alignment bugs in prom_init

[Olaf Hering]

Some G5 and pSeries models dont boot with recent kernels. The reason is
likely the casting of pointers of stack variables to u32. One example is
the prom_getprop() call in prom_init_stdout().

sp is 0x0023e784, val is at offset 120, which makes 0x0023e7fc. This
address is casted to u32, which changes it to 0x0023e7f8. The firmware
writes to the wrong addres and things go downhill very quick.

c00000000040baa8:       3b 21 00 78     addi    r25,r1,120
..
c00000000040baf4:       57 28 00 38     rlwinm  r8,r25,0,0,28
..
c00000000040bb10:       4b ff d3 3d     bl      c000000000408e4c <.call_prom>

If I remove the casts and pass the pointer as is, everything starts to
work as expected? Why is all this (u32)(unsigned long) casting in
arch/powerpc/kernel/prom_init.c required?

Does -Os vs -O2 make a difference here?