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

[Sven Peter]

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