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

Akhil R
2026-03-04T10:38:00Z

On Tue, 3 Mar 2026 17:34:00 +0000, Jon Hunter wrote:
> On 03/03/2026 17:14, Akhil R wrote:
>> 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.
>
> Is that a valid configuration? Above we said it is a 1-to-1 mapping
> which would imply the mapping is always constant. Ie. same channels maps
> to name SID. Is that not the case?

I think the hypervisor configuration can determinte which stream IDs
are assigned to each VM, so the mapping can vary across platforms.
By 1-to-1, I meant that each channel maps to one IOMMU ID, but the
specific IDs themselves may not be fixed. If we prefer a constant
mapping instead, we could document that only IDs in the range 0x801 to
0x81f should be allocated to a Linux VM. Happy to go either way. Let me
know your thoughts.

Regards,
Akhil