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authorCassio Neri <cassio.neri@gmail.com>2021-06-24 23:13:43 +0300
committerAlexandre Belloni <alexandre.belloni@bootlin.com>2021-08-10 01:09:21 +0300
commit1d1bb12a8b1805ddeef9793ebeb920179fb0fa38 (patch)
tree883add72e000a924a060c2a0a07283ae1ee6f120 /drivers/rtc/lib.c
parentfffd603ae9f6ee1da47fa4ae4c70c324323bc201 (diff)
downloadlinux-1d1bb12a8b1805ddeef9793ebeb920179fb0fa38.tar.xz
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
Diffstat (limited to 'drivers/rtc/lib.c')
-rw-r--r--drivers/rtc/lib.c107
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;