Question on all SPARC BSP get nanoseconds handler

[Nick Withers]

On Mon, 2015-02-02 at 11:45 -0600, Joel Sherrill wrote:
> Hi
> 
> Last week while teaching an RTEMS class, we looked at a SPARC BSP's
> get nanoseconds since last tick handler. They all follow this pattern:
> 
>         clicks = Read Counter
>         if ( clock interrupt pending ) {
>          clicks = Read Counter;
>           time =(2 * time per tick) - (clicks);
>         } else {
>           time = time per tick - clicks
>         }
>         
>         return time;
>         
> The code is correct but I would like to add a comment to all three
> versions
> of this code so it is clear what was intended.
> 
> I think it is something like. Read the counter. If there is not an
> interrupt pending,
> then we are in the same clock tick. If there is one pending, then see
> how long
> past that tick we are. The handler has to report from slightly > 0 to
> slightly 
> less than 2 ticks depending on where we are relative to the next clock
> tick.
> 
> Just wanting to make the code more understandable for the next person.
> Help
> writing the comment is appreciated.

For what it's worth, Sebastian and I were looking at similar code in the
PowerPC tree recently and added some comments that are in this sort-of
area:
https://devel.rtems.org/changeset/d11b711b3ed6b4a053a298cf4e9f1209748fae5e/rtems

> -- 
> Joel Sherrill, Ph.D.             Director of Research & Development
> joel.sherrill at OARcorp.com        On-Line Applications Research
> Ask me about RTEMS: a free RTOS  Huntsville AL 35805
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--
Nick Withers

Embedded Systems Programmer
Department of Nuclear Physics
Research School of Physics and Engineering
The Australian National University