rtc: Improve performance of rtc_time64_to_tm(). Add tests.

The current implementation of rtc_time64_to_tm() contains unnecessary
loops, branches and look-up tables. The new one uses an arithmetic-based
algorithm appeared in [1] and is approximately 4.3 times faster (YMMV).

The drawback is that the new code isn't intuitive and contains many 'magic
numbers' (not unusual for this type of algorithm). However, [1] justifies
all those numbers and, given this function's history, the code is unlikely
to need much maintenance, if any at all.

Add a KUnit test case that checks every day in a 160,000 years interval
starting on 1970-01-01 against the expected result. Add a new config
RTC_LIB_KUNIT_TEST symbol to give the option to run this test suite.

[1] Neri, Schneider, "Euclidean Affine Functions and Applications to
Calendar Algorithms". https://arxiv.org/abs/2102.06959

Signed-off-by: Cassio Neri <cassio.neri@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210624201343.85441-1-cassio.neri@gmail.com

1 files changed, 80 insertions, 27 deletions

diff --git a/drivers/rtc/lib.c b/drivers/rtc/lib.c
index 23284580df97..fe361652727a 100644
--- a/drivers/rtc/lib.c
+++ b/drivers/rtc/lib.c
@@ -6,6 +6,8 @@
  * Author: Alessandro Zummo <a.zummo@towertech.it>
  *
  * based on arch/arm/common/rtctime.c and other bits
+ *
+ * Author: Cassio Neri <cassio.neri@gmail.com> (rtc_time64_to_tm)
  */
 
 #include <linux/export.h>
@@ -22,8 +24,6 @@ static const unsigned short rtc_ydays[2][13] = {
 	{ 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 }
 };
 
-#define LEAPS_THRU_END_OF(y) ((y) / 4 - (y) / 100 + (y) / 400)
-
 /*
  * The number of days in the month.
  */
@@ -42,42 +42,95 @@ int rtc_year_days(unsigned int day, unsigned int month, unsigned int year)
 }
 EXPORT_SYMBOL(rtc_year_days);
 
-/*
- * rtc_time64_to_tm - Converts time64_t to rtc_time.
- * Convert seconds since 01-01-1970 00:00:00 to Gregorian date.
+/**
+ * rtc_time64_to_tm - converts time64_t to rtc_time.
+ *
+ * @time:	The number of seconds since 01-01-1970 00:00:00.
+ *		(Must be positive.)
+ * @tm:		Pointer to the struct rtc_time.
  */
 void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 {
-	unsigned int month, year, secs;
+	unsigned int secs;
 	int days;
 
+	u64 u64tmp;
+	u32 u32tmp, udays, century, day_of_century, year_of_century, year,
+		day_of_year, month, day;
+	bool is_Jan_or_Feb, is_leap_year;
+
 	/* time must be positive */
 	days = div_s64_rem(time, 86400, &secs);
 
 	/* day of the week, 1970-01-01 was a Thursday */
 	tm->tm_wday = (days + 4) % 7;
 
-	year = 1970 + days / 365;
-	days -= (year - 1970) * 365
-		+ LEAPS_THRU_END_OF(year - 1)
-		- LEAPS_THRU_END_OF(1970 - 1);
-	while (days < 0) {
-		year -= 1;
-		days += 365 + is_leap_year(year);
-	}
-	tm->tm_year = year - 1900;
-	tm->tm_yday = days + 1;
-
-	for (month = 0; month < 11; month++) {
-		int newdays;
-
-		newdays = days - rtc_month_days(month, year);
-		if (newdays < 0)
-			break;
-		days = newdays;
-	}
-	tm->tm_mon = month;
-	tm->tm_mday = days + 1;
+	/*
+	 * The following algorithm is, basically, Proposition 6.3 of Neri
+	 * and Schneider [1]. In a few words: it works on the computational
+	 * (fictitious) calendar where the year starts in March, month = 2
+	 * (*), and finishes in February, month = 13. This calendar is
+	 * mathematically convenient because the day of the year does not
+	 * depend on whether the year is leap or not. For instance:
+	 *
+	 * March 1st		0-th day of the year;
+	 * ...
+	 * April 1st		31-st day of the year;
+	 * ...
+	 * January 1st		306-th day of the year; (Important!)
+	 * ...
+	 * February 28th	364-th day of the year;
+	 * February 29th	365-th day of the year (if it exists).
+	 *
+	 * After having worked out the date in the computational calendar
+	 * (using just arithmetics) it's easy to convert it to the
+	 * corresponding date in the Gregorian calendar.
+	 *
+	 * [1] "Euclidean Affine Functions and Applications to Calendar
+	 * Algorithms". https://arxiv.org/abs/2102.06959
+	 *
+	 * (*) The numbering of months follows rtc_time more closely and
+	 * thus, is slightly different from [1].
+	 */
+
+	udays		= ((u32) days) + 719468;
+
+	u32tmp		= 4 * udays + 3;
+	century		= u32tmp / 146097;
+	day_of_century	= u32tmp % 146097 / 4;
+
+	u32tmp		= 4 * day_of_century + 3;
+	u64tmp		= 2939745ULL * u32tmp;
+	year_of_century	= upper_32_bits(u64tmp);
+	day_of_year	= lower_32_bits(u64tmp) / 2939745 / 4;
+
+	year		= 100 * century + year_of_century;
+	is_leap_year	= year_of_century != 0 ?
+		year_of_century % 4 == 0 : century % 4 == 0;
+
+	u32tmp		= 2141 * day_of_year + 132377;
+	month		= u32tmp >> 16;
+	day		= ((u16) u32tmp) / 2141;
+
+	/*
+	 * Recall that January 01 is the 306-th day of the year in the
+	 * computational (not Gregorian) calendar.
+	 */
+	is_Jan_or_Feb	= day_of_year >= 306;
+
+	/* Converts to the Gregorian calendar. */
+	year		= year + is_Jan_or_Feb;
+	month		= is_Jan_or_Feb ? month - 12 : month;
+	day		= day + 1;
+
+	day_of_year	= is_Jan_or_Feb ?
+		day_of_year - 306 : day_of_year + 31 + 28 + is_leap_year;
+
+	/* Converts to rtc_time's format. */
+	tm->tm_year	= (int) (year - 1900);
+	tm->tm_mon	= (int) month;
+	tm->tm_mday	= (int) day;
+	tm->tm_yday	= (int) day_of_year + 1;
 
 	tm->tm_hour = secs / 3600;
 	secs -= tm->tm_hour * 3600;