The imx8mn has a 16-bit SDRAM bus width access but the calculation of the memory size treat it as a 32-bit width bus which makes the memory calculation to be wrong (meminfo wrong and memtest fails). There is a difference between the imx7 and the imx8 familys. The imx8 family has a device config field in the master register of the DDRC controller which the imx7 family doesn't have (the bus width is 32-bit as default). The device config field together with the DQ configuration tells us the actual bus width of the device for a correct mem size calculaton.