Re: [PATCH 0/3] Apple M1 DART IOMMU driver

Sven Peter
2021-03-25T20:50:21Z

On Thu, Mar 25, 2021, at 12:50, Robin Murphy wrote:
> On 2021-03-25 07:53, Sven Peter wrote:
> >
> >
> > On Tue, Mar 23, 2021, at 21:53, Rob Herring wrote:
> >> On Sun, Mar 21, 2021 at 05:00:50PM +0100, Mark Kettenis wrote:
> >>>
> >>> 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.
> >>
> >
> > 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.
>

Okay, now it makes sense then!

> > As a concrete example, the PCIe DART IOMMU only allows translations from iovas
> > within 0x00100000...0x3ff00000 to the entire physical address space (though
> > realistically it will only map to 16GB RAM starting at 0x800000000 on the M1).
> >
> > I'm probably just confused or maybe the documentation is outdated but I don't
> > see how I could specify "this device can only use DMA addresses from
> > 0x00100000...0x3ff00000 but can map these via the iommu to any physical
> > address" using 'dma-ranges'.
> >
> > Could you maybe point me to the right direction or give me a small example?
> > That would help a lot!
>
> PCI is easy, since it's already standard practice to use "dma-ranges" to
> describe host bridge inbound windows. Even if the restriction is really
> out in the host-side interconnect rather than in the bridge itself, to
> all intents and purposes it's indistinguishable so can still be
> described the same way.
>
> The case of a standalone device having fewer address bits wired up than
> both its output and the corresponding IOMMU input might expect is a
> little more awkward, since that often *does* require adding an extra
> level of "bus" to explicitly represent that interconnect link in the DT
> model, e.g. [2].
>

Nice, thanks! That's exactly what I was looking for :)

Best,

Sven