us_delay() improvement

[Jonathan Larmour]

Nick Garnett wrote:
> +            if( usecs > usec_per_period/2 )
> +                slice = CYGNUM_KERNEL_COUNTERS_RTC_PERIOD/2;
> +            else if( CYGNUM_KERNEL_COUNTERS_RTC_PERIOD >= 0x7FFFFFFF/usec_per_period )
> +                slice = usecs * ticks_per_usec;
> +            else
> +            {
> +                slice = usecs*CYGNUM_KERNEL_COUNTERS_RTC_PERIOD;
> +                slice /= usec_per_period;
> +            }

I've been thinking about this.... we get overflow if 
usecs*CYGNUM_KERNEL_COUNTERS_RTC_PERIOD overflows, which means that since 
usecs should be at most usec_per_period/2, we have to "play safe" if 
plugging usecs == usec_per_period/2 in to the multiplication would 
overflow, because that's the worst case.

So we get the expression to test as:

(usec_per_period/2)*CYGNUM_KERNEL_COUNTERS_RTC_PERIOD >= 0x7FFFFFFF

which we reorganise to prevent overflow of this expression to become:

CYGNUM_KERNEL_COUNTERS_RTC_PERIOD/2 >= 0x7FFFFFFF/usec_per_period

which should be sufficient. The code as written will unnecessarily catch 
certain configurations I'd think.

> +            // Adjust by elapsed, not slice, since an interrupt may
> +            // have been stalling us for some time.
> +
> +            if( CYGNUM_KERNEL_COUNTERS_RTC_PERIOD >= 0x7FFFFFFF/usec_per_period )
> +                elapsed_usec = elapsed / ticks_per_usec;
> +            else
> +            {
> +                elapsed_usec = elapsed * usec_per_period;
> +                elapsed_usec = elapsed_usec / CYGNUM_KERNEL_COUNTERS_RTC_PERIOD;
> +            }

FAOD the above wouldn't have the same issue because of the additional 
complication that elapsed may be greater than slice, so the risk of 
overflow is even higher, but by an indeterminate amount to make things 
more fun ;-). A factor of two fudge seems as good/bad as any other amount 
then.

Jifl
-- 
eCosCentric    http://www.eCosCentric.com/    The eCos and RedBoot experts
--[ "You can complain because roses have thorns, or you ]--
--[  can rejoice because thorns have roses." -Lincoln   ]-- Opinions==mine