Please provide the document for BT HFP workflow for Android 9 Automotive to IMX8 board.
By default alsa uses the audio_policy_configuration_car.xml . As per this configuration input device is mic and output device is bus0 or bus1.
Thanks & Regards,
You need to review in the schematics of the board that you are using which channel of the PCM of the Bluetooth it is use in order to configure in ALSA the workflow that maybe selected.
By default in our boards we only configure the codec to be the input and output, since Bluetooth it is not in our hardware board, it is externally added, like a module that need to be handle as an external audio board and in the configuration of Gstreamer or ALSA select the proper channel to get the HFP working
You may contact the Bluetooth manufacturer or the Bluetooth module in order to see if the option it is available.