-bit number
    > > wouldn't be able to encode the full aperture size. We could make them
    > > 64-bit numbers in the Linux device tree though and always be explicit
    > > about the size. Older Sun SPARC machines used a single "virtual-dma"
    > > property to encode the aperture. We could do someting similar. You
    > > would use this property to initialize domain->geometry.aperture_start
    > > and domain->geometry.aperture_end in diff 3/3 of this series.
    >
    > 'dma-ranges' is what should be used here.
    >
    > Rob
    >

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