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Date: Fri, 20 Jul 2018 10:46:17 +0200
From: Fredrik Noring <noring@nocrew.org>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Geoff Levand <geoff@infradead.org>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
 "Maciej W. Rozycki" <macro@linux-mips.org>
Subject: Re: Improvements for the PS3
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Hi Geert,

> > That would not work with kernel freezes unfortunately. Debugging those with
> > nondeterministicly invisible kernel prints would be painful, I believe.
> 
> Unfortunately AFAIK the PS3 doesn't have any other "synchronous" output we
> can use for debugging (like flashing the power LED).

Well, one can use lv1_panic to bisect the PowerPC boot process. It either
freezes, in which case the call to lv1_panic isn't reached, or it reboots.
This method yields 1 bit of information for every reflash and reboot, with
a procedure taking about 5 minutes per turn, so 12 bits/hour sustained. ;)

The prime advantage is that lv1_panic works everywhere, including the
earliest boot stages.

I've since learned about the PS3GELIC_UDBG option to broadcast UDP via the
Ethernet port, but I don't know how well it works or its limitations.

One useful option for debugging is to setup a very early graphical console,
more or less from head.S, before any kernel functions are invoked. I have a
simple proof of concept implemented for the PS2 here:

https://github.com/frno7/linux/blob/ps2-v4.17-n3/arch/mips/boot/compressed/dbg.c

This could be done for the PS3 as well. The OtherOS demo

http://mc.pp.se/ps3/oodemo.xhtml

by Marcus Comstedt in 2007 contains the essentials with MMU and hypervisor
initialisation for graphics. I discovered that a small patch is required to
make his demo work with modern GCC:

--- a/source/script.lds
+++ b/source/script.lds
@@ -50,6 +50,7 @@ SECTIONS
  .opd : {
       *(.opd)
       }
+ . = ALIGN(256);
  .got : {
       __toc_start = .;
       *(.got)

Fredrik