What tx_buf expects as input in spi_transfer

Question

I am lookig into a code which is writing some data on SPI line,using spi_transfer structure

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;
};

Now some like this is been done in Code

u8 *cmd
cmd=kmalloc(3,GFP_KERNEL);
cmd[0]=16;
cmd[1]=32;

Now t[0].tx_buf = cmd;

now I am wondering what is being written into this buffer and what tx_buf expects here?? We are writng to some watch dog counter through SPI line.

Answer 1

Looks like you are sending bytes 0x10, 0x20, 0x00 to the SPI device. There should be other code which has defined and initialized the rest of the struct spi_transfer t[]. Notably, t[0].len must be set to 3.

Perhaps your code base is for a kernel module / device driver. Functions spi_message_add_tail() etc. are in kernel source include/linux/spi/spi.h. They are not dynamic modules, but your module will be linked with kernel when your module is loaded.

Check kernel source for examples of SPI infrastructure use, e.g. drivers/base/regmap/regmap-spi.c.