rtc: opal: Handle disabled TPO in opal_get_tpo_time()

On PowerNV platform when Timed-Power-On(TPO) is disabled, read of
stored TPO yields value with all date components set to '0' inside
opal_get_tpo_time(). The function opal_to_tm() then converts it to an
offset from year 1900 yielding alarm-time == "1900-00-01
00:00:00". This causes problems with __rtc_read_alarm() that
expecting an offset from "1970-00-01 00:00:00" and returned alarm-time
results in a -ve value for time64_t. Which ultimately results in this
error reported in kernel logs with a seemingly garbage value:

"rtc rtc0: invalid alarm value: -2-1--1041528741
2005511117:71582844:32"

We fix this by explicitly handling the case of all alarm date-time
components being '0' inside opal_get_tpo_time() and returning -ENOENT
in such a case. This signals generic rtc that no alarm is set and it
bails out from the alarm initialization flow without reporting the
above error.

Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Reported-by: Steve Best <sbest@redhat.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-opal.c

@@ -142,6 +142,16 @@ static int opal_get_tpo_time(struct device *dev, struct rtc_wkalrm *alarm)
 
 	y_m_d = be32_to_cpu(__y_m_d);
 	h_m_s_ms = ((u64)be32_to_cpu(__h_m) << 32);
+
+	/* check if no alarm is set */
+	if (y_m_d == 0 && h_m_s_ms == 0) {
+		pr_debug("No alarm is set\n");
+		rc = -ENOENT;
+		goto exit;
+	} else {
+		pr_debug("Alarm set to %x %llx\n", y_m_d, h_m_s_ms);
+	}
+
 	opal_to_tm(y_m_d, h_m_s_ms, &alarm->time);
 
 exit: