[Xenomai] rtdm_timer_start latency?

Lowell Gilbert kludge at be-well.ilk.org
Mon Feb 12 17:32:01 CET 2018

Lange Norbert <norbert.lange at andritz.com> writes:

> this seems identical to an issue I have. I wanted to start a
> millisecond tick, aligned to seconds, but used the POSIX
> timerfd_settime.  Under Linux this works as intendent, under cobalt it
> doesn't.
> I expect that the first timeout would be somewhat close to the call,
> on a "whole" millisecond.
> What happens is that the timer-function convert the (Nano-)seconds to
> the internal resolution. This has a small error, which normally won't
> matter for short timespans.  The problem here is, that the absolute
> time will get converted with a big (absolute) error - growing bigger
> the longer your system is up.
> What you could do, is reading the current time and fixing it up to a
> whole tick close to it, name this prev_secondtime, then use this as
> initial value.  Now the driver will have to fixup nothing or only a
> small amount, keeping the error low.

It seems like a different problem to me, because I'm working in relative
mode, so such rounding shouldn't have a chance to get very large.

Also, I have a hardware timer that allows me to kick off much more often
than the system tick. Once the timer starts, it fires quite accurately
as often as every 10 microseconds.

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