add support for SPI bit ordering

[cyboflash]

Based on the source code there doesn't seem to be support for bit ordering for SPI, i.e. MSB / LSB first.

[jackmitch]

Hi, how is this manipulated from the user space kernel interface. Do you have an example?

[cyboflash]

At this point I don't know. I had a device where I could do it, so I assumed it is possible to do it in the library. However based on this it seems it depends on the device rather than on the library.

[WereCatf]

Here is a very simplistic example on how to do this:

#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>
#include <stdint.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

#define SPIDEV "/dev/spidev0.1"

int querySpiLsb(int fd, uint8_t *spiLsb){
	if(ioctl(fd, SPI_IOC_RD_LSB_FIRST, spiLsb)){
		fprintf(stderr, "Couldn't query SPI MSB-/LSB-setting!\n");
		return -1;
	}

	printf("The SPI-bus is currently set to use ");
	if(!*spiLsb){
		printf("MSB-ordering.\n");
		*spiLsb = SPI_LSB_FIRST;
	}
	else {
		printf("LSB-ordering.\n");
		*spiLsb = 0;
	}
	return 0;
}

int main(int argc, char *argv[])
{
	int fdSpidev;
	uint8_t spiLsb = 0;

	fdSpidev = open(SPIDEV, O_RDWR);
	if(fdSpidev < 0){
		fprintf(stderr, "Couldn't open SPIDEV %s!\n", SPIDEV);
		exit(1);
	}
	if(querySpiLsb(fdSpidev, &spiLsb)){
		close(fdSpidev);
		exit(1);
	}

	printf("Trying to set the SPI-bus to use ");
	if(!spiLsb) printf("MSB-ordering.\n");
	else printf("LSB-ordering.\n");

	if(ioctl(fdSpidev, SPI_IOC_WR_LSB_FIRST, &spiLsb)){
		fprintf(stderr, "Error performing SPI_IOC_WR_LSB_FIRST ioctl() (hardware doesn't support this?)!\n");
		close(fdSpidev);
		exit(1);
	}

	if(querySpiLsb(fdSpidev, &spiLsb)){
		close(fdSpidev);
		exit(1);
	}
}

The Raspberry Pi, for example, does not seem to support changing bit-ordering and the ioctl()-call fails, but on the Orange Pi Plus 2E, on the other hand, this works fine.