diff options
Diffstat (limited to 'drivers/rtc/lib.c')
| -rw-r--r-- | drivers/rtc/lib.c | 40 |
1 files changed, 19 insertions, 21 deletions
diff --git a/drivers/rtc/lib.c b/drivers/rtc/lib.c index 13b5b1f20465..f7051592a6e3 100644 --- a/drivers/rtc/lib.c +++ b/drivers/rtc/lib.c @@ -51,7 +51,7 @@ EXPORT_SYMBOL(rtc_year_days); */ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm) { - int days, secs; + int secs; u64 u64tmp; u32 u32tmp, udays, century, day_of_century, year_of_century, year, @@ -59,28 +59,26 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm) bool is_Jan_or_Feb, is_leap_year; /* - * Get days and seconds while preserving the sign to - * handle negative time values (dates before 1970-01-01) + * The time represented by `time` is given in seconds since 1970-01-01 + * (UTC). As the division done below might misbehave for negative + * values, we convert it to seconds since 0000-03-01 and then assume it + * will be non-negative. + * Below we do 4 * udays + 3 which should fit into a 32 bit unsigned + * variable. So the latest date this algorithm works for is 1073741823 + * days after 0000-03-01 which is in the year 2939805. */ - days = div_s64_rem(time, 86400, &secs); + time += (u64)719468 * 86400; + + udays = div_s64_rem(time, 86400, &secs); /* - * We need 0 <= secs < 86400 which isn't given for negative - * values of time. Fixup accordingly. + * day of the week, 0000-03-01 was a Wednesday (in the proleptic + * Gregorian calendar) */ - if (secs < 0) { - days -= 1; - secs += 86400; - } - - /* day of the week, 1970-01-01 was a Thursday */ - tm->tm_wday = (days + 4) % 7; - /* Ensure tm_wday is always positive */ - if (tm->tm_wday < 0) - tm->tm_wday += 7; + tm->tm_wday = (udays + 3) % 7; /* - * The following algorithm is, basically, Proposition 6.3 of Neri + * The following algorithm is, basically, Figure 12 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 @@ -100,15 +98,15 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm) * (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 + * [1] Neri C, Schneider L. Euclidean affine functions and their + * application to calendar algorithms. Softw Pract Exper. + * 2023;53(4):937-970. doi: 10.1002/spe.3172 + * https://doi.org/10.1002/spe.3172 * * (*) The numbering of months follows rtc_time more closely and * thus, is slightly different from [1]. */ - udays = days + 719468; - u32tmp = 4 * udays + 3; century = u32tmp / 146097; day_of_century = u32tmp % 146097 / 4; |