[U-Boot] [PATCH] tegra: Specify debugging serial port at boot.

[Graeme Russ]

Hi Simon,

On Fri, Mar 23, 2012 at 10:03 AM, Simon Glass <sjg at google.com> wrote:

> I am going to have a similar problem in the SPL soon - how to deal
> with panic(). Advice gratefully accepted.

Take a leaf out of the Linux x86 source....

arch/x86/boot/main.c - This is the start of the 16-bit 'real-mode' code
which is entered by the boot loader (so it's like SPL - IPL being the
BIOS/Bootloader combo)

void main(void)
{
        /* First, copy the boot header into the "zeropage" */
        copy_boot_params();

        /* Initialize the early-boot console */
        console_init();
...


arch/x86/boot/early_serial_console.c

void console_init(void)
{
        parse_earlyprintk();

        if (!early_serial_base)
                parse_console_uart8250();
}

early_serial_base is a global defined in boot.h:

extern int early_serial_base;

You can do the same in SPL, but you are not going to have the luxury of
having it configurable unless you can do so in some non-volatile memory
or hardware configuration (dedicated GPIO pins etc)

It's a chicken and egg scenario - If you want your default console port to
be configurable, you need code to determine the configuration. But you
cannot spew out debug messages for the code which determines the console
configuration. Linux x86 has the same problem, there is no way to ouput
debug messages in copy_boot_params() or console_init()

In U-Boot we kind of 'cheat' - We define a board specific default console
in the board config (hard coded in the U-Boot binary) which we use unitl
environment variables are available. Linux x86 could do the same - have
compile time options which allow the serial console to be enabled before
copy_boot_params(), but the amount of code which is 'dark' is so small
it's not worth it.

U-Boot is different, the amount of code that is 'dark' before the
environment variables are available is rather large. And pre console
buffer covers the 'dark' code just prior to the hard-coded console being
available (but of course pre console buffer does not help if there is a
hang or crash before the hard-coded console is available)

I should check - Is it possible to not have a default console, and
therefor pre console buffer is used all the way up to the console defined
in the environment being intialised?

Regards,

Graeme