STRUCT SPI_TRANSFER(9) Serial Peripheral Interface (S STRUCT SPI_TRANSFER(9)NAMEstruct_spi_transfer - a read/write buffer pair
SYNOPSIS
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;
};
MEMBERS
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
DESCRIPTION
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.
COPYRIGHTKernel Hackers Manual 2.6. November 2013 STRUCT SPI_TRANSFER(9)
