MIPS: BCM47XX: improve memory size detection

The memory size is detected by finding a place where it repeats in
memory. Currently we are just checking when the function prom_init is
seen again, but it is better to check for a bigger part of the memory
to decrease the chance of wrong results.

This should fix a problem we saw in OpenWrt, where the detected
available memory decreed on some devices when doing a soft reboot.
Signed-off-by: NHauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4362Signed-off-by: NJohn Crispin <blogic@openwrt.org>

arch/mips/bcm47xx/prom.c

 /*
  *  Copyright (C) 2004 Florian Schirmer <jolt@tuxbox.org>
  *  Copyright (C) 2007 Aurelien Jarno <aurelien@aurel32.net>
+ *  Copyright (C) 2010-2012 Hauke Mehrtens <hauke@hauke-m.de>
  *
  *  This program is free software; you can redistribute  it and/or modify it
  *  under  the terms of  the GNU General  Public License as published by the
@@ -128,6 +129,7 @@ static __init void prom_init_mem(void)
 {
 	unsigned long mem;
 	unsigned long max;
+	unsigned long off;
 	struct cpuinfo_mips *c = &current_cpu_data;
 
 	/* Figure out memory size by finding aliases.
@@ -145,15 +147,15 @@ static __init void prom_init_mem(void)
 	 * max contains the biggest possible address supported by the platform.
 	 * If the method wants to try something above we assume 128MB ram.
 	 */
-	max = ((unsigned long)(prom_init) | ((128 << 20) - 1));
+	off = (unsigned long)prom_init;
+	max = off | ((128 << 20) - 1);
 	for (mem = (1 << 20); mem < (128 << 20); mem += (1 << 20)) {
-		if (((unsigned long)(prom_init) + mem) > max) {
+		if ((off + mem) > max) {
 			mem = (128 << 20);
 			printk(KERN_DEBUG "assume 128MB RAM\n");
 			break;
 		}
-		if (*(unsigned long *)((unsigned long)(prom_init) + mem) ==
-		    *(unsigned long *)(prom_init))
+		if (!memcmp(prom_init, prom_init + mem, 32))
 			break;
 	}