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I am trying to configure my Raspberry Zero W SPI interface to communicate lsb-first. I've modified spidev_test.c, but when I run it, I get an error:

uint8_t lsb_first = 1;
ret = ioctl(fd, SPI_IOC_WR_LSB_FIRST, &lsb_first);
if (ret == -1)
    pabort("can't set lsbfirst");

can't set lsbfirst: Invalid argument

If I set it to zero, it works.

Does the Raspberry Pi not support lsb-first? And if so, why is this apparently not documented anywhere?

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    TBH I've no idea but as a workaround you can reverse the bit order in the byte you are sending and then transfer using the MSB code... Ugly 'hack' but if it keeps you on track...
    – user130616
    Mar 21 at 16:07
  • Yeah, I considered that. Fastest example I found was to define a look-up table. In this particular case, it's probably simpler just to change the firmware of the slave device (I wrote it). But I want to know if the Pi does actually lack this basic functionality. Mar 21 at 16:15
  • Can the coward who downvoted my question please tell me why? Apr 22 at 19:13
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For something to be documented you need to read the documentation.

The main SPI device (GPIO 7-11) has no support for changing the bit order.

The auxiliary SPI device (GPIO 16-21) supports changing the bit order.

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