rtc: jz4740: rework invalid time detection

The scratchpad register is used to detect an invalid time when power to the
RTC has been lost. Instead of deleting that precious information and set
the time to the UNIX epoch, forward it to userspace.
Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-jz4740.c

@@ -157,6 +157,9 @@ static int jz4740_rtc_read_time(struct device *dev, struct rtc_time *time)
 	uint32_t secs, secs2;
 	int timeout = 5;
 
+	if (jz4740_rtc_reg_read(rtc, JZ_REG_RTC_SCRATCHPAD) != 0x12345678)
+		return -EINVAL;
+
 	/* If the seconds register is read while it is updated, it can contain a
 	 * bogus value. This can be avoided by making sure that two consecutive
 	 * reads have the same value.
@@ -180,8 +183,13 @@ static int jz4740_rtc_read_time(struct device *dev, struct rtc_time *time)
 static int jz4740_rtc_set_time(struct device *dev, struct rtc_time *time)
 {
 	struct jz4740_rtc *rtc = dev_get_drvdata(dev);
+	int ret;
+
+	ret = jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SEC, rtc_tm_to_time64(time));
+	if (ret)
+		return ret;
 
-	return jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SEC, rtc_tm_to_time64(time));
+	return jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SCRATCHPAD, 0x12345678);
 }
 
 static int jz4740_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
@@ -308,7 +316,6 @@ static int jz4740_rtc_probe(struct platform_device *pdev)
 {
 	int ret;
 	struct jz4740_rtc *rtc;
-	uint32_t scratchpad;
 	struct resource *mem;
 	const struct platform_device_id *id = platform_get_device_id(pdev);
 	const struct of_device_id *of_id = of_match_device(
@@ -376,16 +383,6 @@ static int jz4740_rtc_probe(struct platform_device *pdev)
 		return ret;
 	}
 
-	scratchpad = jz4740_rtc_reg_read(rtc, JZ_REG_RTC_SCRATCHPAD);
-	if (scratchpad != 0x12345678) {
-		ret = jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SCRATCHPAD, 0x12345678);
-		ret = jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SEC, 0);
-		if (ret) {
-			dev_err(&pdev->dev, "Could not write to RTC registers\n");
-			return ret;
-		}
-	}
-
 	if (np && of_device_is_system_power_controller(np)) {
 		if (!pm_power_off) {
 			/* Default: 60ms */