• Julius Werner's avatar
    rtc: rk808: Compensate for Rockchip calendar deviation on November 31st · f076ef44
    Julius Werner authored
    In A.D. 1582 Pope Gregory XIII found that the existing Julian calendar
    insufficiently represented reality, and changed the rules about
    calculating leap years to account for this. Similarly, in A.D. 2013
    Rockchip hardware engineers found that the new Gregorian calendar still
    contained flaws, and that the month of November should be counted up to
    31 days instead. Unfortunately it takes a long time for calendar changes
    to gain widespread adoption, and just like more than 300 years went by
    before the last Protestant nation implemented Greg's proposal, we will
    have to wait a while until all religions and operating system kernels
    acknowledge the inherent advantages of the Rockchip system. Until then
    we need to translate dates read from (and written to) Rockchip hardware
    back to the Gregorian format.
    
    This patch works by defining Jan 1st, 2016 as the arbitrary anchor date
    on which Rockchip and Gregorian calendars are in sync. From that we can
    translate arbitrary later dates back and forth by counting the number
    of November/December transitons since the anchor date to determine the
    offset between the calendars. We choose this method (rather than trying
    to regularly "correct" the date stored in hardware) since it's the only
    way to ensure perfect time-keeping even if the system may be shut down
    for an unknown number of years. The drawback is that other software
    reading the same hardware (e.g. mainboard firmware) must use the same
    translation convention (including the same anchor date) to be able to
    read and write correct timestamps from/to the RTC.
    Signed-off-by: default avatarJulius Werner <jwerner@chromium.org>
    Reviewed-by: default avatarDouglas Anderson <dianders@chromium.org>
    Signed-off-by: default avatarAlexandre Belloni <alexandre.belloni@free-electrons.com>
    f076ef44
rtc-rk808.c 13.3 KB