For some reason Bluetooth driver (rtl8821) did not want to load on my BPI-M4 (Linux kernel 4.9.119) due to missing RTL8821 firmware. Here’s how I solved it:
Download ’ rtl8821cu_fw’ and ’ rtl8821cu_config’ (https://github.com/BPI-SINOVOIP/BPI_WiFi_Firmware/tree/master/rtl8821cu) and copy them to your BPI-M4 Linux folder ‘/lib/firmware’.
Verify Bluetooth firmware could be loaded: dmesg | grep -Ei ‘blue|firm’
List Bluetooth devices: sudo hcitool dev
Feel free to ask questions or post comments