suse man page for spi_write_then_read

SPI_WRITE_THEN_READ(9)					  Serial Peripheral Interface (S				    SPI_WRITE_THEN_READ(9)

NAME
       spi_write_then_read - SPI synchronous write followed by read

SYNOPSIS
       int spi_write_then_read(struct spi_device * spi, const u8 * txbuf, unsigned n_tx, u8 * rxbuf, unsigned n_rx);

ARGUMENTS
       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

CONTEXT
       can sleep

DESCRIPTION
       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.

COPYRIGHT
Kernel Hackers Manual 2.6.					     July 2010						    SPI_WRITE_THEN_READ(9)