Re: [PATCH v2 01/12] dt-bindings: media: qcom,venus: Add context bank subnodes to common schema

[Dmitry Baryshkov]

On Thu, Aug 06, 2026 at 05:26:51PM +0530, Vikash Garodia wrote:
> 
> On 8/6/2026 4:05 PM, Dmitry Baryshkov wrote:
> > On Thu, Aug 06, 2026 at 11:36:45AM +0200, Krzysztof Kozlowski wrote:
> > > On 06/08/2026 11:22, Dmitry Baryshkov wrote:
> > > > On Thu, Aug 06, 2026 at 11:11:09AM +0200, Krzysztof Kozlowski wrote:
> > > > > On 06/08/2026 10:46, Dmitry Baryshkov wrote:
> > > > > > 
> > > > > > > dma-ranges tell how this bus - so venus/iris - performs DMA translation
> > > > > > > in respective to parent. Address/size-cells are obviously also needed if
> > > > > > > this is a bus with addressing.
> > > > > > > 
> > > > > > > But there are no children with addressing, thus what sort of bus would
> > > > > > > it be?
> > > > > > > 
> > > > > > > It looks to me that having here both:
> > > > > > > 1. dma-ranges + address/size-cells
> > > > > > > 2. children without bus addressing
> > > > > > > is some sort of abuse of the DT syntax. It is allowed, but does not
> > > > > > > really represent hardware.
> > > > > > > 
> > > > > > > IOW, dma-ranges alone feels okay, although unusual, and it states proper
> > > > > > > DMA translation for this bus. If you add address/size-cells, it means
> > > > > > > this bus HAS addressing and thus YOU MUST use addressing.
> > > > > > > 
> > > > > > > If my understanding is correct, then solution would be to add addressing
> > > > > > > to the children (so unit address and "reg" property) or drop
> > > > > > > address/size-cells as Rob pointed out. [1]
> > > > > > 
> > > > > > Doesn't dma-ranges require address/size cells? In the end, how can you
> > > > > 
> > > > > I think it does not require, at least how I understood the DT spec,
> > > > > unless you provide actual addresses to the property.
> > > > > 
> > > > > IOW, this requires address/size-cells:
> > > > > 	dma-ranges = <0 0 0 0 0x10 0>;
> > > > > 
> > > > > > specify the DMA address if the device doesn't have addressing at all (or
> > > > > > MMIO-style addressing)?
> > > > > 
> > > > > Yeah, that's why having here children without bus addressing is
> > > > > confusing. I would interpret it that, children are not on MMIO bus, thus
> > > > > the venus/iris is some sort of proprietary bus with no mapping between
> > > > > parent MMIO and children nodes.
> > > > > 
> > > > > If there is no mapping, then we do not have 'ranges' property. But I
> > > > > could imagine that such no-mapping bus still provides access to system RAM?
> > > > > 
> > > > > Actually this feels like a huge stretch, so I tend to think that the
> > > > > only reasonable option is to have children with MMIO, which would make
> > > > > it explicit: Venus/iris is a bus which provides translation of both MMIO
> > > > > and DMA addresses to the parent.
> > > > 
> > > > But there are no separate addresses for those subnodes. Would you prefer
> > > 
> > > There might be some or maybe these should be the addresses of DMA?
> > > 
> > > > them to duplicate the addresses of the parent node? Or would the
> > > > 'ranges' be enough?
> > > 
> > > I made mistake earlier - 'dma-ranges' without values is not described in
> > > DT spec explicitly, but should be treated as 'ranges' without values,
> > > thus direct mapping from parent to the child.
> > 
> > Documentation/devicetree/bindings/iommu/iommu.txt:
> > 
> >    An empty "dma-ranges" property means that there is a 1:1 mapping from
> >    IOMMU to memory.
> > 
> > > 
> > > So it also requires address/size-cells, just like 'dma-ranges =
> > > <mapping>'. And dtc checks/reports that.
> > > 
> > > I think therefore now that the binding is unusual (because no bus
> > > addresses of children) but actually correct.
> > > 
> > > > 
> > > > Or, thinking about it, if Venus / Iris have 32-bit addressing, then
> > > > dma-range should probably define the limited DMA range.
> > > 
> > > That's another point which I also raised to Vikash already - mapping
> > > should be restricted to 32-bit if this is how the child devices operate.
> > 
> > Souds so. Then we need a non-empty dma-ranges.
> > 
> > 
> 
> based on this discussion, below works for schema check and dtc
> 
> schema:
>   '#address-cells':
>     const: 1
> 
>   '#size-cells':
>     const: 1
> 
>   dma-ranges:
>     maxItems: 1

If this works, then LGTM.

> 
> 
> Now in the schema examples, soc is added to match the reg/dma-ranges of
> parent (iris)
> 
>   examples
>      soc {
>           #address-cells = <2>;
>           #size-cells = <2>;
> 
>           video-codec@aa00000 {
>             compatible = "qcom,sm8550-iris";
>             reg = <0x0 0x0aa00000 0x0 0xf0000>;
> 
> DTS
> 
> iris {
> ....
>   #address-cells = <1>;
>   #size-cells = <1>;
>   dma-ranges = <0x0 0x0 0x0 0xe0000000>;
> 
>   iris_non_pixel: non-pixel {
> 	iommus = <&apps_smmu 0x1940 0x0>;
> 	memory-region = <&iris_resv>;
>   };
> 
>   pixel {
> 	iommus = <&apps_smmu 0x1947 0x0>;
>   };
> 
> Please review if anything is missed out.
> 
> Regards,
> Vikash

-- 
With best wishes
Dmitry