Open firmware seems to be available here for a minimal boot. I have no idea if this is reverse engineered or provided by Broadcom, it seems like broadcom has at least released all the headers under some bsd-like license. The firmware here is gplv2+.

https://github.com/christinaa/rpi-open-firmware

Here is a report of someone booting using only the free firmware dated Jan 2017 while the most recent commits are from April.

http://crna.cc/b/11

If I recall correctly the kernel driver was already open-sourced in 2014. The VC4 mesa driver made and upstreamed by a broadcom emploee here. I'm kind of confused since the vpu has to be used for the rpi to boot even if you don't want video acceleration, but this is an foss drm driver for the vc4 silicon only... what's going on with the part of the blob that used to do the initialization?

https://github.com/anholt/mesa/wiki/VC4

And some documentation

https://www.kernel.org/doc/html/latest/gpu/vc4.html

Is this enough ? Or close to enough for a fully libre BCM2837 system to make it worth taking a look ? It seems like with the community around the rpi any bugs etc would be very easy to spot and squash in the future. I still don't know what's going on with the wifi in that SoC and I won't pretend that I understand everything I just wrote, but it certainly seems like there has been some movement from the "the 2mb userland blob does everything" stage. I found somewhere that board schematics still require an nda though.