rtc: ds1307: Detect oscillator fail on mcp794xx

This patch enables the detection of the oscillator failure on mcp794xx chips.

Co-developed-by: Szentendrei, Tamás <szentendrei.tamas@prolan.hu>
Signed-off-by: Szentendrei, Tamás <szentendrei.tamas@prolan.hu>
Signed-off-by: Csókás, Bence <csokas.bence@prolan.hu>
Link: https://lore.kernel.org/r/20240611072411.671600-1-csokas.bence@prolan.hu
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: Csókás, Bence

drivers/rtc/rtc-ds1307.c

@@ -65,6 +65,7 @@ enum ds_type {
 #	define DS1340_BIT_CENTURY_EN	0x80	/* in REG_HOUR */
 #	define DS1340_BIT_CENTURY	0x40	/* in REG_HOUR */
 #define DS1307_REG_WDAY		0x03	/* 01-07 */
+#	define MCP794XX_BIT_OSCRUN	BIT(5)
 #	define MCP794XX_BIT_VBATEN	0x08
 #define DS1307_REG_MDAY		0x04	/* 01-31 */
 #define DS1307_REG_MONTH	0x05	/* 01-12 */
@@ -242,6 +243,10 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 	    regs[DS1307_REG_MIN] & M41T0_BIT_OF) {
 		dev_warn_once(dev, "oscillator failed, set time!\n");
 		return -EINVAL;
+	} else if (ds1307->type == mcp794xx &&
+	    !(regs[DS1307_REG_WDAY] & MCP794XX_BIT_OSCRUN)) {
+		dev_warn_once(dev, "oscillator failed, set time!\n");
+		return -EINVAL;
 	}
 
 	tmp = regs[DS1307_REG_SECS];