Man page 9 : spi_write_then_read

spi_write_then_read - SPI synchronous write followed by read

int spi_write_then_read(struct spi_device *  spi , const void *  txbuf , unsigned  n_tx , void *  rxbuf , unsigned  n_rx );

spi
    device with which data will be exchanged

txbuf
    data to be written (need not be dma-safe)

n_tx
    size of txbuf, in bytes

rxbuf
    buffer into which data will be read (need not be dma-safe)

n_rx
    size of rxbuf, in bytes

can sleep

This performs a half duplex MicroWire style transaction with the device, sending txbuf and then reading rxbuf. The return value is zero for success, else a negative errno status code. This call may only be used from a context that may sleep.

Parameters to this routine are always copied using a small buffer; portable code should never use this for more than 32 bytes. Performance-sensitive or bulk transfer code should instead use spi_{async,sync}() calls with dma-safe buffers.