Re: [PATCH v5 02/13] dt-bindings: media: qcom,sm8550-iris: Add context bank subnodes

[Bryan O'Donoghue <bod@kernel.org>]

On 26/09/2026 07:34, 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/
> 
> Update example to the subnode form. Doing so, it picks up the supporting
> properties needed on the video-codec node, address-cells, size-cells and
> dma-ranges to declare 1:1 DMA translation into the parent.
> 
> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
> Co-developed-by: Vishnu Reddy <busanna.reddy@oss.qualcomm.com>
> Signed-off-by: Vishnu Reddy <busanna.reddy@oss.qualcomm.com>
> Signed-off-by: Vikash Garodia <vikash.garodia@oss.qualcomm.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>