powerpc: Ensure that ppc_md is empty before probing for machine type

Anything in there will be overwritten, so it helps catching nasty
bugs if we check that it's indeed full of NULL's before we do so.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/setup-common.c

@@ -575,13 +575,25 @@ void probe_machine(void)
 {
 	extern struct machdep_calls __machine_desc_start;
 	extern struct machdep_calls __machine_desc_end;
+	unsigned int i;
 
 	/*
 	 * Iterate all ppc_md structures until we find the proper
 	 * one for the current machine type
 	 */
 	DBG("Probing machine type ...\n");
 
+	/*
+	 * Check ppc_md is empty, if not we have a bug, ie, we setup an
+	 * entry before probe_machine() which will be overwritten
+	 */
+	for (i = 0; i < (sizeof(ppc_md) / sizeof(void *)); i++) {
+		if (((void **)&ppc_md)[i]) {
+			printk(KERN_ERR "Entry %d in ppc_md non empty before"
+			       " machine probe !\n", i);
+		}
+	}
+
 	for (machine_id = &__machine_desc_start;
 	     machine_id < &__machine_desc_end;
 	     machine_id++) {