[PATCH 0/3] Apple M1 DART IOMMU driver

Sven Peter
2021-03-20T15:20:45Z

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.
I'm obviously also open to more options :-)

Best regards,

Sven

[1] https://lore.kernel.org/linux-arch/20210304213902.83903-1-marcan@marcan.st/
[2] https://developer.apple.com/library/archive/documentation/DeviceDrivers/Conceptual/IOKitFundamentals/DataMgmt/DataMgmt.html
[3] https://github.com/svenpeter42/m1n1/commit/1e2661abf5ea2c820297b3ff591235c408d19a34
[4] https://github.com/svenpeter42/m1n1/tree/usb-uartproxy-console-wip