Re: [PATCH] dma-iommu: Introduce API to reserve IOVA regions for dynamically created devices

[Jason Gunthorpe]

On Wed, Jun 10, 2026 at 07:57:50PM +0530, Vishnu Reddy wrote:

>   +--------------------------------------------------+
>   |                  VPU Hardware                    |
>   |                                                  |
>   |  +------------+   SID-0   IOVA: 600MB - 3500MB   |
>   |  |  Block 0   |                                  |
>   |  +------------+                                  |
>   |                                                  |
>   |  +------------+   SID-1   IOVA: 0MB  - 3500MB    |
>   |  |  Block 1   |                                  |
>   |  +------------+                                  |
>   |                                                  |
>   |  +------------+   SID-2   IOVA: 16MB - 600MB     |
>   |  |  Block 2   |                                  |
>   |  +------------+                                  |
>   +--------------------------------------------------+
> 
> Each Stream ID maps to a distinct IOMMU context bank, and each context
> bank enforces a different IOVA range.

I think Robin is saying you have to describe your HW properly in
device tree. In Linux a single struct device should not own multiple
*different* IOMMU contexts.

So your DT should describe all those blocks as unique DT nodes with
the proper dma ranges and related data so they can do DMA
correctly. Then the parent device has to assemble itself from that
collection of struct devices.

> These are synthetic child devices created at runtime and do not have their
> own of_node. Inheriting the parent of_node might not be the correct way.

Why would you create child devices at runtime? Linux doesn't really
have a good way to create a fully DMA capable struct device at runtime
without a DT backing description. The fact you immediately hit API
problems like this is a big clue :)

Jason