Re: [PATCH v2 1/9] dt-bindings: dma: nvidia,tegra186-gpc-dma: Add iommu-map property

[Krzysztof Kozlowski <krzk@kernel.org>]

On 03/03/2026 09:40, Akhil R wrote:
> On Tue, 3 Mar 2026 07:39:58 +0100 Krzysztof Kozlowski wrote:
>> On Mon, Mar 02, 2026 at 06:02:31PM +0530, Akhil R wrote:
>>> Add iommu-map property to specify separate stream IDs for each DMA
>>> channel. This enables each channel to be in its own IOMMU domain,
>>> keeping memory isolated from other devices sharing the same DMA
>>> controller.
>>>
>>> Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
>>> ---
>>>  .../devicetree/bindings/dma/nvidia,tegra186-gpc-dma.yaml     | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/Documentation/devicetree/bindings/dma/nvidia,tegra186-gpc-dma.yaml b/Documentation/devicetree/bindings/dma/nvidia,tegra186-gpc-dma.yaml
>>> index 0dabe9bbb219..1e7b5ddd4658 100644
>>> --- a/Documentation/devicetree/bindings/dma/nvidia,tegra186-gpc-dma.yaml
>>> +++ b/Documentation/devicetree/bindings/dma/nvidia,tegra186-gpc-dma.yaml
>>> @@ -14,6 +14,7 @@ description: |
>>>  maintainers:
>>>    - Jon Hunter <jonathanh@nvidia.com>
>>>    - Rajesh Gumasta <rgumasta@nvidia.com>
>>> +  - Akhil R <akhilrajeev@nvidia.com>
>>>  
>>>  allOf:
>>>    - $ref: dma-controller.yaml#
>>> @@ -51,6 +52,10 @@ properties:
>>>    iommus:
>>>      maxItems: 1
>>>  
>>> +  iommu-map:
>>> +    minItems: 1
>>> +    maxItems: 32
>>
>> Why is this flexible? If it is, means usually items are distinctive, so
>> I would expect defining/listing them. If they are not distinctive,
>> commit msg is incorrect. If the list is as simple as 1-to-1 channel
>> mapping, just add it in the description how they are ordered.
> 
> Yes, it is a 1-to-1 channel mapping to an IOMMU ID. The intent of making
> it flexible is to allow non-consecutive IOMMU ID assignments as well.

You cannot skip items if these have such meaning of mapping per channel.
The list order is the ABI - index of the list is the channel here.

> This is particularly needed in virtualised environments where the
> hypervisor may reserve certain stream IDs, and the guest VM can map only
> the permitted ones. Shall I add a description here mentioning this
> use-case?

You can also look at recent works for iris video codec from Qualcomm
solving something similar.


Best regards,
Krzysztof