How the kernel printk works before do console_setup.

[Benjamin Herrenschmidt]

> 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...

Cheers,
Ben.


> See http://elinux.org/Kernel_Debugging_Tips#Accessing_the_printk_buffer_after_a_silent_hang_on_boot
> for some tips on accessing the log buffer of a previous boot.
> 
> Note also, that if the serial console does not come up,
> but the kernel successfully boots, and you can get a network
> login on the machine, you can always print out the kernel log
> buffer messages using 'dmesg' at a user-space shell prompt.
> 
> Hope this helps!
>  -- Tim
> 
> =============================
> Tim Bird
> Architecture Group Chair, CE Linux Forum
> Senior Staff Engineer, Sony Corporation of America
> =============================
> 
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