..which is a huge thing because AFAIK most Cortex-A SoCs on the market are full of undocumented peripherals. Cortex-Ms are usually sufficiently documented that you can bring them up from scratch. Once you want a MMU it's either "use our mainlined Linux drivers full of dark magic" (if you are very lucky - perfect if you want Linux, less so if you are developing your own OS), "use our Linux kernel full of binary blobs" (if you are less lucky) or, as a rule, "sign a NDA and don't even bother us if you aren't a billion-dollar corporation".
(Started it as a minor edit, then decided to elaborate so moved it to a new comment).
alt Hacker News
..which is a huge thing because AFAIK most Cortex-A SoCs on the market are full of undocumented peripherals. Cortex-Ms are usually sufficiently documented that you can bring them up from scratch. Once you want a MMU it's either "use our mainlined Linux drivers full of dark magic" (if you are very lucky - perfect if you want Linux, less so if you are developing your own OS), "use our Linux kernel full of binary blobs" (if you are less lucky) or, as a rule, "sign a NDA and don't even bother us if you aren't a billion-dollar corporation".
(Started it as a minor edit, then decided to elaborate so moved it to a new comment).