• John Stultz's avatar
    rtc: Avoid accumulating time drift in suspend/resume · 3dcad5ff
    John Stultz authored
    Because the RTC interface is only a second granular interface,
    each time we read from the RTC for suspend/resume, we introduce a
    half second (on average) of error.
    
    In order to avoid this error accumulating as the system is suspended
    over and over, this patch measures the time delta between the RTC
    and the system CLOCK_REALTIME.
    
    If the delta is less then 2 seconds from the last suspend, we compensate
    by using the previous time delta (keeping it close). If it is larger
    then 2 seconds, we assume the clock was set or has been changed, so we
    do no correction and update the delta.
    
    Note: If NTP is running, ths could seem to "fight" with the NTP corrected
    time, where as if the system time was off by 1 second, and NTP slewed the
    value in, a suspend/resume cycle could undo this correction, by trying to
    restore the previous offset from the RTC. However, without this patch,
    since each read could cause almost a full second worth of error, its
    possible to get almost 2 seconds of error just from the suspend/resume
    cycle alone, so this about equal to any offset added by the compensation.
    
    Further on systems that suspend/resume frequently, this should keep time
    closer then NTP could compensate for if the errors were allowed to
    accumulate.
    
    Credits to Arve Hjønnevåg for suggesting this solution.
    
    This patch also improves some of the variable names and adds more clear
    comments.
    
    CC: Arve Hjønnevåg <arve@android.com>
    CC: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
    3dcad5ff
class.c 6.81 KB