struct spi_transfer — a read/write buffer pair
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;
u8 bits_per_word;
u16 delay_usecs;
u32 speed_hz;
struct list_head transfer_list;
}; data to be written (dma-safe memory), or NULL
data to be read (dma-safe memory), or NULL
size of rx and tx buffers (in bytes)
DMA address of tx_buf, if spi_message.is_dma_mapped
DMA address of rx_buf, if spi_message.is_dma_mapped
affects chipselect after this transfer completes
select a bits_per_word other than the device default
for this transfer. If 0 the default (from spi_device) is used.
microseconds to delay after this transfer before
(optionally) changing the chipselect status, then starting
the next transfer or completing this spi_message.
Select a speed other than the device default for this
transfer. If 0 the default (from spi_device) is used.
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).
It's 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 isn't 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 don't set up explicitly, to insulate against future API updates. After you submit a message and its transfers, ignore them until its completion callback.