The i.MX53 System Reset Controller provides information about reset sources in the SRSR register. Mainline u-boot as of v2013.01 reads this register and resets information after reading to enable clean info at next boot. Decoding is done for single reset sources only. If your baseboard contains an supervisor which is connected to the reset input of the i.MX53 two reset sources (POR and IPP USER) are detected but not decoded - the reset source is reportes an “unknown”.