IP32 prom crashes due to __pa() funkiness

[Kumba <kumba@gentoo.org>]

Initially, booting a straight git checkout on an IP32 will cause it to prom 
crash, usually somewhere in between init_bootmem() and init_bootmem_core().  I 
bisected git to trace this back to one of the inital __pa() introduction patches 
from commit d4df6d4 (get ride of CPHYSADDR()).  It actually appears that the 
actual commit that broke things was 620a480 (Make __pa() aware of XKPHYS/CKSEG0 
address mix for 64 bit kernels).

The [short-term] fix highlighted by Ilya is to make __pa() unconditionally be 
defined to "((unsigned long)(x) < CKSEG0 ? PAGE_OFFSET : CKSEG0)"; Discovered by 
building IP32 with CONFIG_BUILD_ELF64=n.

Normally, this shouldn't be possible, as CONFIG_BUILD_ELF64=n was originally 
only allowed by using the old o64 hack, which has subsequently died and been 
replaced with the newer -msym32 form.  As far as I know, CONFIG_BUILD_ELF64 is 
apparently supposed to be removed at some point in the future, since I believe 
it existed only for quirky 64bit-in-32bit kernels for IP22 and more commonly, 
IP32.  From 2.6.17 onwards, I've been building IP32 kernels with 
CONFIG_BUILD_ELF64=y and using gcc-4, and haven't had problems up until now.

Anyways, I'm not sure if this is an IP32-specific oddity or not (probably is), 
but it needs the define highlighted above to work properly.  Plain PAGE_OFFSET 
won't work for these machines.  Given the same trick os -msym32 is used for the 
rare IP22 64bit kernel, I would not be surprised if the same problem and fix 
both occur and work for those machines as well.  Something to maybe test later, 
I suppose.

But for now, anyone got thoughts as to a sane workaround for this?  Perhaps some 
conditional tweaks in mach-ip32/*.h files somewheres, would it be simpler to 
just switch to:

#if defined(CONFIG_64BIT) && (!defined(CONFIG_BUILD_ELF64) || 
defined(CONFIG_SGI_IP32))

(assuming that IP22 doesn't need it; I'll find out later)



--Kumba

-- 
Gentoo/MIPS Team Lead

"Such is oft the course of deeds that move the wheels of the world: small hands 
do them because they must, while the eyes of the great are elsewhere."  --Elrond