Gilles Chanteperdrix wrote:
On Fri, Apr 4, 2008 at 12:45 PM, Jan Kiszka <[EMAIL PROTECTED]> wrote:
Sebastian Smolorz wrote:

Jan Kiszka wrote:

Sebastian Smolorz wrote:

Jan Kiszka wrote:

This patch may do the trick: it uses the inverted tsc-to-ns function
instead of the frequency-based one. Be warned, it is totally untested inside
Xenomai, I just ran it in a user space test program. But it may give an
idea.
Your patch needed two minor corrections (ns instead of ts in functions
xnarch_ns_to_tsc()) in order to compile. A short run (30 minutes) of latency
-t1 seems to prove your bug-fix: There seems to be no drift.
That's good to hear.


If I got your patch correctly, it doesn't make xnarch_tsc_to_ns more
precise but introduces a new function xnarch_ns_to_tsc() which is also less
precise than the generic xnarch_ns_to_tsc(), right?
Yes. It is now precisely the inverse imprecision, so to say. :)


So isn't there still the danger of getting wrong values when calling
xnarch_tsc_to_ns()  not in combination with xnarch_ns_to_tsc()?
Only if the user decides to implement his own conversion. Xenomai with
all its skins and both in kernel and user space should always run through
the xnarch_* path.
OK, would you commit the patch?

 Will do unless someone else has concerns. Gilles, Philippe? ARM and
Blackfin then need to be fixed similarly, full patch attached.

Well, I am sorry, but I do not like this solution;
- the aim of scaled math is to avoid divisions, and with this patch we
end up using divisions;

Please check again, no new division due to my patch, just different parameters for the existing one.

- with scaled math we do wrong calculations, and making a wrong
xnarch_ns_to_tsc only works for values which should be passed to
xnarch_tsc_to_ns.

IMHO, the error is within the range of the clock's precision, if not even below. So struggling for mathematically precise conversion of imprecise physical values makes no sense to me. Therefore I once proposed the scaled-math optimization.

Jan

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