Tried now this loopback test on JetPack 4.3 and am reading zeros. Pins 19 and 21 on J41 header are connected. /dev/spidev0.0, -0.1, -1.0 and -1.1 are present in the system, the loopback test shows no data through either. sudo cat /sys/kernel/debug/tegra_gpio shows 00 in row C, indicating that the SPI pins are muxed to the SPI function. What has to be done on an out-of-the-box JetPack 4.3 to get SPI to work?