The iMX8m has ECSPI peripherals that according to the reference manual support up to four SS lines. However, in the reference manual I can only find information about how to use SS0 (e.g. via the IOMUXC_SW_MUX_CTL_PAD_ECSPI1_SS0 register for ECSPI1).
Where can I find information about how to use the SS1-SS2 signals?
I have found an example (DART-MX8M SPI - Variscite Wiki ) which suggests that I can use GPIO1_IO12 and GPIO1_IO15, but this doesn't seem to be documented (in the IOMUXC_SW_MUX_CTL_PAD_GPIO1_IO12 and IOMUXC_SW_MUX_CTL_PAD_GPIO1_IO15 registers).