HSPI slave mode seems to not allow simple transfer of data. Setting SPI_USER to zero does nothing. It will still go through its built in high level protcol with "address" and "command" phase.
I was able to abuse the API_ADDR register to transfer one byte MOSI, but there is no equivalent back channel since the high level protocol transmits all 1's during the 'address phase'
Maybe someone has already ventured into this and got SPI Slave mode working in a usable way?
Maybe there's a hidden way to make it transmit data during the 'address phase' ?