rtc: stm32: fix issues of stm32_rtc_valid_alrm function

stm32_rtc_valid_alrm function has some issues :
- arithmetical operations are impossible on BCD values
- "cur_mon + 1" can overflow
- the use case with the next month, the same day/hour/minutes went wrong

To solve that, we prefer to use timestamp comparison.
e.g. : On 5 Dec. 2021, the alarm limit is 5 Jan. 2022 (+31 days)
       On 31 Jan 2021, the alarm limit is 28 Feb. 2022 (+28 days)
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20230705174357.353616-7-valentin.caron@foss.st.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-stm32.c

@@ -90,6 +90,9 @@
 /* Max STM32 RTC register offset is 0x3FC */
 #define UNDEF_REG			0xFFFF
 
+/* STM32 RTC driver time helpers */
+#define SEC_PER_DAY		(24 * 60 * 60)
+
 struct stm32_rtc;
 
 struct stm32_rtc_registers {
@@ -427,40 +430,42 @@ static int stm32_rtc_alarm_irq_enable(struct device *dev, unsigned int enabled)
 	return 0;
 }
 
-static int stm32_rtc_valid_alrm(struct stm32_rtc *rtc, struct rtc_time *tm)
+static int stm32_rtc_valid_alrm(struct device *dev, struct rtc_time *tm)
 {
-	const struct stm32_rtc_registers *regs = &rtc->data->regs;
-	int cur_day, cur_mon, cur_year, cur_hour, cur_min, cur_sec;
-	unsigned int tr = readl_relaxed(rtc->base + regs->tr);
-	unsigned int dr = readl_relaxed(rtc->base + regs->dr);
-
-	cur_day = (dr & STM32_RTC_DR_DATE) >> STM32_RTC_DR_DATE_SHIFT;
-	cur_mon = (dr & STM32_RTC_DR_MONTH) >> STM32_RTC_DR_MONTH_SHIFT;
-	cur_year = (dr & STM32_RTC_DR_YEAR) >> STM32_RTC_DR_YEAR_SHIFT;
-	cur_sec = (tr & STM32_RTC_TR_SEC) >> STM32_RTC_TR_SEC_SHIFT;
-	cur_min = (tr & STM32_RTC_TR_MIN) >> STM32_RTC_TR_MIN_SHIFT;
-	cur_hour = (tr & STM32_RTC_TR_HOUR) >> STM32_RTC_TR_HOUR_SHIFT;
+	static struct rtc_time now;
+	time64_t max_alarm_time64;
+	int max_day_forward;
+	int next_month;
+	int next_year;
 
 	/*
 	 * Assuming current date is M-D-Y H:M:S.
 	 * RTC alarm can't be set on a specific month and year.
 	 * So the valid alarm range is:
 	 *	M-D-Y H:M:S < alarm <= (M+1)-D-Y H:M:S
-	 * with a specific case for December...
 	 */
-	if ((((tm->tm_year > cur_year) &&
-	      (tm->tm_mon == 0x1) && (cur_mon == 0x12)) ||
-	     ((tm->tm_year == cur_year) &&
-	      (tm->tm_mon <= cur_mon + 1))) &&
-	    ((tm->tm_mday > cur_day) ||
-	     ((tm->tm_mday == cur_day) &&
-	     ((tm->tm_hour > cur_hour) ||
-	      ((tm->tm_hour == cur_hour) && (tm->tm_min > cur_min)) ||
-	      ((tm->tm_hour == cur_hour) && (tm->tm_min == cur_min) &&
-	       (tm->tm_sec >= cur_sec))))))
-		return 0;
+	stm32_rtc_read_time(dev, &now);
+
+	/*
+	 * Find the next month and the year of the next month.
+	 * Note: tm_mon and next_month are from 0 to 11
+	 */
+	next_month = now.tm_mon + 1;
+	if (next_month == 12) {
+		next_month = 0;
+		next_year = now.tm_year + 1;
+	} else {
+		next_year = now.tm_year;
+	}
 
-	return -EINVAL;
+	/* Find the maximum limit of alarm in days. */
+	max_day_forward = rtc_month_days(now.tm_mon, now.tm_year)
+			 - now.tm_mday
+			 + min(rtc_month_days(next_month, next_year), now.tm_mday);
+
+	/* Convert to timestamp and compare the alarm time and its upper limit */
+	max_alarm_time64 = rtc_tm_to_time64(&now) + max_day_forward * SEC_PER_DAY;
+	return rtc_tm_to_time64(tm) <= max_alarm_time64 ? 0 : -EINVAL;
 }
 
 static int stm32_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
@@ -471,17 +476,17 @@ static int stm32_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	unsigned int cr, isr, alrmar;
 	int ret = 0;
 
-	tm2bcd(tm);
-
 	/*
 	 * RTC alarm can't be set on a specific date, unless this date is
 	 * up to the same day of month next month.
 	 */
-	if (stm32_rtc_valid_alrm(rtc, tm) < 0) {
+	if (stm32_rtc_valid_alrm(dev, tm) < 0) {
 		dev_err(dev, "Alarm can be set only on upcoming month.\n");
 		return -EINVAL;
 	}
 
+	tm2bcd(tm);
+
 	alrmar = 0;
 	/* tm_year and tm_mon are not used because not supported by RTC */
 	alrmar |= (tm->tm_mday << STM32_RTC_ALRMXR_DATE_SHIFT) &