Apparently Apple provides closed firmware and other binary blobs with relatively stable interfaces that can be used by Linux, which should reduce the effort of porting to newer silicon generations. Not great for open-source, but this is also increasingly done on x86 by Intel (FSP), AMD (PSP/AGESA) and Nvidia (GPU firmware).
https://twitter.com/marcan42/status/1394159806731063298
> Apple have put in a ton of development effort into _allowing_ Linux and other OSes to run on M1 Macs ... There's a whole bespoke boot policy whose entire purpose is to allow this, securely and with user control and consent, with per-OS security policies which even allow you to mix and match security levels. And updates since the M1 launch have gotten more open, not less.
https://twitter.com/marcan42/status/1380849768394485763
> The firmware gets loaded first for security reasons (it's also signed like Nvidia's), but the entire boot process is designed to allow third party OSes and at least one person on the team that designed it has said as much publicly ... they invested a significant amount of development effort into _opening_ the M1 platform, down to supporting multiple security contexts for multiple installed OSes, which is something not even Android phones that bill themselves as open have.