單看 console 的部份。
在 start_kernel 的最後 reset_init 用 kernel_thread 執行 kernel_init
console 好像是 kernel 中很重要的東西(?)，在 kerne/printk.c 中。
說明 cosole 依照開機的階段，分為兩種，這說明寫在 regisgter_console( ) 的前面：
/*
* The console driver calls this routine during kernel initialization
* to register the console printing procedure with printk() and to
* print any messages that were printed by the kernel before the
* console driver was initialized.
*
* This can happen pretty early during the boot process (because of
* early_printk) - sometimes before setup_arch() completes - be careful
* of what kernel features are used - they may not be initialised yet.
*
* There are two types of consoles - bootconsoles (early_printk) and
* "real" consoles (everything which is not a bootconsole) which are
* handled differently.
* - Any number of bootconsoles can be registered at any time.
* - As soon as a "real" console is registered, all bootconsoles
* will be unregistered automatically.
* - Once a "real" console is registered, any attempt to register a
* bootconsoles will be rejected
*/
大概翻譯一下：
為了要讓 printk, early_printk能正常送出 message，要儘早 call register_console( ) 把 console 掛起來。 console 分為兩種： 1. bootconsole : 為了應付kernel boot 前段，真正的 (real) console 還沒決定的時候，所使用的 console 2. real consol : 就真正的 console。 在real console 被 register 後，bootconsole 就會自動被 unregister，然後就再也不允許有人註冊 boot_console 了。在 arch/arm/kernal/ 下有 early_prink.c，但是要
obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
才會 build 進去。
在 arch/arm/Kconfig include 的 Kconfig.debug 有option:
config EARLY_PRINTK bool "Early printk" depends on DEBUG_LL help Say Y here if you want to have an early console using the kernel low-level debugging functions. Add earlyprintk to your kernel parameters to enable this console.所以要 enable DEBUG_LL (Kernel low-level debugging functions)，才會顯示出來。
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