run_main_loop(void)

This function calls main_loop().

static int run_main_loop(void)
{
#ifdef CONFIG_SANDBOX
	sandbox_main_loop_init();
#endif
	/* main_loop() can return to retry autoboot, if so just run it again */
	for (;;)
		main_loop();
	return 0;
}

main_loop(void)

This is the place to receive command and execute it.

/* We come here after U-Boot is initialised and ready to process commands */
void main_loop(void)
{
	const char *s;

	bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");

#ifdef CONFIG_VERSION_VARIABLE
	env_set("ver", version_string);  /* set version variable */
#endif /* CONFIG_VERSION_VARIABLE */

	cli_init();

	run_preboot_environment_command();

#if defined(CONFIG_UPDATE_TFTP)
	update_tftp(0UL, NULL, NULL);
#endif /* CONFIG_UPDATE_TFTP */

	s = bootdelay_process();
	if (cli_process_fdt(&s))
		cli_secure_boot_cmd(s);

	autoboot_command(s);

	cli_loop();
	panic("No CLI available");
}