[PATCH] RTC: M41T80: Century Bit support

[Maciej W. Rozycki]

 Make use of the Century Bit to support years in the range from 1970 to 
2169.  Enable toggling of the bit at the end of a century on a clock 
update.
---
 The clock is used with the Broadcom SWARM and the D-Link DNS-323 
platform.

 I have verified correct operation with the SWARM -- the firmware assumes
19YY when CB is clear and 20YY otherwise.  Which means years 1900-1969
will be shown as 2100-2169 in Linux.  I think this is a feature rather
than a problem.  The firmware does not set the CEB bit itself and does not 
care of what its state is.

 I will be happy to hear from a DNS-323 user to confirm or deny whether
such an interpretation is compatible.

 Otherwise, please apply.

  Maciej

patch-2.6.26-rc1-20080505-m41t80-century-4
diff -up --recursive --new-file linux-2.6.26-rc1-20080505.macro/drivers/rtc/rtc-m41t80.c linux-2.6.26-rc1-20080505/drivers/rtc/rtc-m41t80.c
--- linux-2.6.26-rc1-20080505.macro/drivers/rtc/rtc-m41t80.c	2008-05-05 02:55:40.000000000 +0000
+++ linux-2.6.26-rc1-20080505/drivers/rtc/rtc-m41t80.c	2008-05-16 21:16:48.000000000 +0000
@@ -52,6 +52,8 @@
 	(M41T80_REG_ALARM_SEC + 1 - M41T80_REG_ALARM_MON)
 
 #define M41T80_SEC_ST		(1 << 7)	/* ST: Stop Bit */
+#define M41T80_HOUR_CEB		(1 << 7)	/* CEB: Century Enable Bit */
+#define M41T80_HOUR_CB		(1 << 6)	/* CB: Century Bit */
 #define M41T80_ALMON_AFE	(1 << 7)	/* AFE: AF Enable Bit */
 #define M41T80_ALMON_SQWE	(1 << 6)	/* SQWE: SQW Enable Bit */
 #define M41T80_ALHOUR_HT	(1 << 6)	/* HT: Halt Update Bit */
@@ -164,8 +166,12 @@ static int m41t80_get_datetime(struct i2
 	tm->tm_wday = buf[M41T80_REG_WDAY] & 0x07;
 	tm->tm_mon = BCD2BIN(buf[M41T80_REG_MON] & 0x1f) - 1;
 
-	/* assume 20YY not 19YY, and ignore the Century Bit */
-	tm->tm_year = BCD2BIN(buf[M41T80_REG_YEAR]) + 100;
+	/* Assume 1970-2169, taking the Century Bit into account. */
+	tm->tm_year = BCD2BIN(buf[M41T80_REG_YEAR]);
+	if ((buf[M41T80_REG_HOUR] & M41T80_HOUR_CB) != 0)
+		tm->tm_year += 100;
+	if (tm->tm_year < 70)
+		tm->tm_year += 200;
 	return 0;
 }
 
@@ -196,8 +202,14 @@ static int m41t80_set_datetime(struct i2
 		BIN2BCD(tm->tm_mday) | (buf[M41T80_REG_DAY] & ~0x3f);
 	buf[M41T80_REG_MON] =
 		BIN2BCD(tm->tm_mon + 1) | (buf[M41T80_REG_MON] & ~0x1f);
-	/* assume 20YY not 19YY */
+	/* Assume 1970-2169 and set the Century Bit for 19YY/21YY. */
 	buf[M41T80_REG_YEAR] = BIN2BCD(tm->tm_year % 100);
+	if ((tm->tm_year / 100) % 2 != 0)
+		buf[M41T80_REG_HOUR] |= M41T80_HOUR_CB;
+	else
+		buf[M41T80_REG_HOUR] &= ~M41T80_HOUR_CB;
+	/* Enable toggling of the Century Bit at the end of a century. ;-) */
+	buf[M41T80_REG_HOUR] |= M41T80_HOUR_CEB;
 
 	if (m41t80_write_block_data(client, M41T80_REG_SSEC,
 				    M41T80_DATETIME_REG_SIZE, buf) < 0) {
--
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