powerpc/e500: fix breakage with fsl_rio_mcheck_exception

The wrong MCSR bit was being used on e500mc.  MCSR_BUS_RBERR only exists
on e500v1/v2.  Use MCSR_LD on e500mc, and remove all MCSR checking
in fsl_rio_mcheck_exception as we now no longer call that function
if the appropriate bit in MCSR is not set.

If RIO support was enabled at compile-time, but was never probed, just
return from fsl_rio_mcheck_exception rather than dereference a NULL
pointer.

TODO: There is still a remaining, though comparitively minor, issue in
that this recovery mechanism will falsely engage if there's an unrelated
MCSR_LD event at the same time as a RIO error.
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/kernel/traps.c

@@ -425,7 +425,7 @@ int machine_check_e500mc(struct pt_regs *regs)
 	unsigned long reason = mcsr;
 	int recoverable = 1;
 
-	if (reason & MCSR_BUS_RBERR) {
+	if (reason & MCSR_LD) {
 		recoverable = fsl_rio_mcheck_exception(regs);
 		if (recoverable == 1)
 			goto silent_out;

arch/powerpc/sysdev/fsl_rio.c

@@ -283,10 +283,12 @@ static void __iomem *rio_regs_win;
 #ifdef CONFIG_E500
 int fsl_rio_mcheck_exception(struct pt_regs *regs)
 {
-	const struct exception_table_entry *entry = NULL;
-	unsigned long reason = mfspr(SPRN_MCSR);
+	const struct exception_table_entry *entry;
+	unsigned long reason;
+
+	if (!rio_regs_win)
+		return 0;
 
-	if (reason & MCSR_BUS_RBERR) {
 	reason = in_be32((u32 *)(rio_regs_win + RIO_LTLEDCSR));
 	if (reason & (RIO_LTLEDCSR_IER | RIO_LTLEDCSR_PRT)) {
 		/* Check if we are prepared to handle this fault */
@@ -301,7 +303,6 @@ int fsl_rio_mcheck_exception(struct pt_regs *regs)
 			return 1;
 		}
 	}
-	}
 
 	return 0;
 }