> 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@xxxxxxxxxxxxxxxx > https://lists.ozlabs.org/listinfo/linuxppc-dev -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
