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

[Akhil R]

On Tue, 3 Mar 2026 13:09:00 +0000, Jon Hunter wrote:
> On 03/03/2026 08:40, Akhil R wrote:
>
> ...
>
>>> 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.
>> 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?
>
> Isn't this already handled by the 'dma-channel-mask' property? The 
> driver will skip over any channels that are not in specified by the mask.

dma-channel-mask would not help if a channel is exposed, and the
corresponding IOMMU ID is not exposed. For instance say channel 15 is
available for a VM, but not the stream ID 0x80f.

Regards,
Akhil