Commit 1d1bb12a authored by Cassio Neri's avatar Cassio Neri Committed by Alexandre Belloni
Browse files

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: default avatarCassio Neri <cassio.neri@gmail.com>
Reported-by: default avatarkernel test robot <lkp@intel.com>
Signed-off-by: default avatarAlexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210624201343.85441-1-cassio.neri@gmail.com
parent fffd603a

drivers/rtc/Kconfig

+10 −0
Original line number Diff line number Diff line
@@ -10,6 +10,16 @@ config RTC_MC146818_LIB 
	bool

	select RTC_LIB



config RTC_LIB_KUNIT_TEST

	tristate "KUnit test for RTC lib functions" if !KUNIT_ALL_TESTS

	depends on KUNIT

	default KUNIT_ALL_TESTS

	select RTC_LIB

	help

	  Enable this option to test RTC library functions.



	  If unsure, say N.



menuconfig RTC_CLASS

	bool "Real Time Clock"

	default n

drivers/rtc/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -178,3 +178,4 @@ obj-$(CONFIG_RTC_DRV_WM8350) += rtc-wm8350.o 
obj-$(CONFIG_RTC_DRV_X1205)	+= rtc-x1205.o

obj-$(CONFIG_RTC_DRV_XGENE)	+= rtc-xgene.o

obj-$(CONFIG_RTC_DRV_ZYNQMP)	+= rtc-zynqmp.o

obj-$(CONFIG_RTC_LIB_KUNIT_TEST)	+= lib_test.o

drivers/rtc/lib.c

+80 −27
Original line number Diff line number Diff line
@@ -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;

	/*

	 * 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].

	 */



	for (month = 0; month < 11; month++) {

		int newdays;

	udays		= ((u32) days) + 719468;



		newdays = days - rtc_month_days(month, year);

		if (newdays < 0)

			break;

		days = newdays;

	}

	tm->tm_mon = month;

	tm->tm_mday = days + 1;

	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;

drivers/rtc/lib_test.c

0 → 100644
+79 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: LGPL-2.1+



#include <kunit/test.h>

#include <linux/rtc.h>



/*

 * Advance a date by one day.

 */

static void advance_date(int *year, int *month, int *mday, int *yday)

{

	if (*mday != rtc_month_days(*month - 1, *year)) {

		++*mday;

		++*yday;

		return;

	}



	*mday = 1;

	if (*month != 12) {

		++*month;

		++*yday;

		return;

	}



	*month = 1;

	*yday  = 1;

	++*year;

}



/*

 * Checks every day in a 160000 years interval starting on 1970-01-01

 * against the expected result.

 */

static void rtc_time64_to_tm_test_date_range(struct kunit *test)

{

	/*

	 * 160000 years	= (160000 / 400) * 400 years

	 *		= (160000 / 400) * 146097 days

	 *		= (160000 / 400) * 146097 * 86400 seconds

	 */

	time64_t total_secs = ((time64_t) 160000) / 400 * 146097 * 86400;



	int year	= 1970;

	int month	= 1;

	int mday	= 1;

	int yday	= 1;



	struct rtc_time result;

	time64_t secs;

	s64 days;



	for (secs = 0; secs <= total_secs; secs += 86400) {



		rtc_time64_to_tm(secs, &result);



		days = div_s64(secs, 86400);



		#define FAIL_MSG "%d/%02d/%02d (%2d) : %ld", \

			year, month, mday, yday, days



		KUNIT_ASSERT_EQ_MSG(test, year - 1900, result.tm_year, FAIL_MSG);

		KUNIT_ASSERT_EQ_MSG(test, month - 1, result.tm_mon, FAIL_MSG);

		KUNIT_ASSERT_EQ_MSG(test, mday, result.tm_mday, FAIL_MSG);

		KUNIT_ASSERT_EQ_MSG(test, yday, result.tm_yday, FAIL_MSG);



		advance_date(&year, &month, &mday, &yday);

	}

}



static struct kunit_case rtc_lib_test_cases[] = {

	KUNIT_CASE(rtc_time64_to_tm_test_date_range),

	{}

};



static struct kunit_suite rtc_lib_test_suite = {

	.name = "rtc_lib_test_cases",

	.test_cases = rtc_lib_test_cases,

};



kunit_test_suite(rtc_lib_test_suite);