Re: [PATCH] clocksource: document some basic concepts

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-11-15 at 15:06 -0500, Nicolas Pitre wrote:
> On Mon, 15 Nov 2010, Peter Zijlstra wrote:
> 
> > On Mon, 2010-11-15 at 11:33 +0100, Linus Walleij wrote:
> > > +The sched_clock() function may wrap only on unsigned long long boundaries,
> > > +i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps
> > > +after circa 585 years. (For most practical systems this means "never".)
> 
> This is not necessarily the case.  Some implementations require a 
> scaling factor too, making the number of remaining bits smaller than 64.  
> See arch/arm/mach-pxa/time.c:sched_clock() for example, which has a 
> maximum range of 208 days.  Of course, in practice we don't really care 
> if sched_clock() wraps each 208 days, unlike for clock-source.

Right, its like sched_clock() would go backwards and we loose some
precision during that jiffy (assuming the arch uses
HAVE_UNSTABLE_SCHED_CLOCK), nothing too horrible.

> > Currently true, John Stultz was going to look into ammending this by
> > teaching the kernel/sched_clock.c bits about early wraps (and a way for
> > architectures to specify this)
> > 
> > #define SCHED_CLOCK_WRAP_BITS 48
> > 
> > ...
> > 
> > #ifdef SCHED_CLOCK_WRAP_BITS
> >   /* handle short wraps */
> > #endif
> 
> Is this worth supporting?  I'd simply use the low 32 bits and extend it 
> to 63 bits using cnt32_to_63(). If the low 32 bits are wrapping too 
> fast, then just shifting them down a few positions first should do the 
> trick.  That certainly would have a much faster result.

Whatever works, dealing with the wrap is only a few shifts.