powerpc/powernv/idle: Clear r12 on wakeup from stop lite

pnv_wakeup_noloss() expects r12 to contain SRR1 value to determine if the wakeup
reason is an HMI in CHECK_HMI_INTERRUPT.

When we wakeup with ESL=0, SRR1 will not contain the wakeup reason, so there is
no point setting r12 to SRR1.

However, we don't set r12 at all so r12 contains garbage (likely a kernel
pointer), and is still used to check HMI assuming that it contained SRR1. This
causes the OPAL msglog to be filled with the following print:

  HMI: Received HMI interrupt: HMER = 0x0040000000000000

This patch clears r12 after waking up from stop with ESL=EC=0, so that we don't
accidentally enter the HMI handler in pnv_wakeup_noloss() if the value of
r12[42:45] corresponds to HMI as wakeup reason.

Prior to commit 9d29250136f6 ("powerpc/64s/idle: Avoid SRR usage in idle
sleep/wake paths") this bug existed, in that we would incorrectly look at SRR1
to check for a HMI when SRR1 didn't contain a wakeup reason. However the SRR1
value would just happen to never have bits 42:45 set.

Fixes: 9d29250136f6 ("powerpc/64s/idle: Avoid SRR usage in idle sleep/wake paths")
Signed-off-by: Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Change log and comment massaging]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/idle_book3s.S

@@ -256,6 +256,19 @@ power_enter_stop:
 	bne	 .Lhandle_esl_ec_set
 	IDLE_STATE_ENTER_SEQ(PPC_STOP)
 	li	r3,0  /* Since we didn't lose state, return 0 */
+
+	/*
+	 * pnv_wakeup_noloss() expects r12 to contain the SRR1 value so
+	 * it can determine if the wakeup reason is an HMI in
+	 * CHECK_HMI_INTERRUPT.
+	 *
+	 * However, when we wakeup with ESL=0, SRR1 will not contain the wakeup
+	 * reason, so there is no point setting r12 to SRR1.
+	 *
+	 * Further, we clear r12 here, so that we don't accidentally enter the
+	 * HMI in pnv_wakeup_noloss() if the value of r12[42:45] == WAKE_HMI.
+	 */
+	li	r12, 0
 	b 	pnv_wakeup_noloss
 
 .Lhandle_esl_ec_set: