Did you solve this? I use my ADC as my I2S master (driving raspberry pi I2S including DAC chip) and this is one outstanding issue I would like to test before moving the project from raspberry pi to radxa. I use a dummy DTO and control my ADC/I2S via I2C outside of ALSA. All my dummy DTO does is set I2S of the pi to slave mode. I have no radxa experience so I’m hoping this is possible or I have no use for radxa devices. Hoping to ditch broadcom asap.
Edit: also referencing this topic here which confirms that (at some point) radxa only supports master mode and cannot be a slave. Hoping this has changed. Basic question about kernel modules