[PATCH] rtc: rtc-stmp3xxx: detect power failure on backup power domain

[Michael Thalmeier <michael.thalmeier@hale.at>]

To detect when the backup power domain has lost power a software defined bit
is set in one of the general purpose persistent registers when writing a new
time into the rtc.
When reading the time this bit is checked to determine if a power fail has
happened since the last time the rtc time was set.

When we detect a power fail we return -ENODATA.

Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>
---
 drivers/rtc/rtc-stmp3xxx.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/rtc/rtc-stmp3xxx.c b/drivers/rtc/rtc-stmp3xxx.c
index d578e40..51330ec 100644
--- a/drivers/rtc/rtc-stmp3xxx.c
+++ b/drivers/rtc/rtc-stmp3xxx.c
@@ -62,6 +62,9 @@
 /* missing bitmask in headers */
 #define STMP3XXX_RTC_PERSISTENT1_FORCE_UPDATER	0x80000000
 
+#define STMP3XXX_RTC_PERSISTENT2		0x80
+#define STMP3XXX_RTC_PERSISTENT2_VALID_TIME	0x01
+
 struct stmp3xxx_rtc_data {
 	struct rtc_device *rtc;
 	void __iomem *io;
@@ -160,6 +163,10 @@ static int stmp3xxx_rtc_gettime(struct device *dev, struct rtc_time *rtc_tm)
 	if (ret)
 		return ret;
 
+	if (!(readl(rtc_data->io + STMP3XXX_RTC_PERSISTENT2) &
+				STMP3XXX_RTC_PERSISTENT2_VALID_TIME))
+		return -ENODATA;
+
 	rtc_time_to_tm(readl(rtc_data->io + STMP3XXX_RTC_SECONDS), rtc_tm);
 	return 0;
 }
@@ -169,6 +176,8 @@ static int stmp3xxx_rtc_set_mmss(struct device *dev, unsigned long t)
 	struct stmp3xxx_rtc_data *rtc_data = dev_get_drvdata(dev);
 
 	writel(t, rtc_data->io + STMP3XXX_RTC_SECONDS);
+	writel(STMP3XXX_RTC_PERSISTENT2_VALID_TIME,
+		rtc_data->io + STMP3XXX_RTC_PERSISTENT2 + STMP_OFFSET_REG_SET);
 	return stmp3xxx_wait_time(rtc_data);
 }
 
-- 
2.9.2