Hi Gerd
for examples of StartUp of the Cortex-M4 one can look at
uboot function imx8qxp_wake_secondary_cores()
cpu.c\imx8\mach-imx\arm\arch - uboot-imx - i.MX U-Boot
m4ctrl tool
m4ctrl - M4 Control Tool for i.MX platforms
Please pay attention to sw package revisions as they should fit to each other:
refer to Table 3. BSP and Multimedia standard packages attached Release Notes
and sect.1.2 System Controller Firmware (SCFW) Requirements
i.MX 8QuadXPlus and 8DualXPlus Automotive and Infotainment Applications Processors

All documentation (Release Notes, Linux Manual) for each software release can be found on link:
i.MX Software and Development Tools | NXP
Best regards
igor
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