[PATCH v6 03/17] rtc: rzn1: Fix weekday underflow when alarm crosses month boundary

[Prabhakar]

From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>

rzn1_rtc_set_alarm() calculates the alarm weekday from the difference
between the alarm day and the current day of the month. When the alarm
crosses a month boundary, this difference can become negative. Since
days_ahead is unsigned, it underflows and results in an incorrect
weekday being programmed into RZN1_RTC_ALW.

The RTC core already provides a fully populated struct rtc_time for
the alarm, including the correct tm_wday. Use tm->tm_wday directly
instead of recalculating the weekday from the day-of-month.

This avoids the underflow and ensures alarms scheduled across a month
boundary use the correct weekday.

Fixes: b5ad1bf00d2c4 ("rtc: rzn1: Add alarm support")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
Suggested-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>
Tested-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>
---
v5->v6:
- No changes.

v4->v5:
- Added Tested-by tag from Wolfram.

v3->v4:
- Made use of tm->tm_wday directly instead of recalculating the weekday
  from the day-of-month.
- Updated commit message.

v2->v3:
- New patch to fix weekday underflow when alarm crosses month boundary.
---
 drivers/rtc/rtc-rzn1.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/drivers/rtc/rtc-rzn1.c b/drivers/rtc/rtc-rzn1.c
index f81d691c8b9a..56284a4320ae 100644
--- a/drivers/rtc/rtc-rzn1.c
+++ b/drivers/rtc/rtc-rzn1.c
@@ -261,7 +261,6 @@ static int rzn1_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	struct rzn1_rtc *rtc = dev_get_drvdata(dev);
 	struct rtc_time *tm = &alrm->time, tm_now;
 	unsigned long alarm, farest;
-	unsigned int days_ahead, wday;
 	int ret;
 
 	ret = rzn1_rtc_read_time(dev, &tm_now);
@@ -274,13 +273,9 @@ static int rzn1_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	if (time_after(alarm, farest))
 		return -ERANGE;
 
-	/* Convert alarm day into week day */
-	days_ahead = tm->tm_mday - tm_now.tm_mday;
-	wday = (tm_now.tm_wday + days_ahead) % 7;
-
 	writel(bin2bcd(tm->tm_min), rtc->base + RZN1_RTC_ALM);
 	writel(bin2bcd(tm->tm_hour), rtc->base + RZN1_RTC_ALH);
-	writel(BIT(wday), rtc->base + RZN1_RTC_ALW);
+	writel(BIT(tm->tm_wday), rtc->base + RZN1_RTC_ALW);
 
 	rtc->tm_alarm = alrm->time;
 
-- 
2.43.0