Merge tag 'io_uring-5.15-2021-09-11' of git://git.kernel.dk/linux-block

[linux-2.6-microblaze.git] / drivers / rtc / lib.c

diff --git a/drivers/rtc/lib.c b/drivers/rtc/lib.c
index 2328458..fe36165 100644 (file)
--- 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;