I'm working with NRF5 SDK. Most of its source is available (no idea if it's free software or not, but sources are there). The most glaring exception is softdevi

I'm working with NRF5 SDK. Most of its source is available (no idea if it's free software or not, but sources are there). The most glaring exception is softdevice - that's BLE implementation, it's huge binary blob, taking control over most CPU. Their newer SDK based on Zephyr RTOS, I didn't work with it, but I think it's mostly open source as well.