Re: [PATCH v2 05/15] iommu: Add QDA compute context bank bus to iommu_buses

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Aug 18, 2026 at 10:40:25AM +0530, Ekansh Gupta wrote:

> The CB child nodes are not separate addressable hardware — they are SMMU
> stream-ID assignments described as sub-resources of the parent DSP
> interface. 

So this is exactly the same thing.

Read my remarks here:
https://lore.kernel.org/linux-iommu/20260618151745.GD231643@ziepe.ca/

The DT modeling for devices that have multiple stream-IDs is to list
them all in iommus list.

If you don't like that DT modeling then you need to start out by
explaining why not very clearly.

There is undeniably a Linux gap where it forces all streams in the
iommus property to share a translation. This is a *linux* problem in
the iommu subsystem and DMA API. In an ideal world it should not be
fixed by hacking up new busses or mangling the DT.

But of the easy options I think hacking in new busses is the worst
option and has already been basically NAK'd for the hamoa project so
please don't use it here for a diferent qualcomm IP.

Given several qualcomm chips now have this same issue I strongly
suggest qualcomm consider trying to fix the root cause and provide
some way for a Linux driver to progmatically "unbundle" the multiple
streams.

It would be much easier if the HW properly supported actual sub
streams as we do already have a programming model for PASID. Maybe
some kind of driver thing to reconstruct the iommus so that each entry
is exposed as a PASID in the API instead of an alias or something like
that would be an easy direction.

Jason