rtc: rk808: Compensate for Rockchip calendar deviation on November 31st

In A.D. 1582 Pope Gregory XIII found that the existing Julian calendar
insufficiently represented reality, and changed the rules about
calculating leap years to account for this. Similarly, in A.D. 2013
Rockchip hardware engineers found that the new Gregorian calendar still
contained flaws, and that the month of November should be counted up to
31 days instead. Unfortunately it takes a long time for calendar changes
to gain widespread adoption, and just like more than 300 years went by
before the last Protestant nation implemented Greg's proposal, we will
have to wait a while until all religions and operating system kernels
acknowledge the inherent advantages of the Rockchip system. Until then
we need to translate dates read from (and written to) Rockchip hardware
back to the Gregorian format.

This patch works by defining Jan 1st, 2016 as the arbitrary anchor date
on which Rockchip and Gregorian calendars are in sync. From that we can
translate arbitrary later dates back and forth by counting the number
of November/December transitons since the anchor date to determine the
offset between the calendars. We choose this method (rather than trying
to regularly "correct" the date stored in hardware) since it's the only
way to ensure perfect time-keeping even if the system may be shut down
for an unknown number of years. The drawback is that other software
reading the same hardware (e.g. mainboard firmware) must use the same
translation convention (including the same anchor date) to be able to
read and write correct timestamps from/to the RTC.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-rk808.c

@@ -56,6 +56,42 @@ struct rk808_rtc {
 	int irq;
 };
 
+/*
+ * The Rockchip calendar used by the RK808 counts November with 31 days. We use
+ * these translation functions to convert its dates to/from the Gregorian
+ * calendar used by the rest of the world. We arbitrarily define Jan 1st, 2016
+ * as the day when both calendars were in sync, and treat all other dates
+ * relative to that.
+ * NOTE: Other system software (e.g. firmware) that reads the same hardware must
+ * implement this exact same conversion algorithm, with the same anchor date.
+ */
+static time64_t nov2dec_transitions(struct rtc_time *tm)
+{
+	return (tm->tm_year + 1900) - 2016 + (tm->tm_mon + 1 > 11 ? 1 : 0);
+}
+
+static void rockchip_to_gregorian(struct rtc_time *tm)
+{
+	/* If it's Nov 31st, rtc_tm_to_time64() will count that like Dec 1st */
+	time64_t time = rtc_tm_to_time64(tm);
+	rtc_time64_to_tm(time + nov2dec_transitions(tm) * 86400, tm);
+}
+
+static void gregorian_to_rockchip(struct rtc_time *tm)
+{
+	time64_t extra_days = nov2dec_transitions(tm);
+	time64_t time = rtc_tm_to_time64(tm);
+	rtc_time64_to_tm(time - extra_days * 86400, tm);
+
+	/* Compensate if we went back over Nov 31st (will work up to 2381) */
+	if (nov2dec_transitions(tm) < extra_days) {
+		if (tm->tm_mon + 1 == 11)
+			tm->tm_mday++;	/* This may result in 31! */
+		else
+			rtc_time64_to_tm(time - (extra_days - 1) * 86400, tm);
+	}
+}
+
 /* Read current time and date in RTC */
 static int rk808_rtc_readtime(struct device *dev, struct rtc_time *tm)
 {
@@ -101,9 +137,10 @@ static int rk808_rtc_readtime(struct device *dev, struct rtc_time *tm)
 	tm->tm_mon = (bcd2bin(rtc_data[4] & MONTHS_REG_MSK)) - 1;
 	tm->tm_year = (bcd2bin(rtc_data[5] & YEARS_REG_MSK)) + 100;
 	tm->tm_wday = bcd2bin(rtc_data[6] & WEEKS_REG_MSK);
+	rockchip_to_gregorian(tm);
 	dev_dbg(dev, "RTC date/time %4d-%02d-%02d(%d) %02d:%02d:%02d\n",
 		1900 + tm->tm_year, tm->tm_mon + 1, tm->tm_mday,
-		tm->tm_wday, tm->tm_hour , tm->tm_min, tm->tm_sec);
+		tm->tm_wday, tm->tm_hour, tm->tm_min, tm->tm_sec);
 
 	return ret;
 }
@@ -116,6 +153,10 @@ static int rk808_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	u8 rtc_data[NUM_TIME_REGS];
 	int ret;
 
+	dev_dbg(dev, "set RTC date/time %4d-%02d-%02d(%d) %02d:%02d:%02d\n",
+		1900 + tm->tm_year, tm->tm_mon + 1, tm->tm_mday,
+		tm->tm_wday, tm->tm_hour, tm->tm_min, tm->tm_sec);
+	gregorian_to_rockchip(tm);
 	rtc_data[0] = bin2bcd(tm->tm_sec);
 	rtc_data[1] = bin2bcd(tm->tm_min);
 	rtc_data[2] = bin2bcd(tm->tm_hour);
@@ -123,9 +164,6 @@ static int rk808_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	rtc_data[4] = bin2bcd(tm->tm_mon + 1);
 	rtc_data[5] = bin2bcd(tm->tm_year - 100);
 	rtc_data[6] = bin2bcd(tm->tm_wday);
-	dev_dbg(dev, "set RTC date/time %4d-%02d-%02d(%d) %02d:%02d:%02d\n",
-		1900 + tm->tm_year, tm->tm_mon + 1, tm->tm_mday,
-		tm->tm_wday, tm->tm_hour , tm->tm_min, tm->tm_sec);
 
 	/* Stop RTC while updating the RTC registers */
 	ret = regmap_update_bits(rk808->regmap, RK808_RTC_CTRL_REG,
@@ -170,6 +208,7 @@ static int rk808_rtc_readalarm(struct device *dev, struct rtc_wkalrm *alrm)
 	alrm->time.tm_mday = bcd2bin(alrm_data[3] & DAYS_REG_MSK);
 	alrm->time.tm_mon = (bcd2bin(alrm_data[4] & MONTHS_REG_MSK)) - 1;
 	alrm->time.tm_year = (bcd2bin(alrm_data[5] & YEARS_REG_MSK)) + 100;
+	rockchip_to_gregorian(&alrm->time);
 
 	ret = regmap_read(rk808->regmap, RK808_RTC_INT_REG, &int_reg);
 	if (ret) {
@@ -227,6 +266,7 @@ static int rk808_rtc_setalarm(struct device *dev, struct rtc_wkalrm *alrm)
 		alrm->time.tm_mday, alrm->time.tm_wday, alrm->time.tm_hour,
 		alrm->time.tm_min, alrm->time.tm_sec);
 
+	gregorian_to_rockchip(&alrm->time);
 	alrm_data[0] = bin2bcd(alrm->time.tm_sec);
 	alrm_data[1] = bin2bcd(alrm->time.tm_min);
 	alrm_data[2] = bin2bcd(alrm->time.tm_hour);