rtc: opal: Enable alarms only when opal supports tpo

rtc-opal driver provides support for rtc alarms via
timed-power-on(tpo). However some Power platforms like BML use a fake
rtc clock and don't support tpo. Such platforms are indicated by the
missing 'has-tpo' property in the device tree.

Current implementation however enables callback for
rtc_class_ops.read/set alarm irrespective of the tpo support from the
platform. This results in a failed opal call when kernel tries to read
an existing alarms via opal_get_tpo_time during rtc device registration.

This patch fixes this issue by setting opal_rtc_ops.read/set_alarm
callback pointers only when tpo is supported.

Acked-by: Michael Neuling <mikey@neuling.org>
Acked-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-opal.c

@@ -190,11 +190,9 @@ exit:
 	return rc;
 }
 
-static const struct rtc_class_ops opal_rtc_ops = {
+static struct rtc_class_ops opal_rtc_ops = {
 	.read_time	= opal_get_rtc_time,
 	.set_time	= opal_set_rtc_time,
-	.read_alarm	= opal_get_tpo_time,
-	.set_alarm	= opal_set_tpo_time,
 };
 
 static int opal_rtc_probe(struct platform_device *pdev)
@@ -202,8 +200,11 @@ static int opal_rtc_probe(struct platform_device *pdev)
 	struct rtc_device *rtc;
 
 	if (pdev->dev.of_node && of_get_property(pdev->dev.of_node, "has-tpo",
-						 NULL))
+						 NULL)) {
 		device_set_wakeup_capable(&pdev->dev, true);
+		opal_rtc_ops.read_alarm	= opal_get_tpo_time;
+		opal_rtc_ops.set_alarm = opal_set_tpo_time;
+	}
 
 	rtc = devm_rtc_device_register(&pdev->dev, DRVNAME, &opal_rtc_ops,
 				       THIS_MODULE);