From mboxrd@z Thu Jan  1 00:00:00 1970
From: john@jjdev.com (John de la Garza)
Date: Sun, 24 Aug 2014 18:02:29 -0400
Subject: serial console in early boot
Message-ID: <20140824220227.GA9505@vega.jjdev.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

I'm using qemu like this:

qemu  -nographic -kernel /home/john/dev-linux/arch/x86/boot/bzImage \
-append "console=ttyS0"

I noticed that I can do a printk in arch/x86/kernel/head64.c right
after the call to clear_bss (line 162 in x86_64_start_kernel).

I see the output of the printk on the serial console.

This seems to be before any uart/console has been initialized.  I'm curious
how the printk can make it to the serial console.

Anyone know why printk is sending output to the serial console this
early in the boot process?

I would expect that the serial line wouldn't be ready and that qemu
would behave like actual hardware.