From iMX 3.1x kernel, all kernel debug messages will be print to debug serial port after UART driver loaded, so if the kernel hang up before tty console driver ready, there will be no kernel boot up messages.
The attached patch can be used to enable the iMX serial debug console in early time, then kernel will not buffer the debug messages.
Note: the default patch is for UART1 (tty0) as the debug port, if you need use other debug port, please modify the code "early_console_setup()" with correct UART port base address.
This patch is based on L3.10.53_GA1.1.0 release, it can support iMX6S/DL/D/Q.
This patch is based on L3.14.52_GA1.1.0 release, it can support iMX6S/DL/D/Q, iMX6SL, iMX6SX, iMX6UL and iMX7.