(am335x, kernel 4.19-ti)

Note: earlyprintk parameter ignores argument
      earlycon optionally takes argument, the flow below assumes no argument


========== earlyprintk =========================================================

CONFIG_DEBUG_KERNEL=y
CONFIG_DEBUG_LL=y
CONFIG_DEBUG_AM33XXUART1=y  "Kernel low-level debugging messages via AM33XX UART1"
	(note that this is in fact uart0, not uart1 as it claims)
kernel parameter: earlyprintk

(config):

arch/arm/Kconfig.debug                 config DEBUG_LL_INCLUDE
arch/arm/Kconfig.debug                         default "debug/8250.S" if DEBUG_LL_UART_8250 || DEBUG_UART_8250

(init):

arch/arm/kernel/early_printk.c          [match] early_param("earlyprintk", setup_early_printk);
arch/arm/kernel/early_printk.c          setup_early_printk()

(write):

arch/arm/kernel/early_printk.c          early_console_write()
arch/arm/kernel/early_printk.c           early_write()
arch/arm/kernel/debug.S                   ENTRY(printascii)
arch/arm/include/debug/8250.S              .macro  addruart


========== earlycon (no argument) ==============================================

CONFIG_SERIAL_EARLYCON=y
kernel parameter: earlycon

(init):

drivers/tty/serial/earlycon.c           [match] early_param("earlycon", param_setup_earlycon);
drivers/tty/serial/earlycon.c           param_setup_earlycon()
drivers/of/fdt.c:                        early_init_dt_scan_chosen_stdout()
drivers/tty/serial/8250/8250_early.c      [match] OF_EARLYCON_DECLARE(omap8250, "ti,omap3-uart", early_omap8250_setup);
drivers/tty/serial/earlycon.c             of_setup_earlycon()
drivers/tty/serial/earlycon.c              earlycon_init()
drivers/tty/serial/8250/8250_early.c       early_omap8250_setup()

(write):

drivers/tty/serial/8250/8250_early.c    early_serial8250_write
drivers/tty/serial/serial_core.c         uart_console_write
drivers/tty/serial/8250/8250_early.c      serial_putc