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

From: Robin Murphy
Date: Thu Mar 25 2021 - 07:51:57 EST

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:
Date: Sat, 20 Mar 2021 15:19:33 +0000
From: Sven Peter <sven@xxxxxxxxxxxxx>
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.


Good point, I'll keep that in mind and give option 2 a try.


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.

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].

Robin.

[1] https://lore.kernel.org/linux-arm-kernel/720d0a9a42e33148fcac45cd39a727093a32bf32.1614965598.git.robin.murphy@xxxxxxx/
[2] https://lore.kernel.org/linux-arm-kernel/20180926132247.10971-23-laurentiu.tudor@xxxxxxx/