I've been trying to find some documentation on the status of all GPIOs during boot. I have a relay board permanently connected to the GPIOs, and I've noticed that during boot, there is a lot of switching actions on the relays, meaning the GPIOs must be switching between high/low a lot and pretty quickly.
I'm working with Raspbian Stretch 4.9 with desktop, and would like to know exactly what the status of all the GPIOs is by default at boot, and how it changes during boot, before the desktop etc is loaded. Info regarding Raspbian Jessie would be good too (if different from the info relative to Stretch).
I've found a lot of "user submitted" answers googling around, regarding different patches of Raspbian and quite inconsistent with each other, and not pointing to up to date documentation.
Does anyone know where I could find this information, possibly from official documentation?