rtc: ds1307: fix century bit support

Add an option to properly support the century bit of ds1337 and compatibles
and ds1340.
Because the driver had a bug until now, it is not possible to switch users
to the fixed code directly as RTCs in the field will wrongly have the
century bit set.
Acked-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/Kconfig

@@ -234,6 +234,20 @@ config RTC_DRV_DS1307_HWMON
 	  Say Y here if you want to expose temperature sensor data on
 	  rtc-ds1307 (only DS3231)
 
+config RTC_DRV_DS1307_CENTURY
+	bool "Century bit support for rtc-ds1307"
+	depends on RTC_DRV_DS1307
+	default n
+	help
+	  The DS1307 driver suffered from a bug where it was enabling the
+	  century bit inconditionnally but never used it when reading the time.
+	  It made the driver unable to support dates beyond 2099.
+	  Setting this option will add proper support for the century bit but if
+	  the time was previously set using a kernel predating this option,
+	  reading the date will return a date in the next century.
+	  To solve that, you could boot a kernel without this option set, set
+	  the RTC date and then boot a kernel with this option set.
+
 config RTC_DRV_DS1374
 	tristate "Dallas/Maxim DS1374"
 	help

drivers/rtc/rtc-ds1307.c

@@ -382,10 +382,25 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 	t->tm_mday = bcd2bin(ds1307->regs[DS1307_REG_MDAY] & 0x3f);
 	tmp = ds1307->regs[DS1307_REG_MONTH] & 0x1f;
 	t->tm_mon = bcd2bin(tmp) - 1;
-
-	/* assume 20YY not 19YY, and ignore DS1337_BIT_CENTURY */
 	t->tm_year = bcd2bin(ds1307->regs[DS1307_REG_YEAR]) + 100;
 
+#ifdef CONFIG_RTC_DRV_DS1307_CENTURY
+	switch (ds1307->type) {
+	case ds_1337:
+	case ds_1339:
+	case ds_3231:
+		if (ds1307->regs[DS1307_REG_MONTH] & DS1337_BIT_CENTURY)
+			t->tm_year += 100;
+		break;
+	case ds_1340:
+		if (ds1307->regs[DS1307_REG_HOUR] & DS1340_BIT_CENTURY)
+			t->tm_year += 100;
+		break;
+	default:
+		break;
+	}
+#endif
+
 	dev_dbg(dev, "%s secs=%d, mins=%d, "
 		"hours=%d, mday=%d, mon=%d, year=%d, wday=%d\n",
 		"read", t->tm_sec, t->tm_min,
@@ -409,6 +424,27 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
 		t->tm_hour, t->tm_mday,
 		t->tm_mon, t->tm_year, t->tm_wday);
 
+#ifdef CONFIG_RTC_DRV_DS1307_CENTURY
+	if (t->tm_year < 100)
+		return -EINVAL;
+
+	switch (ds1307->type) {
+	case ds_1337:
+	case ds_1339:
+	case ds_3231:
+	case ds_1340:
+		if (t->tm_year > 299)
+			return -EINVAL;
+	default:
+		if (t->tm_year > 199)
+			return -EINVAL;
+		break;
+	}
+#else
+	if (t->tm_year < 100 || t->tm_year > 199)
+		return -EINVAL;
+#endif
+
 	buf[DS1307_REG_SECS] = bin2bcd(t->tm_sec);
 	buf[DS1307_REG_MIN] = bin2bcd(t->tm_min);
 	buf[DS1307_REG_HOUR] = bin2bcd(t->tm_hour);
@@ -424,11 +460,13 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
 	case ds_1337:
 	case ds_1339:
 	case ds_3231:
-		buf[DS1307_REG_MONTH] |= DS1337_BIT_CENTURY;
+		if (t->tm_year > 199)
+			buf[DS1307_REG_MONTH] |= DS1337_BIT_CENTURY;
 		break;
 	case ds_1340:
-		buf[DS1307_REG_HOUR] |= DS1340_BIT_CENTURY_EN
-				| DS1340_BIT_CENTURY;
+		buf[DS1307_REG_HOUR] |= DS1340_BIT_CENTURY_EN;
+		if (t->tm_year > 199)
+			buf[DS1307_REG_HOUR] |= DS1340_BIT_CENTURY;
 		break;
 	case mcp794xx:
 		/*