[PATCH] correct the comment about stackpointer alignment in __boot_from_prom

The address of variable val in prom_init_stdout is passed to prom_getprop.
prom_getprop casts the pointer to u32 and passes it to call_prom in the hope
that OpenFirmware stores something there.
But the pointer is truncated in the lower bits and the expected value is
stored somewhere else.

In my testing I had a stackpointer of 0x0023e6b4. val was at offset 120,
wich has address 0x0023e72c. But the value passed to OF was 0x0023e728.

c00000000040b710:       3b 01 00 78     addi    r24,r1,120
...
c00000000040b754:       57 08 00 38     rlwinm  r8,r24,0,0,28
...
c00000000040b784:       80 01 00 78     lwz     r0,120(r1)
...
c00000000040b798:       90 1b 00 0c     stw     r0,12(r27)
...

The stackpointer came from 32bit code.
The chain was yaboot -> zImage -> vmlinux

PowerMac OpenFirmware does appearently not handle the ELF sections
correctly.  If yaboot was compiled in
/usr/src/packages/BUILD/lilo-10.1.1/yaboot, then the stackpointer is
unaligned. But the stackpointer is correct if yaboot is compiled in
/tmp/yaboot.

This bug triggered since 2.6.15, now prom_getprop is an inline
function. gcc clears the lower bits, instead of just clearing the
upper 32 bits.
Signed-off-by: NOlaf Hering <olh@suse.de>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/head_64.S

@@ -1544,7 +1544,11 @@ _STATIC(__boot_from_prom)
 	mr	r28,r6
 	mr	r27,r7
 
-	/* Align the stack to 16-byte boundary for broken yaboot */
+	/*
+	 * Align the stack to 16-byte boundary
+	 * Depending on the size and layout of the ELF sections in the initial
+	 * boot binary, the stack pointer will be unalignet on PowerMac
+	 */
 	rldicr	r1,r1,0,59
 
 	/* Make sure we are running in 64 bits mode */