Different type iommus integrated in a SoC

[Robin Murphy]

On 2021-05-27 03:37, xxm at rock-chips.com wrote:
> Hi all,
> 
> I have a SoC integrate with two different types of iommus, one is ARM SMMU, serves the PCIe/SATA/USB,
> the others are vendor specific iommus, serves display device and multimedia device.
> 
> In the current linux kernel, the iommu framework seems only support one type iommu at runtime, if enable both types iommu, only one type can work.
> Is there any way to support this kind of SoC?

Hooray! I've been forecasting this for years, but the cases we regularly 
hit with internal FPGA prototyping (nor the secret unused MMU-400 I 
found on RK3288) have never really been a strong enough argument to 
stand behind.

Based on what I remember from looking into this a few years ago, 
converting *most* of the API to per-device ops (now via dev->iommu) is 
trivial; the main challenge will be getting the per-device data 
bootstrapped in iommu_probe_device(), which would probably need to rely 
on the fwspec and/or list of registered IOMMU instances.

The other notable thing which will need to change is the domain 
allocation interface, but in practice I think everyone who calls 
iommu_domain_alloc() today is in fact doing so for a specific device, so 
I don't think it's as big a problem as it might first appear.

Robin.