spi_dma_xchange()

The spi_dma_xchange() function uses DMA to exchange data between the SPI master and an SPI device. The prototype for this function is:

int spi_dma_xchange( int fd,
                     uint32_t device,
                     void *wbuf,
                     void *rbuf,
                     int len );

The arguments are:

fd

The file descriptor returned by spi_open().

device

The device ID with at most one of the following flags optionally ORed in:

• SPI_DEV_LOCK
• SPI_DEV_UNLOCK

wbuf

A pointer to the send buffer, or NULL if there's no data to send.

rbuf

A pointer to the receive buffer, or NULL if there's no data to receive.

len

The exchange length, in bytes.

This function calls spi_dma_xfer() to do the actual DMA. If you're using the same buffer repeatedly, it's more efficient to call spi_dma_xfer() directly.

The spi_dma_xchange() function returns the number of bytes of data that it successfully exchanged. If an error occurred, the function returns -1 and sets errno:

EIO

The write to the device failed, or a hardware error occurred.

EINVAL

The device ID is invalid, or you're trying to unlock a device that isn't locked, or the buffer address is invalid.

ENOMEM

Insufficient memory.

EPERM

The device is locked by another connection.

ENOTSUP

DMA isn't supported.

An SPI driver typically considers it to be an error if the number of bytes returned by this function isn't the same as the number of bytes it asked the function to exchange.