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

[Wu Zhangjin <wuzhangjin@gmail.com>]

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, 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.

The core idea here is to get a smaller mult to let (cyc * mult) not
overflow that quickly but also not get a 'bad' precision, of course, we
can try to implement the 128bit arithmatic in 32bit system, but that may
increase the overhead(not tested it yet, perhaps will be worse than the
original getnstimeofday()).

Regards,
	Wu Zhangjin