drivers/rtc/rtc-ds1302.c: handle write protection

This chip has a control register and can prevent altering saved clock.
Without this patch we could have:

(arm)root@pac14:~# date
Tue May 21 03:08:27 MSK 2013
(arm)root@pac14:~# /etc/init.d/hwclock.sh show
Tue May 21 11:13:58 2013  -0.067322 seconds
(arm)root@pac14:~# /etc/init.d/hwclock.sh stop
[info] Saving the system clock.
[info] Hardware Clock updated to Tue May 21 03:09:01 MSK 2013.
(arm)root@pac14:~# /etc/init.d/hwclock.sh show
Tue May 21 11:14:15 2013  -0.624272 seconds

The patch enables write access to rtc before the driver tries to write
time and re-disables when time data is written.
Signed-off-by: Sergey Yanovich <ynvich@gmail.com>
Acked-by: Marc Zyngier <maz@misterjones.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-ds1302.c

@@ -23,8 +23,12 @@
 #define	RTC_CMD_READ	0x81		/* Read command */
 #define	RTC_CMD_WRITE	0x80		/* Write command */
 
+#define	RTC_CMD_WRITE_ENABLE	0x00		/* Write enable */
+#define	RTC_CMD_WRITE_DISABLE	0x80		/* Write disable */
+
 #define RTC_ADDR_RAM0	0x20		/* Address of RAM0 */
 #define RTC_ADDR_TCR	0x08		/* Address of trickle charge register */
+#define	RTC_ADDR_CTRL	0x07		/* Address of control register */
 #define	RTC_ADDR_YEAR	0x06		/* Address of year register */
 #define	RTC_ADDR_DAY	0x05		/* Address of day of week register */
 #define	RTC_ADDR_MON	0x04		/* Address of month register */
@@ -161,6 +165,7 @@ static int ds1302_rtc_read_time(struct device *dev, struct rtc_time *tm)
 
 static int ds1302_rtc_set_time(struct device *dev, struct rtc_time *tm)
 {
+	ds1302_writebyte(RTC_ADDR_CTRL, RTC_CMD_WRITE_ENABLE);
 	/* Stop RTC */
 	ds1302_writebyte(RTC_ADDR_SEC, ds1302_readbyte(RTC_ADDR_SEC) | 0x80);
 
@@ -175,6 +180,8 @@ static int ds1302_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	/* Start RTC */
 	ds1302_writebyte(RTC_ADDR_SEC, ds1302_readbyte(RTC_ADDR_SEC) & ~0x80);
 
+	ds1302_writebyte(RTC_ADDR_CTRL, RTC_CMD_WRITE_DISABLE);
+
 	return 0;
 }