Re: [Xenomai-core] xenomai 2.5 rc1 timing computations.

[Philippe Gerum <rpm@xenomai.org>]

On Tue, 2009-04-28 at 07:50 +0200, Gilles Chanteperdrix wrote:
> Hi,
> 
> currently, the situation is this:
> - the timing core uses an approximate value of the cpu frequency (using
> xnarch_llmulshft) to do conversions between tsc and ns;
> - the APIC timer reprogrammation still uses imuldiv, that is a more
> exact cpu frequency, coupled with an approximate APIC frequency
> (obtained from ipipe_request_tickdev), to do the conversion between tsc
> delays and APIC delays;
> - the posix skin uses llimd to do conversions between timespec and
> timeval structures and tsc values
> - the posix and native skins in user-space still use llimd to do
> conversions between ns and tsc (only if using tsc in user-space).
> 
> These are causes of unprecisions, I think we should fix this, though it
> is unlikely to be the cause of the high jitters observed by Vikesh with
> 2.5-rc1.
> 

Ack, we definitely should. Working on allowing userland to use
llmulshift for timing computations for the native skin here, as a first
step. For that purpose, xnsysinfo_t has just been extended to carry the
pre-scaled values needed for llmulshft. I just don't want to have to
issue a syscall for those conversions to take place.

> Regards.
> 
-- 
Philippe.