CODEWITHSUNDEEP
device-tree
71GA
71GA

Reputation: 1391

Devicetree - how are "ranges" mmapped to device nodes?

Let's say we have this example devicetree:

enter image description here

I just want to know what mechanism ranges uses so that it gets properly mapped to the device nodes (ethernet@0,0, i2c@1,0, flash@2,0).

Does it use:

to properly figure out mapping?

Upvotes: 1

Views: 1226

Answers (1)

sawdust
sawdust

Reputation: 17047

Does it use:
...
to properly figure out mapping?

The ordering of DT nodes is not significant and typically done for esthetics, so "order of appearance" can be immediately ruled out. See The order in which the device-tree text file is written, does it matter?

The range property is supposed to be a 3-tuple of <child-bus-address>, <parent-bus-address>, and <length> values.
However the #address-cells property (of external-bus) specifies that the (external bus) <address> value will consist of two <u32> values. This "double-length" is applied only to <child-bus-address>, while <parent-bus-address> is a single <u32> value per the #address-cells property of the parent node. The #size-cells property specifies that the <length> value will consist of a single <u32> value.
Therefore the rangle property will apparently in your example consist of "triplets" composed of 4 values, the first two for <child-bus-address>, the next for <parent-bus-address>, and the last for <length>.

The reg property is supposed to be a pairs of <address> and <length> values.
Since the external-bus #address-cells property specifies (and/or the default value is 2 anyway) that the <address> value will consist of two <u32> values , and the #size-cells property specifies that the <length> value will consist of a single <u32> value, the reg property will in your example consist of a "pair" composed of 3 values, the first two for <address> and the last for <length>.

Note that the DT Specification requires that the <address> in the reg property must match the "@<unit-address>" in the node name. The additional ",0" in the <unit-address> is what what you guessed, the later half of the <address> in each reg.

So the answer to your question would be the <child-bus-address>, which is related to the chip select value.
However, rather than cite the <unit-address> in the node name, the salient linkage "mechanism" in the device node is the <address> in the reg property.

Upvotes: 1

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