powerpc: prom_init exception when updating core value

Since the CPU is generating an exception when accessing unaligned word, and
as this exception is not yet handled when running prom_init, data should be
copied from the architecture vector byte per byte.
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/prom_init.c

@@ -858,7 +858,8 @@ static void __init prom_send_capabilities(void)
 {
 	ihandle root;
 	prom_arg_t ret;
-	__be32 *cores;
+	u32 cores;
+	unsigned char *ptcores;
 
 	root = call_prom("open", 1, 1, ADDR("/"));
 	if (root != 0) {
@@ -868,15 +869,30 @@ static void __init prom_send_capabilities(void)
 		 * (we assume this is the same for all cores) and use it to
 		 * divide NR_CPUS.
 		 */
-		cores = (__be32 *)&ibm_architecture_vec[IBM_ARCH_VEC_NRCORES_OFFSET];
-		if (be32_to_cpup(cores) != NR_CPUS) {
+
+		/* The core value may start at an odd address. If such a word
+		 * access is made at a cache line boundary, this leads to an
+		 * exception which may not be handled at this time.
+		 * Forcing a per byte access to avoid exception.
+		 */
+		ptcores = &ibm_architecture_vec[IBM_ARCH_VEC_NRCORES_OFFSET];
+		cores = 0;
+		cores |= ptcores[0] << 24;
+		cores |= ptcores[1] << 16;
+		cores |= ptcores[2] << 8;
+		cores |= ptcores[3];
+		if (cores != NR_CPUS) {
 			prom_printf("WARNING ! "
 				    "ibm_architecture_vec structure inconsistent: %lu!\n",
-				    be32_to_cpup(cores));
+				    cores);
 		} else {
-			*cores = cpu_to_be32(DIV_ROUND_UP(NR_CPUS, prom_count_smt_threads()));
+			cores = DIV_ROUND_UP(NR_CPUS, prom_count_smt_threads());
 			prom_printf("Max number of cores passed to firmware: %lu (NR_CPUS = %lu)\n",
-				    be32_to_cpup(cores), NR_CPUS);
+				    cores, NR_CPUS);
+			ptcores[0] = (cores >> 24) & 0xff;
+			ptcores[1] = (cores >> 16) & 0xff;
+			ptcores[2] = (cores >> 8) & 0xff;
+			ptcores[3] = cores & 0xff;
 		}
 
 		/* try calling the ibm,client-architecture-support method */