Re: [PATCH v3 11/13] arm64: dts: qcom: monaco: Add Iris context bank subnodes

[Vikash Garodia <vikash.garodia@oss.qualcomm.com>]

On 9/10/2026 12:33 PM, Krzysztof Kozlowski wrote:
> On Wed, Sep 09, 2026 at 08:27:18PM +0530, Vikash Garodia wrote:
>> The VPU issues DMA through several SMMU streams, and the hardware does
>> not give every stream the same addressable range. The non-pixel stream
>> cannot address the low 600MB of IOVA space, while the pixel stream can
>> address the full range:
>>      +-----------------------------------------------------------+
>>      | non-pixel stream addressable range (600 MB - 3.5 GB)      |
>>      | 0x25800000 - 0xe0000000                                   |
>>      +-----------------------------------------------------------+
>>      | pixel stream addressable range (0 - 3.5 GB)               |
>>      | 0x00000000 - 0xe0000000                                   |
>>      +-----------------------------------------------------------+
>> A single "iommus" property on the video-codec node puts every stream in
>> one IOMMU domain sharing one IOVA allocator, so nothing restricts a
>> non-pixel buffer to avoid 0 to 600MB. Once an allocation lands below
>> that boundary the hardware faults, which shows up as unhandled SMMU page
>> faults and spontaneous reboots:
>> https://gitlab.freedesktop.org/drm/msm/-/work_items/100
>>
>> Given that the address range restriction is for specific VPU stream, it
>> should be ideally be moved to that stream. To achieve the same, a subset
>> of streams is now represented as subnodes, so that each can be
>> associated with its respective addressable range. The design was discussed
>> and agreed by mainatiners here
>> https://lore.kernel.org/all/c7b956a9-d3e8-4e18-b780-5d08f5cd2ca1@kernel.org
>>
>> In the past, this limitation was addressed with an iommu-map approach,
>> with the iris driver dynamically creating the devices. That approach
>> was later concluded to be a hack to avoid having subnodes, and was NAKed
>> by the iommu maintainers. It was discussed in detail here:
>> https://lore.kernel.org/all/c7b956a9-d3e8-4e18-b780-5d08f5cd2ca1@kernel.org/
>>
>> The video-codec node includes address-cells, size-cells and dma-ranges
>> to declare 1:1 mapping for DMA translation to the parent.
>>
>> Tested-by: Daniel J Blueman <daniel@quora.org>
> 
> I do not believe the testing actually occured, except some case, so
> please apply it ONLY to the commits where applicable.
> 
> I already asked this at v2.
> 
> If Daniel did not confirm where this was tested, then the tag DOES NOT
> apply to any of the DTS patches.

you are right, it was not clearly confirmed where it was tested. I would 
drop the tag.

> 
> Best regards,
> Krzysztof