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I'm struggling to understand where to get the address of a device on a device tree? As an example how do I know that I should set <0x00900000 0x20000> in here.

Is memory mapped IO done in the hardware (the processor itself) or in software and do I just have to pass the right address in the device tree?

Is the address hardcoded on the processor or can I just set an arbitrary address? I cannot find anything in my reference manual about setting a certain address in the device tree

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These kind of addresses can be found in the Reference Manual of the processor.

You can find the link here.

Take a look at the chapter 48 (OCRAM On-chip RAM Memory Controller) and more specifically at the section 48.2.1 (page 4118):

The total on-chip RAM size for the chip is 128 Kbytes, organized as 16K x 64 bits,mapped from 0x00900000 to 0x0091FFFF

This is where come from the values <0x00900000 0x20000> from the dtsi file, corresponding to the base address and the offset.

These values are in dts/dtsi file provided by the chip maker.

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