[powerstation-owners] questions about boot debugging

[James Bottomley]

On Mon, 2009-06-08 at 18:43 +0900, Tejun Heo wrote:
> Hello,
> 
> I'm messing with early boot code in Linux and the kernel dies after
> prom access goes away but before printk comes online.  early_setup()
> finishes fine but gets stuck after rfid enables RI/DI.  The machine
> simply hangs and I have no way of finding out where the problem
> occurred (my last resort on x86 is combination of infinite loop and
> reset by doublefault but dunno how to do something like that on
> powerpc).
> 
> The udbg stuff looks like the equivalent of early_printk() on x86 and
> RTAS_PANEL/CONSOLE seems to be the right one but the machine hangs if
> I enable it.  I ordered rj45 to db9 gender hoping that I could get
> more information from the debug port, so my questions are...
> 
> 1. The rj45 gender needs to be wired.  There seems to be several
>    different rj45 to db9 mappings.  How should I wire it?

Let me see if I can dig this up:  I had to build a special RJ-45 plug to
get this to work (of course, I did it ~8 months ago and now don't have
the details to hand).

> 2. Is the debug port gonna help at all?

Sort of ... once you get it up, you see all the BMC and early boot
output.  It *seems* to be writing console output to it before early
printk is enabled, but I can't be sure of this.

It also responds to the power on/power off BMC keystrokes (if you have
the updated BMC installed)

> 3. Any ideas on how to get early debug output?  At this point, I'll be
>    _VERY_ happy if I can just get the a double word value written in
>    some nv storage and then retrieve it after hard reset.

Not tried, but looking at the output, yaboot can certainly write to it
(you have to set the open firmware output device for all of this to
happen) so I'm guessing the kernel can too.

James