How the kernel printk works before do console_setup.

How the kernel printk works before do console_setup.

[Johnny Hung]

Hi All:
    I have a powerpc arch platform. The default console is ttyS1 not
ttyS0 in the platform. My question is how the kernel print debug
message
like DBG before parse kernel command line and do console_setup
function. The command line passed to kernel about console info is
console=ttyS1.
So kernel can not print anything before parse cmd line and initial
console but the result is against. Anyone point me or give me a clue
is appreciate.

BR, H. Johnny

Re: How the kernel printk works before do console_setup.

[Tim Bird]

Johnny Hung wrote:
> Hi All:
>     I have a powerpc arch platform. The default console is ttyS1 not
> ttyS0 in the platform. My question is how the kernel print debug
> message
> like DBG before parse kernel command line and do console_setup
> function. The command line passed to kernel about console info is
> console=ttyS1.
> So kernel can not print anything before parse cmd line and initial
> console but the result is against. Anyone point me or give me a clue
> is appreciate.

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.

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
=============================

Re: 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
> =============================
> 
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@lists.ozlabs.org
> https://lists.ozlabs.org/listinfo/linuxppc-dev

Re: How the kernel printk works before do console_setup.

[Johnny Hung]

2009/6/25 Benjamin Herrenschmidt <benh@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

Re: How the kernel printk works before do console_setup.

[Michael Ellerman]

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On Thu, 2009-06-25 at 14:27 +0800, Johnny Hung wrote:
> 2009/6/25 Benjamin Herrenschmidt <benh@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.

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

cheers

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Re: How the kernel printk works before do console_setup.

[Freeman]

Johnny Hung <johnny.hacking <at> gmail.com> writes:

> 
> Hi All:
>     I have a powerpc arch platform. The default console is ttyS1 not
> ttyS0 in the platform. My question is how the kernel print debug
> message
> like DBG before parse kernel command line and do console_setup
> function. The command line passed to kernel about console info is
> console=ttyS1.
> So kernel can not print anything before parse cmd line and initial
> console but the result is against. Anyone point me or give me a clue
> is appreciate.
> 
> BR, H. Johnny
> 

Don't worry, printk uses a ring buffer so it allows the kernel to print some, 
hopefully enough, out before console is registered/initialized. This is from 
printk.c

197 /*
198  * The console driver calls this routine during kernel initialization
199  * to register the console printing procedure with printk() and to
200  * print any messages that were printed by the kernel before the
201  * console driver was initialized.
202  */
203 void register_console(void (*proc)(const char *))

Freeman