HI @somethingsomething
I guess you already read AN5122 STM32MP1 Series DDR memory routing guidelines
To ease your work, you will find some ready-made Altium projects in STM32MP15x Series DDR memory routing guidelines examples as well as an excel file for each package to check DDR length Vs guidelines (including length skew introduced by package).
Component placement, DDR routing, PCB stack-up and layer usage are very complex stuff and you should carefully prepare the job to avoid painful PCB rework afterward.
You will find also some help from:
AN5168 How to configure DDR on STM32MP1 MPUs
and
AN5031 Getting started with STM32MP151, STM32MP153 and STM32MP157 line hardware development
Regards.
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