In the process of creating a Linux module for the Raspberry PI I wrote this slave/master loop SPI tester for it using a F8722 and F25K22. The ADC slave module is independent so it could be used as the basis for a SPI based ADC IO expander.
https://github.com/nsaspook/daq_gert/tree/master/RPi_PIC
It works well as a signal generator but YMMV...
https://github.com/nsaspook/daq_gert/tree/master/RPi_PIC
It works well as a signal generator but YMMV...