rtc: ds1307: Fix relying on reset value for weekday

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: 1d1945d261a2af "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>

1 files changed, 27 insertions, 1 deletions

diff --git a/drivers/rtc/rtc-ds1307.c b/drivers/rtc/rtc-ds1307.c
index 821d9c089cdb..f25f7dce6e1f 100644
--- a/drivers/rtc/rtc-ds1307.c
+++ b/drivers/rtc/rtc-ds1307.c
@@ -602,6 +602,8 @@ static const struct rtc_class_ops ds13xx_rtc_ops = {
  * Alarm support for mcp794xx devices.
  */
 
+#define MCP794XX_REG_WEEKDAY            0x3
+#define MCP794XX_REG_WEEKDAY_WDAY_MASK  0x7
 #define MCP794XX_REG_CONTROL            0x07
 #       define MCP794XX_BIT_ALM0_EN     0x10
 #       define MCP794XX_BIT_ALM1_EN     0x20
@@ -1231,13 +1233,16 @@ static int ds1307_probe(struct i2c_client *client,
 {
         struct ds1307           *ds1307;
         int                     err = -ENODEV;
-        int                     tmp;
+        int                     tmp, wday;
         struct chip_desc        *chip = &chips[id->driver_data];
         struct i2c_adapter      *adapter = to_i2c_adapter(client->dev.parent);
         bool                    want_irq = false;
         bool                    ds1307_can_wakeup_device = false;
         unsigned char           *buf;
         struct ds1307_platform_data *pdata = dev_get_platdata(&client->dev);
+        struct rtc_time         tm;
+        unsigned long           timestamp;
+
         irq_handler_t   irq_handler = ds1307_irq;
 
         static const int        bbsqi_bitpos[] = {
@@ -1526,6 +1531,27 @@ read_rtc:
                                 bin2bcd(tmp));
         }
 
+        /*
+         * 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;
+        timestamp = rtc_tm_to_time64(&tm);
+        rtc_time64_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) {
                 device_set_wakeup_capable(&client->dev, true);
                 set_bit(HAS_ALARM, &ds1307->flags);