Re: [PATCH v2 1/3] MIPS: add a common mips_cyc2ns()

[Wu Zhangjin <wuzhangjin@gmail.com>]

On Thu, 2010-04-08 at 17:32 +0800, Wu Zhangjin wrote:
> On Wed, 2010-04-07 at 09:50 -0700, David Daney wrote:
> [...]
> > > +static inline unsigned long long mips_cyc2ns(u64 cyc, u64 mult, u64 shift)
> > > +{
> > > +#ifdef CONFIG_32BIT
> > > +	/*
> > > +	 * To balance the overhead of 128bit-arithematic and the precision
> > > +	 * lost, we choose a smaller shift to avoid the quick overflow as the
> > > +	 * X86&  ARM does. please refer to arch/x86/kernel/tsc.c and
> > > +	 * arch/arm/plat-orion/time.c
> > > +	 */
> > > +	return (cyc * mult)>>  shift;
> > 
> > Have you tested that on a 32-bit kernel?  I think it may overflow for 
> > many cases.
> > 
> 
> Yes, I have done some basic testing ;)
> 
> Since a c0 count with 400MHz clock frequency will overflow after about
> more than 1 hour with the scaling factor 10,

Exactly, with 10, it will overflow after counting 2^51, which means it
will overflow at 3127 hours(about 130 days), which is enough.

>  I think it is enough for
> the generic debugging, such as Ftrace, If it is not enough, perhaps we
> can choose a smaller scaling factor, such as 8.

With 8, it will overflow after 12510 hours(about 521 days).

So, I will choose 8 in the next revision.

PS: ...

#include <stdio.h>

#define NSEC_PER_SEC 1000000000	/* 10^9 */
#define CLOCK_FREQ 400000000	/* 400 M*/
#define CYC2NS_SHIFT 8

int main(void)
{
	unsigned long long mult, v;
	unsigned long long ullint_max = ~0;
	unsigned long long tmp = 2ULL<<53;
	double t_ns;
	int t_h, t_d;

        v = NSEC_PER_SEC;
        v <<= CYC2NS_SHIFT;
        v += CLOCK_FREQ/2;
        v = v / CLOCK_FREQ;
        mult = v;

	printf("sizeof(unsigned long long): %d\n", sizeof(unsigned long long));
	printf("%lld (max of cycles)\n", ullint_max/mult);
	printf("%lld (2^53)\n", tmp);

	t_h = (double)tmp / CLOCK_FREQ / 3600;
	t_d = t_h / 24;
	printf("%d hours, %d days\n", t_h, t_d);

	return 0;
}

$ gcc -o clock clock.c
$ $ ./clock 
sizeof(unsigned long long): 8
28823037615171174 (max of cycles)
18014398509481984 (2^53)
12509 hours, 521 days

Regards,
	Wu Zhangjin