Re: Ranges - Intended use case

On Thu, Feb 12, 2026 at 01:14:33PM +0000, Kyle Bonnici wrote:
> Hi 
> 
> I have been trying to understand how ranges should/should not be used.
> 
> Looking at the spec 
> 
> > The ranges property provides a means of defining a mapping or translation between the address space of the
> bus (the child address space) and the address space of the bus node’s parent (the parent address space).
> 
> This to me the spec leave some space for interpretation on if ranges should be used on just buses or not. Below are some examples to try to get a better understanding.

I'd like to say ranges literally specifies the translation process
between the children address space and the parent one, regardless
whether the parent is a "bus".

For example, reserved-memory nodes aren't really "buses", but often
come with ranges property, as shown in the example of dt-schema[1].

However it might be a signature of bad device-tree model designs if you
have to use a ranges property for a random non-bus node.

> 
> ```
> / {
> 	foo { // not a bus
> 		#address-cells = <1>;
> 		#size-cells = <1>;
> 		ranges; // does this ranges have any meaning?

I'd like to say, yes.

> 		bar@10 {
> 			reg<0x10 …> 
> 		};
> 	};
> };
> ```
> 
> ```
> / {
> 	foo { // not a bus
> 		#address-cells = <1>;
> 		#size-cells = <1>;
> 		reg = <0x100 0x50>;
> 		ranges = <0x0 0x100 0x50>; // is this an intended use case?

Depending on what the child node represents, but IMHO it comes with a
clear meaning.

> 		bar@10 {
> 			reg<0x10 …> // hence abs address is 0x110. 
> 		};
> 	};
> };
> ```
> 
> ```
> / {
> 	foo { // bus
> 		compatible = <simple-bus>;
> 		#address-cells = <1>;
> 		#size-cells = <1>;
> 		ranges; // this I understand to declare that children will have direct mapping
> 
> 		bar {
> 			ranges; // is this needed? Or is it implied that the foobar is in the same address space of bar?

It's necessary. I don't think ranges property is ever implied.

> 			foobar@10 {
> 				reg<0x10 …>  // hence abs address is 0x10. 
> 			};
> 		};
> 	};
> };
> ```
> 
> ```
> / {
> 	soc { // bus
> 		compatible = <simple-bus>;
> 		#address-cells = <1>;
> 		#size-cells = <1>;
> 		ranges; // this I understand to declare that children will have direct mapping
> 
> 		flash@1000 {
> 			reg<0x1000 …> 
> 			ranges = <0x0 0x1000 …>;
> 		
> 			partitions {
> 				ranges;  // is this needed? Or is it implied that the partition1 is in the same address space of partitions?

Personally I think it SHOULD be needed, though existing dt-bindings for
fixed partition layout don't enforce such.

I'm not very familiar with flash partitions and their dt patterns, so
will leave the question for others :)

> 				partition1@10 {
> 					reg<0x10 …>   // hence abs address is 0x1010. 
> 				};
> 			};
> 		};
> 	};
> };
> ```
> 
> Looking forward to explanations on this topic to extend my understanding of devicetree
> 
> Regards
> Kyle

Best regards,
Yao Zi

[1]: https://github.com/devicetree-org/dt-schema/blob/d16dc68b093e59ec4ae6a32a2e1179cb9cc0fada/dtschema/schemas/reserved-memory/reserved-memory.yaml#L147