rtc: ds1307: Fix relying on reset value for weekday

commit e29385fa upstream.

The reset value of weekday is 0x1. This is wrong since
the reset values of the day/month/year make up to Jan 1 2001.
When computed weekday comes out to be Monday. On a scale
of 1-7(Sunday - Saturday) it should be 0x2. So we should not
be relying on the reset value.

Hence compute the wday using the current date/month/year values.
Check if reset wday is any different from the computed wday,
If different then set the wday which we computed using
date/month/year values.

Document Referred:
http://ww1.microchip.com/downloads/en/DeviceDoc/20002266F.pdf

Fixes: 1d1945d2 "drivers/rtc/rtc-ds1307.c: add alarm support for mcp7941x chips"
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[bwh: Backported to 3.16:
 - No 64-time rtc_time functions available
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/rtc/rtc-ds1307.c

@@ -611,6 +611,8 @@ static const struct rtc_class_ops ds13xx_rtc_ops = {
  * Alarm support for mcp7941x devices.
  */
 
+#define MCP794XX_REG_WEEKDAY		0x3
+#define MCP794XX_REG_WEEKDAY_WDAY_MASK	0x7
 #define MCP7941X_REG_CONTROL		0x07
 #	define MCP7941X_BIT_ALM0_EN	0x10
 #	define MCP7941X_BIT_ALM1_EN	0x20
@@ -840,12 +842,15 @@ static int ds1307_probe(struct i2c_client *client,
 {
 	struct ds1307		*ds1307;
 	int			err = -ENODEV;
-	int			tmp;
+	int			tmp, wday;
 	const struct chip_desc	*chip = &chips[id->driver_data];
 	struct i2c_adapter	*adapter = to_i2c_adapter(client->dev.parent);
 	bool			want_irq = false;
 	unsigned char		*buf;
 	struct ds1307_platform_data *pdata = dev_get_platdata(&client->dev);
+	struct rtc_time		tm;
+	unsigned long		timestamp;
+
 	static const int	bbsqi_bitpos[] = {
 		[ds_1337] = 0,
 		[ds_1339] = DS1339_BIT_BBSQI,
@@ -1115,6 +1120,27 @@ static int ds1307_probe(struct i2c_client *client,
 		return PTR_ERR(ds1307->rtc);
 	}
 
+	/*
+	 * Some IPs have weekday reset value = 0x1 which might not correct
+	 * hence compute the wday using the current date/month/year values
+	 */
+	ds1307_get_time(&client->dev, &tm);
+	wday = tm.tm_wday;
+	rtc_tm_to_time(&tm, &timestamp);
+	rtc_time_to_tm(timestamp, &tm);
+
+	/*
+	 * Check if reset wday is different from the computed wday
+	 * If different then set the wday which we computed using
+	 * timestamp
+	 */
+	if (wday != tm.tm_wday) {
+		wday = i2c_smbus_read_byte_data(client, MCP794XX_REG_WEEKDAY);
+		wday = wday & ~MCP794XX_REG_WEEKDAY_WDAY_MASK;
+		wday = wday | (tm.tm_wday + 1);
+		i2c_smbus_write_byte_data(client, MCP794XX_REG_WEEKDAY, wday);
+	}
+
 	if (want_irq) {
 		err = request_irq(client->irq, ds1307_irq, IRQF_SHARED,
 			  ds1307->rtc->name, client);