A build of kernel 5.1.0-rc2 resulted in a failure to boot on my PowerBook G4 
Aluminum. The bootstrap loads the initial kernel and issues the appropriate 
start, but the machine hangs at that point.

The problem does not depend on the choice of PPC32 processor type. This machine 
has a 7447A according to /proc/cpuinfo.

The problem was bisected to the following:

commit 0df977eafc792a5365a7f81d8d5920132e03afad
Author: Christophe Leroy <christophe.leroy@c-s.fr>
Date:   Thu Feb 21 10:37:54 2019 +0000

     powerpc/6xx: Don't use SPRN_SPRG2 for storing stack pointer while in RTAS

     When calling RTAS, the stack pointer is stored in SPRN_SPRG2
     in order to be able to restore it in case of machine check in RTAS.

     As machine check is not a perfomance critical path, this patch
     frees SPRN_SPRG2 by using a field in thread struct instead.

     Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

I reverted this patch and found that the system began execution, and then 
failed, likely due to the reassignment of SPRN_SPRG2.

I had found this problem with 5.1.0-rc1, but -rc2 was out by the time I finished 
the bisection. Unfortunately, none of the changes in -rc2 fixed the problem.

Attached is the .config that I used.

Thanks,

Larry