The SPI I/O model consists of a set of queued messages. We submit one or more struct spi_message structures, which are processed and completed synchronously or asynchronously. A single message consists of one or more structspi_transfer objects, each of which represents a full duplex SPI transfer. These are two main structures to exchange data between the driver and the device. They are both defined in include/linux/spi/spi.h:

struct spi_transfer represents a full duplex SPI transfer:

struct spi_transfer { 
    const void  *tx_buf; 
    void *rx_buf; 
    unsigned len; 

    dma_addr_t tx_dma; 
    dma_addr_t rx_dma; 
 
    unsigned cs_change:1; 
    unsigned tx_nbits:3; 
    unsigned rx_nbits:3; 
#define  SPI_NBITS_SINGLE   0x01 /* 1bit transfer */ 
#define  SPI_NBITS_DUAL            0x02 /* 2bits transfer */ 
#define...