Use ESP32 to access A7108, a 433M transceiver, which requires 3-line SPI interface, through spec, the data pin is bi-direction,
I configured GPIO21 as GPIO_MODE_INPUT_OUTPUT mode:

#define GPIO_DATA 21
#define GPIO_IO_PIN_SEL (((uint64_t)1)<<GPIO_DATA)
...
io_conf.pin_bit_mask = GPIO_IO_PIN_SEL ...