Re: [BUG,2.6.28,s390] Fails to boot in Hercules S/390 emulator

Thomas Gleixner
2009-03-12T01:31:21Z

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