Messages in this thread | | | Date | Thu, 12 Mar 2009 02:30:50 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [BUG,2.6.28,s390] Fails to boot in Hercules S/390 emulator |
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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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