From cdab80eecea25859159e7b94ac972ae0320f8ecc Mon Sep 17 00:00:00 2001
From: Johan Lafon <johan.lafon@syslinbit.com>
Date: Mon, 11 Sep 2023 13:45:24 +0200
Subject: [PATCH] drivers: rtc: stm32: avoid reset induced time drift

Calling LL_RTC_Init causes the RTC to stop while being configured thus
inducing time drift. This commit avoids doing it at each reset if the RTC
is already properly configured.

Signed-off-by: Johan Lafon <johan.lafon@syslinbit.com>
---
 drivers/rtc/rtc_ll_stm32.c | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/drivers/rtc/rtc_ll_stm32.c b/drivers/rtc/rtc_ll_stm32.c
index fccbe30f1ce..feb7ff387a2 100644
--- a/drivers/rtc/rtc_ll_stm32.c
+++ b/drivers/rtc/rtc_ll_stm32.c
@@ -66,11 +66,35 @@ struct rtc_stm32_data {
 	/* Currently empty */
 };
 
+static int rtc_stm32_configure(const struct device *dev)
+{
+	const struct rtc_stm32_config *cfg = dev->config;
+
+	uint32_t hour_format     = LL_RTC_GetHourFormat(RTC);
+	uint32_t sync_prescaler  = LL_RTC_GetSynchPrescaler(RTC);
+	uint32_t async_prescaler = LL_RTC_GetAsynchPrescaler(RTC);
+
+	/* configuration process requires to stop the RTC counter so do it
+	 * only if needed to avoid inducing time drift at each reset
+	 */
+	if ((hour_format != cfg->ll_rtc_config.HourFormat) ||
+	    (sync_prescaler != cfg->ll_rtc_config.SynchPrescaler) ||
+	    (async_prescaler != cfg->ll_rtc_config.AsynchPrescaler)) {
+		if (LL_RTC_Init(RTC, ((LL_RTC_InitTypeDef *)&cfg->ll_rtc_config)) != SUCCESS) {
+			return -EIO;
+		}
+	}
+
+	return 0;
+}
+
 static int rtc_stm32_init(const struct device *dev)
 {
 	const struct device *const clk = DEVICE_DT_GET(STM32_CLOCK_CONTROL_NODE);
 	const struct rtc_stm32_config *cfg = dev->config;
 
+	int err = 0;
+
 	if (!device_is_ready(clk)) {
 		LOG_ERR("clock control device not ready");
 		return -ENODEV;
@@ -98,8 +122,9 @@ static int rtc_stm32_init(const struct device *dev)
 
 	z_stm32_hsem_unlock(CFG_HW_RCC_SEMID);
 
-	if (LL_RTC_Init(RTC, ((LL_RTC_InitTypeDef *)&cfg->ll_rtc_config)) != SUCCESS) {
-		return -EIO;
+	err = rtc_stm32_configure(dev);
+	if (err) {
+		return err;
 	}
 
 #ifdef RTC_CR_BYPSHAD
@@ -108,7 +133,7 @@ static int rtc_stm32_init(const struct device *dev)
 	LL_RTC_EnableWriteProtection(RTC);
 #endif /* RTC_CR_BYPSHAD */
 
-	return 0;
+	return err;
 }
 
 static const struct stm32_pclken rtc_clk[] = STM32_DT_INST_CLOCKS(0);