struct_spi_message (9)

struct spi_message {
  struct list_head transfers;
  struct spi_device * spi;
  unsigned is_dma_mapped:1;
  void (* complete) (void *context);
  void * context;
  unsigned actual_length;
  int status;
  struct list_head queue;
  void * state;
};

transfers

list of transfer segments in this transaction

spi

SPI device to which the transaction is queued

is_dma_mapped

if true, the caller provided both dma and cpu virtual addresses for each transfer buffer

complete

called to report transaction completions

context

the argument to complete when it's called

actual_length

the total number of bytes that were transferred in all successful segments

status

zero for success, else negative errno

queue

for use by whichever driver currently owns the message

state

for use by whichever driver currently owns the message

A spi_message is used to execute an atomic sequence of data transfers, each represented by a struct spi_transfer. The sequence is “atomic” in the sense that no other spi_message may use that SPI bus until that sequence completes. On some systems, many such sequences can execute as as single programmed DMA transfer. On all systems, these messages are queued, and might complete after transactions to other devices. Messages sent to a given spi_device are alway executed in FIFO order.

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.