How to enable early printk

Early printk can only be enabled if debug=y. You may want to enable it if
you are debbuging code that executes before the console is initialized.

Note that selecting this option will limit Xen to a single UART definition.
Attempting to boot Xen image on a different platform *will not work*, so this
option should not be enable for Xens that are intended to be portable.

CONFIG_EARLY_PRINTK=mach
where mach is the name of the machine:
  - vexpress: printk with pl011 for versatile express
  - exynos5250: printk with the second UART
  - midway: printk with the pl011 on Calxeda Midway processors
  - fastmodel: printk on ARM Fastmodel software emulators
  - omap5432: printk with UART3 on TI OMAP5432 processors
  - sun6i: printk with 8250 on Allwinner A31 processors
  - sun7i: printk with 8250 on Allwinner A20 processors

The base address and baud rate is hardcoded in xen/arch/arm/Rules.mk,
see there when adding support for new machines.
If not explicitly requested with "EARLY_PRINTK_INIT_UART := y" in Rules.mk,
the code will not try to initialize the UART, so that bootloader or
firmware settings can be used for maximum compatibility. The baud rate
parameter is ignored in this case.

By default early printk is disabled.
