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[PATCH] RTC: M41T80: Century Bit support

To: Alessandro Zummo <a.zummo@towertech.it>, Herbert Valerio Riedel <hvr@gnu.org>, Andrew Morton <akpm@linux-foundation.org>
Subject: [PATCH] RTC: M41T80: Century Bit support
From: "Maciej W. Rozycki" <macro@linux-mips.org>
Date: Sat, 17 May 2008 01:25:18 +0100 (BST)
Cc: rtc-linux@googlegroups.com, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
 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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