It is not a relatively simple matter of porting it. The Cortex M cores are microcontrollers, and are much simpler than the Cortex A series.
In particular, they do not have a memory management unit (MMU), which is necessary for normal Linux.
There is a variant of Linux (uCLinux) which technically works on the Cortex M cores, but the chip we are using likely does not have enough RAM, and uCLinux seems to be a dead project as far as I can tell.
@PINE64 Wow. I wasn't expecting a response. Thank you. I look forward to buying a pinetime and I guess I'll learn a new OS.
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