[PATCH/RFC] powerpc: Fix decrementer setup on 1GHz boards

[Benjamin Herrenschmidt]

On Thu, 2009-10-08 at 16:49 +0200, Stefan Roese wrote:
> We noticed that recent kernels didn't boot on our 1GHz Canyonlands 460EX
> boards anymore. As it seems, patch 8d165db1 [powerpc: Improve
> decrementer accuracy] introduced this problem. The routine div_sc()
> overflows with shift = 32 resulting in this incorrect setup:
> 
> time_init: decrementer frequency = 1000.000012 MHz
> time_init: processor frequency   = 1000.000012 MHz
> clocksource: timebase mult[400000] shift[22] registered
> clockevent: decrementer mult[33] shift[32] cpu[0]
> 
> This patch now introduces a local div_dc64() version of this function
> so that this overflow doesn't happen anymore.

Ugh ? We never expected that the decrementer would be ticking that fast.
This is too fast actually. Somebody at AMCC must be told to stick a
divider in front of the timebase when reaching such frequencies.

Ben.

> 
> Signed-off-by: Stefan Roese <sr at denx.de>
> Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> Cc: Anton Blanchard <anton at samba.org>
> Cc: Detlev Zundel <dzu at denx.de>
> ---
> Ben, Anton, how should we handle this? Is this patch acceptable? Or how
> should this be solved?
> 
>  arch/powerpc/kernel/time.c |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 92dc844..3ad729f 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -882,12 +882,21 @@ static void decrementer_set_mode(enum clock_event_mode mode,
>  		decrementer_set_next_event(DECREMENTER_MAX, dev);
>  }
>  
> +static inline uint64_t div_sc64(unsigned long ticks, unsigned long nsec,
> +				int shift)
> +{
> +	uint64_t tmp = ((uint64_t)ticks) << shift;
> +
> +	do_div(tmp, nsec);
> +	return tmp;
> +}
> +
>  static void __init setup_clockevent_multiplier(unsigned long hz)
>  {
>  	u64 mult, shift = 32;
>  
>  	while (1) {
> -		mult = div_sc(hz, NSEC_PER_SEC, shift);
> +		mult = div_sc64(hz, NSEC_PER_SEC, shift);
>  		if (mult && (mult >> 32UL) == 0UL)
>  			break;
>