I’m testing spi parameters on a Dahlia board with iMX8M-Plus on it. I enabled spidev1.0 in device tree and put a jumper on MISO and MOSI pins on X20 header.
I’m on a nightly build of BSP5 and using preempt patch. Linux localhost 5.4.161-rt67-0+git.75d336c1651a #1 SMP PREEMPT_RT Tue Feb 15 12:08:00 UTC 2022 aarch64 aarch64 aarch64 GNU/Linux
Digging deeper into the SPI driver, nothing could be found related to the LSB mode. Indeed, among the allowed modes, the LSB even can’t be set on the driver.
Also, there is no mention of LSB mode for SPI interfaces on the i.MX8 M Plus Reference Manual from NXP.
So we have several indications that the SPI driver (and probably the hadware) doesn’t support this LSB mode.
For completeness: I misinterpreted the error message spidev spi1.0: setup: unsupported mode bits 8. Actually, this message means that the bit 0x8 (0b1000), that actually represents the LSB mode, is unsupported.