SPI Loopback references: (1) SPI Loopback Test - rpi.org 2019
https://www.raspberrypi.org/documentation/hardware/raspberrypi/spi/README.md

(2) SPI Loopback Test using Python (See Appendix C for the python program) - rpi.stackexchange 2019
https://raspberrypi.stackexchange.com/questions/104479/sim800-gsm-module-to-raspberry-pi-uart-serial-communication-problem

(3) SPI Loopback Test (struct spi_ioc_transfer level) - rpi.org forum 2019 
https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=252431