Man page 9 : struct_spi_transfer

struct_spi_transfer - a read/write buffer pair

tx_buf
    data to be written (dma-safe memory), or NULL

rx_buf
    data to be read (dma-safe memory), or NULL

len
    size of rx and tx buffers (in bytes)

tx_dma
    DMA address of tx_buf, if spi_message.is_dma_mapped

rx_dma
    DMA address of rx_buf, if spi_message.is_dma_mapped

cs_change
    affects chipselect after this transfer completes

bits_per_word
    select a bits_per_word other than the device default for this transfer. If 0 the default (from spi_device) is used.

delay_usecs
    microseconds to delay after this transfer before (optionally) changing the chipselect status, then starting the next transfer or completing this spi_message.

speed_hz
    Select a speed other than the device default for this transfer. If 0 the default (from spi_device) is used.

transfer_list
    transfers are sequenced through spi_message.transfers

SPI transfers always write the same number of bytes as they read. Protocol drivers should always provide rx_buf and/or tx_buf. In some cases, they may also want to provide DMA addresses for the data being transferred; that may reduce overhead, when the underlying driver uses dma.

If the transmit buffer is null, zeroes will be shifted out while filling rx_buf. If the receive buffer is null, the data shifted in will be discarded. Only « len » bytes shift out (or in). ItAqs an error to try to shift out a partial word. (For example, by shifting out three bytes with word size of sixteen or twenty bits; the former uses two bytes per word, the latter uses four bytes.)

In-memory data values are always in native CPU byte order, translated from the wire byte order (big-endian except with SPI_LSB_FIRST). So for example when bits_per_word is sixteen, buffers are 2N bytes long (len = 2N) and hold N sixteen bit words in CPU byte order.

When the word size of the SPI transfer is not a power-of-two multiple of eight bits, those in-memory words include extra bits. In-memory words are always seen by protocol drivers as right-justified, so the undefined (rx) or unused (tx) bits are always the most significant bits.

All SPI transfers start with the relevant chipselect active. Normally it stays selected until after the last transfer in a message. Drivers can affect the chipselect signal using cs_change.

(i) If the transfer isnAqt the last one in the message, this flag is used to make the chipselect briefly go inactive in the middle of the message. Toggling chipselect in this way may be needed to terminate a chip command, letting a single spi_message perform all of group of chip transactions together.

(ii) When the transfer is the last one in the message, the chip may stay selected until the next transfer. On multi-device SPI busses with nothing blocking messages going to other devices, this is just a performance hint; starting a message to another device deselects this one. But in other cases, this can be used to ensure correctness. Some devices need protocol transactions to be built from a series of spi_message submissions, where the content of one message is determined by the results of previous messages and where the whole transaction ends when the chipselect goes intactive.

The code that submits an spi_message (and its spi_transfers) to the lower layers is responsible for managing its memory. Zero-initialize every field you donAqt set up explicitly, to insulate against future API updates. After you submit a message and its transfers, ignore them until its completion callback.