• For the esp32 case it looks like this flag should be set:

    https://esp-idf.readthedocs.io/en/v2.0/a­pi/peripherals/spi_master.html

    SPI_USE_TXDATA
    It says:
    Sometimes, the amount of data is very small making it less than optimal allocating a separate buffer for it. If the data to be transferred is 32 bits or less, it can be stored in the transaction struct itself. For transmitted data, use the tx_data member for this and set the SPI_USE_TXDATA flag on the transmission. For received data, use rx_data and set SPI_USE_RXDATA. In both cases, do not touch the tx_buffer or rx_buffer members, because they use the same memory locations as tx_data and rx_data.

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