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    SubjectRe: [BUG,2.6.28,s390] Fails to boot in Hercules S/390 emulator
    On Wed, 11 Mar 2009, john stultz wrote:
    > For a cleaner version, could you try the following, against 2.6.29-git
    > with no other modification?

    cleaner ?

    > xtime_nsec is expected at times to be negative. Instead of trying to
    > handle all the shifting properly via casts, define it as a s64 instead
    > of a u64.
    >
    > NOT FOR INCLUSION
    > Signed-off-by: John Stultz <johnstul@us.ibm.com>
    >
    > diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
    > index f88d32f..e217000 100644
    > --- a/include/linux/clocksource.h
    > +++ b/include/linux/clocksource.h
    > @@ -86,7 +86,7 @@ struct clocksource {
    > * more than one cache line.
    > */
    > cycle_t cycle_last ____cacheline_aligned_in_smp;
    > - u64 xtime_nsec;
    > + s64 xtime_nsec;
    > s64 error;
    > struct timespec raw_time;
    >
    > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
    > index 900f1b6..387be3c 100644
    > --- a/kernel/time/timekeeping.c
    > +++ b/kernel/time/timekeeping.c
    > @@ -546,7 +546,7 @@ void update_wall_time(void)
    > /* store full nanoseconds into xtime after rounding it up and
    > * add the remainder to the error difference.
    > */
    > - xtime.tv_nsec = ((s64)clock->xtime_nsec >> clock->shift) + 1;
    > + xtime.tv_nsec = (clock->xtime_nsec >> clock->shift) + 1;
    > clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
    > clock->error += clock->xtime_nsec << (NTP_SCALE_SHIFT - clock->shift);

    This code sequence does:

    xtime.tv_nsec = (clock->xtime_nsec >> clock->shift) + 1;
    clock->xtime_nsec -= xtime.tv_nsec << clock->shift;

    Lets substitute a bit:

    a = xtime.tv_nsec
    b = clock->xtime_nsec
    c = clock->shift
    r = result (which ends up in b aka clock->xtime_nsec again for the next round)

    => a = (b >> c) + 1
    r = b - (a << c)

    b >> c = b / 2^c
    a << c = a * 2^c

    => a = (b / 2^c) + 1
    r = b - (a * 2^c)

    => r = b - ((b / 2^c) + 1) * 2^c
    r = b - ((2^c * b / 2^c) + 2^c)
    r = b - (2^c * b / 2^c) - 2^c
    r = b - b - 2^c
    r = -2^c

    => r = -(1 << c)

    So the whole business boils down to:

    clock->xtime_nsec = -(1 << clock->shift);

    Famous last words (see also arch/x86/kernel/tsc.c and
    arch/x86/include/asm/timer.h):

    -johnstul@us.ibm.com "math is hard, lets go shopping!"

    Thanks,

    tglx


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