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

[Jon Hunter]

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?

Jon

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