Re: How the kernel printk works before do console_setup.

On Thu, 2009-06-25 at 14:27 +0800, Johnny Hung wrote:

2009/6/25 Benjamin Herrenschmidt [off-list ref]:

quoted

quoted

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.

setup_arch() is called from start_kernel() in init/main.c

cheers