How the kernel printk works before do console_setup.

[Johnny Hung]

2009/6/25 Benjamin Herrenschmidt <benh at kernel.crashing.org>:
>
>> Before the console is set up, the printk data is formatted
>> and put into the kernel log buffer, but not sent to any console.
>> Any messages printk'ed before that are buffered but do not
>> appear.  When the console is initialized, then all buffered
>> messages are sent to the console, and subsequent printks cause
>> the message to go to the log buffer, but then immediately
>> get sent from there to the console.
>>
>> Under certain conditions you can examine the log buffer of
>> a kernel that failed to initialize it's console, after a
>> warm reset of the machine, using the firmware memory dump
>> command.
>
> On ppc, we have tricks to display things earlier :-)
>
> We can initialize the serial ports way before console_setup() (and we do
> in most cases) and we use what we call the "udbg" console until the real
> one takes over. The "udbg" console is a very small layer which outputs
> via a provided "putc" routine. Platforms can provide their own here, we
> have a collection of standard ones for legacy UARTs (it should be
> automatically setup in that case by the code in legacy_serial), Apple
> ESCCs, etc... We even have compile time options that allow that stuff to
> be initialized before start_kernel...

Thank you. This is what I described and want to understand. The
arch/powerpc/kernel/legacy_serial.c
do find_legacy_serial_ports then take a default serial port by using
open firmware device tree
information. The find_legacy_serial_ports() called form setup_arch but
I don't know who call
setup_arch (setup_32.c)function. Can you give me a hint ? Thanks in advanced.

BRs, H. Johnny