rtc: ds1307: support m41t0 variant

The m41t0 variant is very similar to the already supported m41t00
variant, with the notable exception of the oscillator fail bit.
The data sheet notes:

  If the oscillator fail (OF) bit is internally set to a '1,' this
  indicates that the oscillator has either stopped, or was stopped
  for some period of time and can be used to judge the validity of
  the clock and date data.

The bit will get cleared with a regular write of the system time,
so no changes are needed to clear it.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

2 files changed, 14 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/i2c/trivial-devices.txt b/Documentation/devicetree/bindings/i2c/trivial-devices.txt
index ad10fbe61562..3e0a34c88e07 100644
--- a/Documentation/devicetree/bindings/i2c/trivial-devices.txt
+++ b/Documentation/devicetree/bindings/i2c/trivial-devices.txt
@@ -160,6 +160,7 @@ sii,s35390a		2-wire CMOS real-time clock
 silabs,si7020           Relative Humidity and Temperature Sensors
 skyworks,sky81452       Skyworks SKY81452: Six-Channel White LED Driver with Touch Panel Bias Supply
 st,24c256               i2c serial eeprom  (24cxx)
+st,m41t0                Serial real-time clock (RTC)
 st,m41t00               Serial real-time clock (RTC)
 st,m41t62               Serial real-time clock (RTC) with alarm
 st,m41t80               M41T80 - SERIAL ACCESS RTC WITH ALARMS
diff --git a/drivers/rtc/rtc-ds1307.c b/drivers/rtc/rtc-ds1307.c
index f085b487bbe7..e29481391496 100644
--- a/drivers/rtc/rtc-ds1307.c
+++ b/drivers/rtc/rtc-ds1307.c
@@ -39,6 +39,7 @@ enum ds_type {
         ds_1340,
         ds_1388,
         ds_3231,
+        m41t0,
         m41t00,
         mcp794xx,
         rx_8025,
@@ -53,6 +54,7 @@ enum ds_type {
 #       define DS1340_BIT_nEOSC         0x80
 #       define MCP794XX_BIT_ST          0x80
 #define DS1307_REG_MIN          0x01    /* 00-59 */
+#       define M41T0_BIT_OF             0x80
 #define DS1307_REG_HOUR         0x02    /* 00-23, or 1-12{am,pm} */
 #       define DS1307_BIT_12HR          0x40    /* in REG_HOUR */
 #       define DS1307_BIT_PM            0x20    /* in REG_HOUR */
@@ -183,6 +185,7 @@ static const struct i2c_device_id ds1307_id[] = {
         { "ds1388", ds_1388 },
         { "ds1340", ds_1340 },
         { "ds3231", ds_3231 },
+        { "m41t0", m41t0 },
         { "m41t00", m41t00 },
         { "mcp7940x", mcp794xx },
         { "mcp7941x", mcp794xx },
@@ -261,6 +264,7 @@ static const struct acpi_device_id ds1307_acpi_ids[] = {
         { .id = "DS1388", .driver_data = ds_1388 },
         { .id = "DS1340", .driver_data = ds_1340 },
         { .id = "DS3231", .driver_data = ds_3231 },
+        { .id = "M41T0", .driver_data = m41t0 },
         { .id = "M41T00", .driver_data = m41t00 },
         { .id = "MCP7940X", .driver_data = mcp794xx },
         { .id = "MCP7941X", .driver_data = mcp794xx },
@@ -456,6 +460,13 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 
         dev_dbg(dev, "%s: %7ph\n", "read", ds1307->regs);
 
+        /* if oscillator fail bit is set, no data can be trusted */
+        if (ds1307->type == m41t0 &&
+            ds1307->regs[DS1307_REG_MIN] & M41T0_BIT_OF) {
+                dev_warn_once(dev, "oscillator failed, set time!\n");
+                return -EINVAL;
+        }
+
         t->tm_sec = bcd2bin(ds1307->regs[DS1307_REG_SECS] & 0x7f);
         t->tm_min = bcd2bin(ds1307->regs[DS1307_REG_MIN] & 0x7f);
         tmp = ds1307->regs[DS1307_REG_HOUR] & 0x3f;
@@ -1578,6 +1589,7 @@ read_rtc:
         tmp = ds1307->regs[DS1307_REG_SECS];
         switch (ds1307->type) {
         case ds_1307:
+        case m41t0:
         case m41t00:
                 /* clock halted?  turn it on, so clock can tick. */
                 if (tmp & DS1307_BIT_CH) {
@@ -1642,6 +1654,7 @@ read_rtc:
         tmp = ds1307->regs[DS1307_REG_HOUR];
         switch (ds1307->type) {
         case ds_1340:
+        case m41t0:
         case m41t00:
                 /*
                  * NOTE: ignores century bits; fix before deploying