Can you publish open source firmware for closed/commercial hardware (such as routers, motherboards or basically any device) without asking the manufacturer?A few examples which seem to indicate that you can: DD-WRT (a Linux based alternative OpenSource firmware), OpenWrt, OpenBIOS, etc.
Can you open source firmware for closed hardware?
law;hardware;firmware
To answer your question with a question: Can you build the firmware without needing to violate the copyright or trade secret protection of the target hardware?' The answer to this question is the answer to your question.So, if the target hardware is 'just an ARM chip with a conventional boot process', sure, you can write new code for it.One tricky part is that the most important part of your target is probably not the processor, but rather the peripherals. If you have to reverse-engineer to figure out how to communicate with the radio or ethernet or disk drive or whatever, you may be in dubious territory with respect to the shrink-wrap license you 'accepted' when you unwrapped the box.