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    SubjectRe: [PATCH] timekeeping: Prevent time going backwards on resume
    On Wed, 12 Sep 2007 00:36:34 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:

    > Timekeeping resume adjusts xtime by adding the slept time in seconds and
    > resets the reference value of the clock source (clock->cycle_last).
    > clock->cycle last is used to calculate the delta between the last xtime
    > update and the readout of the clock source in __get_nsec_offset(). xtime
    > plus the offset is the current time. The resume code ignores the delta
    > which had already elapsed between the last xtime update and the actual
    > time of suspend. If the suspend time is short, then we can see time
    > going backwards on resume.
    >
    > Suspend:
    > offs_s = clock->read() - clock->cycle_last;
    > now = xtime + offs_s;
    > timekeeping_suspend_time = read_rtc();
    >
    > Resume:
    > sleep_time = read_rtc() - timekeeping_suspend_time;
    > xtime.tv_sec += sleep_time;
    > clock->cycle_last = clock->read();
    > offs_r = clock->read() - clock->cycle_last;
    > now = xtime + offs_r;
    >
    > if sleep_time_seconds == 0 and offs_r < offs_s, then time goes
    > backwards.
    >
    > Fix this by storing the offset from the last xtime update and add it to
    > xtime during resume, when we reset clock->cycle_last:
    >
    > sleep_time = read_rtc() - timekeeping_suspend_time;
    > xtime.tv_sec += sleep_time;
    > xtime += offs_s; /* Fixup xtime offset at suspend time */
    > clock->cycle_last = clock->read();
    > offs_r = clock->read() - clock->cycle_last;
    > now = xtime + offs_r;
    >
    > Thanks to Marcelo for tracking this down on the OLPC and providing the
    > necessary details to analyze the root cause.
    >
    > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    >
    > --- a/kernel/time/timekeeping.c
    > +++ b/kernel/time/timekeeping.c
    > @@ -280,6 +280,8 @@ void __init timekeeping_init(void)
    > static int timekeeping_suspended;
    > /* time in seconds when suspend began */
    > static unsigned long timekeeping_suspend_time;
    > +/* xtime offset when we went into suspend */
    > +static s64 timekeeping_suspend_offset;
    >
    > /**
    > * timekeeping_resume - Resumes the generic timekeeping subsystem.
    > @@ -305,6 +307,8 @@ static int timekeeping_resume(struct sys_device *dev)
    > wall_to_monotonic.tv_sec -= sleep_length;
    > total_sleep_time += sleep_length;
    > }
    > + /* Make sure that we have the correct xtime reference */
    > + timespec_add_ns(&xtime, timekeeping_suspend_offset);
    > /* re-base the last cycle value */
    > clock->cycle_last = clocksource_read(clock);
    > clock->error = 0;
    > @@ -326,6 +330,8 @@ static int timekeeping_suspend(struct sys_device *dev, pm_message_t state)
    > unsigned long flags;
    >
    > write_seqlock_irqsave(&xtime_lock, flags);
    > timekeeping_suspended = 1;
    > + /* Get the current xtime offset */
    > + timekeeping_suspend_offset = __get_nsec_offset();
    > timekeeping_suspend_time = read_persistent_clock();
    > write_sequnlock_irqrestore(&xtime_lock, flags);
    >

    kernel/time/timekeeping.c: In function 'timekeeping_suspend':
    kernel/time/timekeeping.c:335: error: implicit declaration of function '__get_nsec_offset'

    sparc32, and presumably anything with CONFIG_GENERIC_TIME=n


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