G5 fan problems return moving to 2.6.15 with dual processor 2.7GHz machine

[Benjamin Herrenschmidt]

On Tue, 2006-02-07 at 11:26 -0800, Brian D. Carlstrom wrote:
> Benjamin Herrenschmidt writes:
>  > Might be something in that prom_init.c fix that broke... it would be
>  > really nice if you could give a try with the console and find out what
>  > it is ... Unfortunately, I don't have access to one of these machines
>  > with the "problem" at the moment...
> 
> Well, I added several prom_printf calls to prom_init.c's
> fixup_device_tree routine. I assumed I would spot these scrolling by
> during boot before what appears to be the video mode switch. However, I
> didn't see anything, but I wasn't sure if it wasn't just going by too
> fast.
> 
> I tried using PROM_BUG to halt the output, but that just resulted in
> returning to an OpenFirmware prompt, although with a white background
> instead of the usual black background when I go their from yaboot with
> 'o'.
> 
> I also tried putting a "while (1) ;" after one of my prom_printf, in
> case the illegal instruction used by PROM_BUG was causing the output to
> be lost, since it was clearing the screen to display the OpenFirmware
> prompt. However then I just got a pure white screen. So clearly in both
> cases it was running my changed code, but I see no output.
> 
> I tried reviewing some OpenFirmware doc, looking at their talk of
> debugging via serial and telnet, but that all seemed to be a dead end,
> although I learned much more about the device tree. :)
> 
> Clearly I could theoretically debug by moving the while(1); around to
> see what branches are being taken, but since I'm away from the machines
> today, I figured I'd ask how I'm expected to use prom_printf, before
> returning to debugging tomorrow. Sorry my lack of ppc experience is
> showing.

prom_printf should work ... try booting manually (from the OF command
line) and maybe comment out the code that opens the displays... (it may
be clearing the screen).... 

Ben.